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Please be advised that this information was generated on 2021-10-06 and may be subject to change. PLACENTAL (К^СGEKÉ S

* Уп^ггі«» ¿л i ι) Дстг R.H.W. LORIJN

^

FETAL OXYGEN CONSUMPTION AND PLACENTAL OXYGEN EXCHANGE PROMOTORES

PROF. DR LD. LONGO

PROF. DR Т.К.A.B. ESKES

CO-REFERENT

DR L.L.H. PEETERS FETAL OXYGEN CONSUMPTION AND PLACENTAL OXYGEN EXCHANGE

PROEFSCHRIFT

ter verkrijging van de graad van doctor in de geneeskunde aan de Katholieke Universiteit te Nijmegen, op gezag van de Rector Magnificus Prof. Dr. P.G.A.B. Wijdeveld, volgens besluit van het College van Decanen in het openbaar ter verdediging op vrijdag 9 mei 1980 des namiddags te 2.00 uur precies

door

RONALD HANS WILMAR LORIJN

geboren te 's Gravenhage

Krips Repro Meppel 1980 This study was performed during a two year research fellowship in the Department of Perinatal Biology, Loma Linda University, School of Medicine, Loma Linda, California 92350, U.S.A., under the guidance of Prof. Dr. L. D. Longo. A Nato Grant awarded by the Netherlands Organization for the Advancement of Pure Research (Z. W. O. ) and U. S. P. H. S. Grant 03807 furnished the financial support of the investigation. "Therefore, one concludes that one and the same soul governs the bodies and one and the same nourishes both. "

Leonardo da Vinci Copyright 1980. All rights reserved. No part of this publication may be reproduced in any form by any photomechanical means without permission from the author. To Hansje To Rosa Maria, Vivienne, Geraldine To my parents

Aknowledgements

The completion of this thesis gives me an excellent opportunity to express my sincere feelings of indebtedness to all who have contributed to it's realization

My parents, who not only gave me a youth free from care, but also stimulated me to accept challenges in order to explore the unknown. The staff members, technicians and research fellows of the Dept. of Perinatal Biology of Loma Linda University, who created an unforgettable and inspiring atmosphere of friendship and dedication to research. Mrs. Helen Little and Mrs. Linda Moore for editorial assistance, while back in Holland Miss Ciska Don helped me to put the finishing touches to the work.

Rosa Maria, a ti en particular debo mucho agradecimiento, por que todo esto ha sido posible por tu apoyo permanente.

CO NTENTS page I. INTRODUCTION 15

II. FETAL OXYGEN CONSUMPTION AND PLACENTAL EXCHANGE 18 UNDER PHYSIOLOGICAL CONDITIONS

1. Maternal Oxygen Affinity and Capacity 18 1.1. General Considerations 18 1.2. 18 1.3. Blood Oxygen Affinity and the Oxyhemoglobin saturation 20 curve 1.4. Blood Oxygen Capacity 23

2. Fetal Blood Oxygen Affinity and Capacity 25 2.1. 25 2.2. Blood Oxygen Affinity 29 2. 3. Blood Oxygen Capacity 32

3. Interrelations of Maternal and Fetal Blood Oxygen Affinity and Capacity 33 3.1. General Considerations 33 3. 2. Species Differences 38

4. Relation of Maternal and Fetal Oxygen Partial Pressures to Placental Oxygen Exchange 38 4.1. Theoretical Considerations 38 4.2. Experimental Studies 43 4.3. Clinical Implications 49 4.3.1. Maternal Oxygen Inhalation 49 4. 3. 2. Hyperbaric Oxygenation 51 4.3.3. Acute Decrease in Inspired Oxygen Tension 52 4.3.4. Chronic Decrease in Maternal Inspired Oxygen - High Altitude 54 4.3.5. Chronic Maternal Hypoxemia - Cyanotic Congeni­ tal Disease 58

5. Relation of Maternal and Fetal Oxygen Affinity to Placental Oxygen Exchange 60 5.1. Theoretical Considerations ¿0

11 page 5.2. Decreasing Fetal Blood Oxygen Affinity by Intrauterine Transfusion 63 5. 3. Changes in Oxygen Affinity due to ^6 5.4. Maternal Hyperventilation and Other Conditions 71

6. Relation of Maternal and Fetal Placental Hemoglobin Flow to Placental Oxygen Exchange 74 6.1. General Considerations 74 6.2. Theoretical Considerations 77 6. 3. Experimental Studies 80 6.4. Clinical Implications 85

7. Interrelations of Maternal and Fetal Placental Flows and Oxygen Exchange 89 7. 1. Distribution of Maternal and Fetal Placental Flows 89 7.1.1. Nonuniform Distribution of Maternal and Fetal Blood Flows 89 7. 1. 2. Distribution of Placental Diffusing Capacity to Blood Flows 9' 7.2. "Sluice" or "Waterfall" Flow in the Placental Vessels 91 7.3. Uterine Contractions and Oxygen Exchange 94

III. TRANSFER OF OXYGEN DURING INCREASED FETAL OXYGEN CONSUMPTION 97

1. Introduction 97 2. Methods and Materials 98 2.1. Surgical Procedure 98 2.2. Experimental Procedure 100 2.2.1. Norepinephrine 100 2.2.2. 3,5,3'-L-Triiodothyronine 101 2.3. Blood Flows and Cardiac Output 101 2.4. Placental Carbon Monoxide Diffusing Capacity 104 2.5. Hormones Determination 106 2.6. Statistical Analysis 107

3. Results 107 3.1. Norepinephrine 107 3.1.1. Plasma Catecholamine concentrations 107

12 page 3.1.2. Fetal Oxygen Consumption 112 3.1.3. Placental Diffusing Capacity 112 3.1.4. Cardiovascular Responses 114 3.1.4.1. Maternal Ш 3.1.4.2. Fetal lU 3.1.4.3. Norepinephrine Dose-Response Relations 114 3.1.4.4. Cardiac Output and Blood Flow 3.2. 3,5,3'-L-Triiodothyronine Changes 117 3.2.1. Changes in Thyroid Hormones 119 3.2.2. Fetal Plasma Cortisol and Catecholamine Concentrations 119 3.2.3. Fetal Oxygen Consumption 119 3.2.4. Placental Carbon Monoxide Diffusing Capacity 121 3.2.5. Cardiovascular Responses 121 3.2.5.1. Maternal 121 3.2.5.2. Fetal 121 3.2.5.3. Blood Flow and Cardiac Output 121 3.2.5.4. Histological Examination of the Fetal Lung 124

DISCUSSION 125 1. Fetal Oxygen Consumption '25 1.1. Previous Estimates of Fetal Oxygen Consumption 125 1.2. Physiological Implications ^5 1.3. Clinical Implications 130

2. Norepinephrine Experiments 13' 2.1. Catecholamine Concentrations in the Mother and Fetus 131 2. 2. Cardiovascular Responses and Results of Previous Studies in the Fetus and Newborn 132

3. 3, 5, 3' - L - Triiodothyronine Experiments 134 3. 1. General Considerations ^35 3. 2. Thyroid Development and Effects upon the Fetus 136 3. 3. Changes in Maternal and Fetal Hormone Concen­ trations 136 page 3.4. Fetal Cardiovascular Responses 139 4. Comparison of Norepinephrine and 3,5,3' - L - Triiodothyro­ nine Results 139 V. SUMMARY 141 VI. BIBLIOGRAPHY 144 CURRICULUM VITAE

14 I. Introduction

The developing fetus presents α paradox of sorts. On one hand, its arterial blood oxygen tension is only 20 to 30 torr as compared with adult values of about 100 torr.

In addition, its rate of oxygen consumption is twice that of the adult per unit weight (8 vs 4 mUmin . kg ), and its oxygen reserve (that amount of oxygen contained in its blood and tissues) is only enough to meet its metabolic needs for one to two minutes. On the other hand, fetal blood oxygen affinity in humans and most other species exceeds that of the adult, and its blood O« capacity (a function of the hemoglobin concentration) is greater than that of the adult, at least during the later part of pregnancy. A complicating factor is that the flow of maternal and fetal blood to the placental exchange area must be reasonably matched for optimal oxygen transfer and transport to the fetal tissues. The question naturally arises how these and other factors interact to fit the fetus for its unique existence and optimal development. An additional question concerns the factors which limit placental oxygen transfer and the fetus' ability to withstand alterations in this exchange process.

Because the integrity of neural and other vital cells require a relatively unin­ terrupted oxygen supply, the processes whereby oxygen cascades by convection and diffusion from the mother's ambient environment to the cristae of the fetal mitochondria must be exquisitely regulated at each step. A critical link in this chain of sequential events is the placenta where a volume of oxygen sufficient to supply fetal needs must diffuse across the membranes from maternal to fetal blood during the short time the two circulations are in close contact. This process is a function of maternal and fetal arterial O- partial pressures, maternal and fetal « affinities, maternal and fetal placental hemoglobin flow rates, the diffusing capacity of the placenta, the vascular relations of maternal to fetal vessels, the amount of CO- exchanged, and several other factors (Longo et al.,

1972).

15 Eoch of these Factors in turn consists of numerous component elements. Tight coupling between the oxygen transport characteristics of both maternal and fetal blood and the respective blood flows is required. This subject is of importance in understanding the physiologic factors which affect placental oxygen exchange, their interactions, dynamics, and specific control mechanisms. In addition, the topic is of clinical importance in understanding the basis of normal and abnormal placental exchange and fetal oxygenation during pregnancy, labor, and delivery.

Therefore the first part of the thesis attempts to synthesize into a coherent whole many of the isolated and seemingly inconsequential findings reported during the past decade or so. It seeks to derive an understanding of the role of maternal and fetal blood oxygen tensions, blood O- affinities and capacities, and blood flows in affecting transplacental oxygen exchange, and thus oxygenation of the fetus.

Specific questions which we will address include the following: 1) How is the physiologic function of maternal hemoglobin correlated with its chemical structure?

2) What changes in maternal blood oxygen affinity and capacity occur during pregnancy, and what is their effect on placental oxygen exchange? 3) What are the biologic and physiologic differences of embryonic and fetal as compared with that of the adult? 4) How do variations in fetal blood oxygen affinity and capacity affect placental O- transfer? 5) What is the relation of maternal and fetal arterial oxygen tensions to placental O- exchange? 6) How is the exchange process affected by acute or chronic maternal hypoxia as during exposure to high altitude or in the presence of cyanotic congenital heart disease? 7) How is placental oxygen exchange affected by changes in maternal or fetal blood Oj affinity, as with fetal intrauterine transfusion or the presence of abnormal hemoglobins? 8) To what extent do maternal and fetal placental blood flows affect oxygen transfer?

9) Of what value is the concept of hemoglobin flow and how do alterations in hemoglobin flow affect the oxygen exchange process? 10) What are the normal

16 relations of maternal and fetal placental blood flows, and how does the distribution of these flows affect respiratory gas exchange? 11) How do variations in hydros­ tatic pressure in the maternal vascular bed affect umbilical flow, and visa versa?

12) To what extent is placental flow and oxygen exchange altered during uterine contractions?, and finally, 13) What important problems in this area require further research?

17 II. Fetal Oxygen Consumption and Placental Exchange under Physiological Conditions

1 . Maternal Blood Oxygen Affinity and Capacity

1.1. General Considerations

Because of its remarkable ability to combine reversibly with oxygen, hemo­

increases the oxygen transport capacity of blood about seventy fold over

that capacity which would exist if the oxygen transported was only that

physically dissolved in plasma. For example, if the entire oxygen requirements

of the maternal organism were met by physically dissolved oxygen the required

cardiac output would equal 100 liters«min . This calculation assumes a normal

oxygen consumption of 300 ml «min , and an arteriovenous O- difference of

3x10 ml »min [(3x10 ml· min )/(3xl0 ml »min )J. With strenuous

exercise the cardiac output would increase five to ten fold, further stressing

the system. Of course some ice fish (Chaenocephalus aceratus) survive in the

polar oceans with neither erythrocytes nor hemoglobin, lacking the need for

this because of their low metabolic rate in the near 0 С

antarctic environment (Ruud, 1965).

1.2. Hemoglobin

The blood of a normal pregnant woman contains 11 to 13 g hemoglo'.in per

deciliter (dl) of blood (Pritchard and Hunt, 1958). The total blood volume of a

near term pregnant woman equals about 5 liters (Pritchard, 1965). Each ml of

blood contains about 3x10 erythrocytes, and in turn each erythrocyte contains о about 2. 8x10 hemoglobin molecules. With a molecular weight of about 6.45

χ 10 Daltons, the concentration of hemoglobin in human blood approximates

2mMoles per liter, (1. 25x1 02 g. Г1 )(103 mM. ml"1 ) / (6. 45 χ 104 g. M-1 )=1. 94.

18 Human hemoglobin, a spheroidal molecule 65 χ 55 χ 50 A in diameter, (Perutz,

1964) is a tetramer containing four polypeptide globin chains. Most consist of two pairs of unlike chains, an alpha chain containing 141 amino- acid residues, and a non-alpha chain of 145 or 146 amino-acid residues. Five types of non-alpha chains have been identified in man (beta, gamma, delta, episolon, and zeta) corresponding to different stages of ontogenesis. The normal adult and fetal hemoglobins of man are designated and F, respectively. Numerous authors have reviewed the properties of these hemoglobins. Inasmuch as the two varieties of globin chains in hemoglobin A (or A, ) have been termed alpha and beta, the gross structure may be represented as0i„ \jl „ where the superscript denotes the hemoglobin source of the chain and the subscript has the usual chemical significance.

In addition to hemoglobin A, adult blood also contains about 2% of hemoglobin A«, A / A which is designatedcO-y о о · Human fetal hemoglobin (Hb F) contains two alpha A F chains in addition to two gamma chains, and is represented asg|/7 У- .

In all globin polypeptide chains an identical group rests near the surface between two helices of a given chain. Each heme contains an iron atom surrounded by a porphyrin ring. And there are many possible resonance structures for this relationship. Of the six coordination bonds of heme-iron, four bind to the porphyrin 87 η 92 ring, the fifth links to the so-called proximal histidineoi» and )i , respectively, and the sixth lodges opposite the distal histidine oL and )i , and represents the site of reversible combination with molecular oxygen. Molecular oxygen combines reversibly with the iron in hemoglobin and other heme pigments by mechanisms which as yet are understood only poorly. Oxygen binds to an iron atom in the 2+ ferrous (Fe ) state in the center of a porphyrin ring without either oxidizing the 3+ iron to the ferric (Fe ) state or becoming reduced. This combination of oxygen with hemoglobin is associated with a shift in the spatial relations among the four component polypeptide chains of the molecule (i.e. its quaternary structure).

19 Because each hemoglobin molecule (tetramer) contains four iron atoms, it can combine with four oxygen molecules. Each mole of hemoglobin thus can combine with four moles of oxygen for a normal blood O« capacity of about 8 mM· I

(1.94 mM hemoglobin·! χ 4 =7.76). Expressed alternatively a gram of hemo­ globin can combine with 1.368 ml oxygen (Scherrer and Bachofen, 1972) for an О, capacity of about 20 ml«dl with a normal hemoglobin concentration of 15 g hemo­ globin· dl

1.3. Blood Oxygen Affinity and the Oxyhemoglobin Saturation Curve

The oxygenation of one of the four enhances the oxygen affinity of the remaining heme moieties, an effect known as heme-heme interaction, or positive cooperativity. Heme-heme interactions constitute the biochemical basis of the sigmoidal shape of the oxyhemoglobin saturation curve which describes the oxy­ hemoglobin concentration at a given oxygen partial pressure.

This oxygen affinity curve may be approximated by the Hill equation:

k.P n k v γ = o2 100 l+k.P" 02 where Y is the percent saturation with oxygen, к is an equilibrium constant, P/-^ η is the oxygen partial pressure, and the exponenfrepresents the number of iron atoms per hemoglobin molecule. The value of n for normal hemoglobin approximates 2. 9.

In 1878 Paul Bert as one of the first described the interrelation of blood О« content and partial pressure while the sigmoid shape of the oxyhemoglobin saturation curve was first demonstrated by Bohr, Hasselbalch, and Krogh in 1904.

Figure 1 depicts the oxyhemoglobin saturation curve for normal adult blood under standard conditions (pH =7.4, Ρ/-/-)? = 40 torr, and 37 C). A useful expression of blood oxygen affinity is the Ρ,-Λ , the partial pressure at which hemoglobin is

50% saturated. For normal adult blood this value equals 26.0 torr (Prystowsky et al.,

1959) (Fig. 1).

20 lUU tA ^—'Ζ^'-ΞΞΞ^^ 90 HEMOGLOBIN IN SOLUTION /' ^/^^PREGNANT ADULT S 80 / / / / І» /V/? PREGNANT

/ / / О I SATURATIO N

/ // OXYHEMOGLOBI N 10

η 'J/ 10 20 30 40 50 60 70 80 90 100 OXYGEN TENSION (torr)

Fig. 1. Oxyhemoglobin saturation curve under standard conditions for normal (pregnant) adult blood. Also shown is the dissociation curve for hemoglobin in solution. ? Pregnant: recent observations by Bauer et al. (1 968;1969) showed a higher P50 for pregnant adult blood.

Some studies suggest that for women this value is around 26.5 torr (Beer et al.,

1955; Hellegers et al., 1961). Since the observation of Bohr et al. (1904) that carbon dioxide alters the position of the dissociation curve it has been discovered that several other factors including hydrogen ion (H ), 2, 3-diphosphoglycerate

(2, 3-DPG), adenosine triphosphate, chloride ion, and carbon monoxide change the affinity of hemoglobin for oxygen. Figure 1 also depicts the oxyhemoglobin

21 dissociation curve for stripped hemoglobin with a P™ of 19 torr, without the

influence of intracellular factors such as H , 2, 3-DPG,etc.

Because pregnancy is associated with a number of maternal respiratory, cardio­

vascular, hematologic, and metabolic changes, the question arises to what extent

pregnancy alters blood O^ affinity. Most studies have indicated that a pregnant

woman's Pen value varies by no more than 1 torr from that of her non-pregnant

sister (Darling et al., 1941 ; Beer et al., 1955; Prystowsky et al., 1 959; Hellegers

and Schruefer, 1961; Lucius et al., 1970). On the other hand several recent obser­

vations challenge this finding.. Bauer et al. (1968 and 1969) reported that maternal

Pen equaled 31 torr under standard conditions, i.e. 4 torr greater than that for normal

adults (Fig. 1). Although these pregnant women had a 2, 3-DPG concentration 10%

greater than controls this could account for an increase in Ρ,-π of less than 1 torr.

R^rth and Bille-Brahe (1972) reported a similar increased Pen during pregnancy,

however, their sample size was small and characterized by a wide scatter in

results. In addition, these later authors noted that some subjects may have been

anemic with an associated increase in 2, 3-DPG concentration. A subsequent

report from these authors substantiated the increase in 2, 3-DPG concentration

from the third month of gestation onward (to about 1.05 from 0.9 moles 2, 3-DPG

per mole hemoglobin tetramer), but the values for alteration in blood Oj affinity

were not given (Bille-Brahe and R^rth, 1979). The issue is not settled as Lucius

et al. (1970) failed to show a different blood oxygen affinity in pregnant women,

and Merlet-Bénichou et al. (1975) reported no difference in pregnant and non-pregnant

guinea pigs.

If indeed the blood of a pregnant subject possesses a significantly higher Pen

(i.e. lower Oy affinity) the mechanism must be explained. A change in the affinity of the hemoglobin molecule itself is theoretically unlikely, and experimentally

Bauer et al. (1969) showed that dialyzed preparations of pregnant and non-pregnant

22 hemoglobin solution possessed identical O-affinities (Pen =19.7 torr). Thus, any difference probably involves dialysable substances in the erythrocytes. Bauer et al.

(1969) reported increases in both sodium and potassium ion, carbonic anhydrase activity, and ATP concentrations, factors which would tend to shift the curve to the right. Bille-Brahe and R^rth (1979) reported that inorganic phosphate concentra­ tions remained relatively constant during pregnancy (although they increased post­ partum). Other factors not measured such as glutathione (Riggs, 1952) or intra- erythrocyte base excess (Rooth and Bartels, 1962) also could have contributed to this effect. Because it commonly is referred to, the question of the possible influence of base excess on maternal blood O^ affinity requires examination. Early in the course of pregnancy, hyperventilation secondary to increased progesterone levels (Heerhaber et al., 1948) results in the arterial CO« tension decreasing to about 32, from a non-pregnant value of 40 torr. With increased renal bicarbonate ion excretion, plasma bicarbonate concentration decreases to about 23 from a non-pregnant value of 26 mEq· I and the respiratory alkalosis becomes compensated. However, this base defecit of 3 or 4 mEq·! , would decrease the Ρ,-η , shifting the saturation curve to the left, only about 0.5 torr. In vivo the maternal oxyhemoglobin curve lies to the left, i.e. has a higher affinity, because of the lowered CO« tension and slightly elevated pH.

In summary, despite about a dozen studies from reputable laboratories, the precise alteration of a pregnant woman's blood oxygen affinity, if any, remain to be determined. Only recently has it been appreciated how many factors influence blood O- affinity and most of these have been examined only superficially, if at all, during gestation.

1. 4. Blood Oxygen Capacity

The capacity of blood for oxygen is the maximum amount of oxygen which can

23 chemically and reversibly bind with hemoglobin. Hüfner (1894) first showed that

1 g hemoglobi η combines with 1. 34 ml О«. More recent studies indicate that the value of this constant equals 1.368 (Scherrer and Bachofen, 1972). For 16,200 g hemoglobin, i.e. the molecular weight of a monomer, the O« capacity equals

22,400 ml (1.368 χ 16,200), or 1 mole of oxygen. Thus, the complete hemoglobin molecule, or tetramer, can combine with 4 moles of О-,

In the normal male and non-pregnant female the hemoglobin concentration equals about 15 and 14 g»dl , respectively, and the O- capacities 20.5 and 19.1 ml.dl" , respectively. During the course of gestation the hemoglobin concentration progressively decreases, due to the hemodilution of a relatively greater increase in plasma volume as compared with red cell mass, to about 11.5 g «dl , with an О- capacity of ,-1 15. 7 ml« dl" 24 10 22 9 20

8 18

7 16 TJ 6 1 14 £ 12 b Ш 1- Z 10 О 4 VJ δ 8 3 ' 6

2 4

1 2

0 0 10 20 30 40 50 60 70 OXYGEN TENSION (torr)

Fig. 2. Oxygen content as a function of 0~ tension for normal pregnant and non-pregnant women.

24 Fig. 2 depicts oxygen content as a function of O« tension for normal pregnant and non-pregnant women. This figure also shows the amount of physically dissolved oxygen in the blood which, according to Henry's Law (concentration =tL'0~ tension) increases as a function of О« tension. The oxygen solubility, Ol equals 3 χ 10 ml O-· ml blood · torr or 1. 3 χ 10 jíM-ml , and is independent of the amount of oxygen bound to hemoglobin.

2. Fetal Blood Oxygen Affinity and Capacity

2. 1. Fetal Hemoglobin

In 1866 Körber first noted that blood from human newborn umbilical vessels denatured less readily in alkaline or acid solutions than did blood from adults, the difference being most pronounced in alkali. This observation has been confirmed by many authors. For instance Wakulenko (1910) reported that human umbilical cord blood was "one hundred times" more resistant to alkaline denaturation than adult blood, and Bischoff (1926) reported a progressive decrease in the alkali resistance of infants blood related to age from conception rather than to age from birth. In 1930 Haurowitz demonstrated that the difference in alkaline denaturation resides in the globin chains of hemoglobin and proposed the presence of two hemo­ in newborn blood, an alkaline susceptible adult fraction and a more resistant fetal fraction. About this time the discovery that fetal blood O« affinity differed from that of the adult (Huggett, 1927) gave renewed impetus to the search for fetal hemoglobin. Brinkman and Jonxis in 1935 first definitely demonstrated the existence of a human fetal hemoglobin and in 1957 Kleihauer and co-workers described a method by which fetal hemoglobin could be demonstrated both with light and elec­ tron microscopy. Schroeder et al. (1963a) first demonstrated the difference in amino acid sequences of human beta and gamma globin chains. These workers also recorded (1963b) that the 141 residues of the alpha chain in hemoglobin F have

25 the same amino acid sequence as the alpha chain of human hemoglobin A. In addition to the two alpha chains, fetal hemoglobin contains two non alpha, so called gamma, chains which resemble the beta chain, containing the same number of amino acids

(146), butdiffering by the replacement of 39 amino acids by other residues.

Each gamma chain contains four isoleucine residues not found in either the alpha or the beta chain, while three proline residues present in the beta chain have been substituted by other amino acids. Blood of the newborn infant contains about 60 percent hemoglobin F, and an additional 15 percent of hemoglobin F, which is characterized by acetylation of the N terminus of the gamma chain (Schroeder et al.,

1962). The newborn infant has about 75 to 84 (Kirschbaum, 1962) percent of fetal hemoglobin.

In addition to containing different fetal hemoglobins, erythrocytes are larger

(Horan, 1950), have a shorter life span (Garby et al., 1964), and differ in ultrastructure. They also vary in their mechanical (Bos, 1955), osmotic (Reimann and Yenen, 1962), temperature (Betke and Kleihauer, 1958), and acid fragility

(Kleihauer, 1963), electrophoretic mobility(Rottino and Angers, 1961), and membrane structure (Hollân et al., 1968).

Drescher and Künzer in 1954 first reported the existence of an embryonic hemo­ globin in the human. Halbracht and co-workers (1956, 1958, 1959) confirmed this finding in embryos and fetuses up to 20 weeks gestation as well as in two full term newborns with multiple malformations and erythroblastosis fetalis. Huehns et al.

(1961, 1964) and Capp et al. (1970) demonstrated that human embryonic hemo­ globin can be differentiated into Hb Gower 1, Hb Gower 2, and Hb Portland, each with different globin chains. These different hemoglobins appear at different stages of development. Recently, Gale et al. (1979) demonstrated that Hb Gower 1 has the structure ζ- f -, which explains the finding that oxygen dissociation studies of embryonic erythrocytes show a full cooperative effect (Huehns and Farooqui,

1975).Apparently the human embryo first elaborates Hb Gower 1 (£9(9) then

26 changes successively to Hb Gower 2 (oCj F-¿Ι, Hb Portland (jf2 VJ, HbF

(0^2 У·}) and Ч^ i01·ο Ρ 2^^'^' ^' ^01^ оп'^ ^0 c'''r'Γeren,' hemoglobins become

manifest, but different sites of hematopoiesis emerge during develop.nent (Fig. 3).

Three weeks after fertilization of the human ovum, clusters of "blood islands"

appear lateral to the primitive stfeak. The peripheral cells of these islets form the

primitive endothelium of a developing vascular network, while central cells

represent the earliest hematopoietic precursors. The cytoplasm of these early cells

contains protohemoglobin whidi probably represents the embryonic varieties, namely

Hb Gower 1, 2, and Hb Portland. After eight weeks of gestation these blood islands

regress, and rapidly enlarging nests of hematopoietic cells appear in the hepatic

sinusoids. By the seventeenth week of gestation both liver and bone marrow

produce HbF and HbA (Thomas et al., 1960; Basch, 1972). Further studies have

shown that individual erythrocytes can contain both HbA and HbF (Shepard et al.,

1962; Thompson et al., 1961; Dan and Hagiwara, 1967; Matioli and Thorell, 1963).

ω ι О ? Ι Ι Fig. 3. Patterns of < J uu- LIVER ^/^T1 St • sites of hematopoiesis and globin chain <50- ^ 03 • SPLEEN/ NW j synthesis. The relative Z LU r\ ι 1 ^ proposions of each 1 1 globin chain produced 1 at each stage of 1 -a--CHAIN 1 gestation are shown. 1 Ι TheOt-chain is expres­ -Y- CHAIN \ À_β-CHAI N (ADULT) sed as 100%; the per­ ( FOETAL) \ / centages of non-o¿- chains produced at each stage are expressed PRESEN T _ С -e- сHAJN(EMBRIONICI/ \ relative to Oi-chain

Ι ι 1 I r6-CHAIN(Hb-A2) r o i> . en (Ρ 0 production, (from: !

% POLYPEPTID E CHAIN S PR Nathan, 1973; Pearson, 1973) > О С u-1 —γ- 1 1 1 1 -e -3 BIRTH 3 6 month 27 Neihher birfh, intrauferine hypoxia (Cook et al. , 1957), or hemolytic disease of the newborn (Schulman and Smith, 1954) affect the change in proportions of HbA and HbF at any given gestational age. However, near term a demand for accelera­ ted erythropoiesis leads preferentially to synthesis of hemoglobin A. The subcellular events (transcription and translation) responsible for the gradual, orderly change from synthesis of gamma to beta chains remain to be elucidated. Probably multiple general factors such as hormones rather than local tissue factors interact to express the beta chain gene in developing erythroblasts (Wood and Weatherall, 1973;

Wood et al., 1976; Nienhuis and Benz, 1977). In addition the decline in gamma diain messenger RNA occurring in human reticulocytes at about 30 weeks of gesta­ tion (and in sheep around 90 days) is not a necessary requirement for increasing beta chain production (Kazazian et al., 1976).

Despite the attention given to the importance of fetal hemoglobin in humans, this hemoglobin type is not required to survive intrauterine life in all species. To date no hemoglobin F has been detected in the horse (Stockell et al., 1961;

Gratzer and Allison, I960), hamster (Gratzer and Allison, 1960), beagle dog

(LeCrone, 1970), cat (Novy and Parer, 1969), or pig (Tautz and Kleihauer, 1972).

In pigs the switches directly to the adult variety (Kleihauer and Stoffler, 1968; Kitchen and Brett, 1974). Early in pregnancy the fetal elephant has the same О™ affinity as the mother, (Pep = 23 torr, ) but later in its relatively long intrauterine existence its О« affinity increases so that the Ρ ¡-г, equals 17 torr

(Riegel et al., 1967). As an aside, hemoglobin F synthesis is increased in intra­ uterine growth retarded infants (Bard, 1974), in D 1 trisomy (Bard, 1972), in beta thalassemia, sickle cell anemia, and a variety of hematologic disorders including megaloblastic anemia, aplastic anemia and chronic myelocytic leukemia. Normal women often synthesize small amounts of fetal hemoglobin for a few weeks during the first trimester of pregnancy (Pembrey et al. , 1973). Hemoglobin F accounts for about 5 per cent of total hemoglobin synthesis during that time and coincides

28 with the peak elaboration of human chorionic gonadotropin (Pembrey et al. , 1973).

2. 2. Blood Oxygen Affinity

In 1927 Huggett demomtrated that the oxyhemoglobin saturation curve of fetal

blood differed from that of the adult. Although Huggett mistakenly held that the

fetal goat blood had a lower O« affinity (Pen = + 40 torr) than that of the adult,

he correctly noted the smaller effect of carbon dioxide on fetal blood oxygen affinity

(). During the subsequent decade, several workers including Eastman (1930),

Haselhorst and Stromberger (1932), Barcroft (1933), McCarthy (1934), Hall (1934,

1935), Roos and Romijn (1937, 1938) and Darling et al. (1941) depicted the fetal oxyhemoglobin saturation curve relatively correct in its relation to that of the mother. Figure 4 portrays this relationship for the human, in which the fetal Ρ,-Λ equals 20 torr under standard conditions (Hellegers and Schruefer, 1961). Numerous authors have confirmed that fetal blood has a higher O« affinity than maternal blood in humans as well as in many others species (see below). Barcroft (1933), Hall (1935) and Roos and Romi¡n (1937) reported a progressive decrease in the blood oxygen affinity (increase in Ρ,-r)) during the course of gestation, such that the fetal curve progressively approximates the maternal curve with approaching parturation. Pre­ sumably this results from an increase in the concentrations of Hgb A relative to that of Hgb F.

The question arises as to the mechanism(s) responsible for the increased oxygen affinity of fetal blood. Hall (1934) first suggested that this might result from a specific difference in hemoglobin, and McCarthy (1943) also noted that fetal erythrocytes per se did not affect the affinity for oxygen of fetal hemoglobin. Beer and co-workers (1958) reported that in human newborns the dissociation curve shifts toward that of the mother as the concentration of hemoglobin F decreases. Also in humans, Kirschbaum (1962) noted a linear correlation between the proportional

29 ===^^A 90 FETUS/^^ ^^^^

S 80 /ьхш//^ X / /INFANT AT 11 / / / MONTHS OF AGE ζ / / / о / / / 5 60 / // at / 1 / э t- / / / I I / S( 50 "la / / Ζ / / / S ¿о / // 2 О 30 / // ш / / / I / / / > 20 / // О / // 10

η J/ 10 20 30 40 50 60 70 80 90 100 OXYGEN TENSION (torr)

Fig. 4. Oxyhemoglobin saturation curve under standard conditions for normal fetal, maternal and infantile blood. composition of fetal hemoglobin and the P,Q at the time of delivery. At term hemoglobin A comprises about 25% of the total hemoglobin and the fetal Ρςη equals about 21.5 torr (Versmold et al., 1973).

Hall (1934 b) first showed that the oxyhemoglobin saturation curve of fetal hemo­ globin in solution differed from that in erythrocytes, possessing a higher O« affinity.

Using dilute solutions, McCarthy (1943) also demonstrated that fetal hemoglobin inherently possesses a slightly lower O« affinity than that of the adult, but that in the erythrocyte this relation is reversed. On the other hand, Allen et al. (1953) reported that in solution the oxygen affinities of hemoglobin A and F are identical, suggesting that the different oxyhemoglobin saturation curves of fetal and adult hemoglobin did not result from structural difference of the hemoglobin molecules.

30 Several other workers have confirmed this finding (Bauer et al., 1968).Seeds et al.

(1969) noted that the Pen for lysed fetal erythrocytes was 5. 5 torr lower than that of fetal whole blood while that for maternal lysed blood was 8.6 torr lower. These workers believed the decreased oxygen affinity of fetal whole blood resulted from a pH difference across the erythrocyte membrane.

Following the discovery of the relation between 2, 3-diphosphoglycerate (2,3-

DPG) and oxygen affinity, several groups (Bauer et aL, 1968; De Verdier and Garby,

1969; Tyuma and Shimizu, 1969) independently demonstrated the modulation of fetal blood O« affinity by 2, 3-DPG. As noted above, in solution (and in the absence of 2, 3-DPG) both fetal and adult hemoglobin possess identical oxygen affinities.

2, 3-DPG stabilizes deoxyhemoglobin A by binding to positively charged residues on each beta chain (the N-terminal valine (ßi)/ histidine ( β -) EF6 lysine

(β 82) and H21 Kistidine (fi 143). Despite almost equal concentrations of 2, 3-DPG in human maternal and fetal blood, hemoglobin F interacts less with 2, 3-DPG

(resulting in its higher O« affinity) as the gamma chains lack one binding site for

2, 3-DPG (position 143:й|іІ5-*У£ег). Thus, a given amount of 2, 3-DPG added to dialyzed fetal blood lowers its O« affinity to a much less extent than it does adult blood. The percentage of hemoglobin A and concentration of 2, 3-DPG most influence fetal blood О« affinity. Orzalesi and Hay (1971) defined the term "functional

DPG fraction" to describe the product of hemoglobin A percentage and 2, 3-DPG concentration. Changes in fetal and postnatal blood O- affinity correlate better with "functional DPG fraction" than with either the concentration of hemoglobin

A or 2, 3-DPG alone.

Although the weak interaction of 2, 3-DPG and fetal hemoglobin accounts for the higher O« affinity of fetal blood as compared with adult in humans (and to a certain extent in the rhesus monkey) (Metcalfe et al. , 1972) this does not hold in all species. For instance, in goats, sheep, and rhesus monkeys, the fetal hemo­ globin possesses a high oxygen affinity per se (Metcalfe, 1972).

31 As noted above, several species including the fetal beagle dog, horse, and pig

only synthesize adult hemoglobin which interacts strongly with 2, 3-DPG. In these

animals the erythrocyte concentration of this cofactor is low before birth producing

a higher O« affinity than after birth when 2, 3-DPG rapidly increases to adult levels

(Dhindsa et al» 1972; Comline and Silver, 1974; Bauman et al, 1973; Tweedale

1973; Novy et al., 1973). On the other hand, the fetal cat which also has no

hemoglobin F, has a higher Ρ,η than that observed in any other fetus (see below)

and represents an exception to the rule that fetal blood possesses a higher O-

affinity than maternal blood (Novy and Parer, 1969).

Following birth, the infant's blood O« affinity decreases rapidly (Delivoria-

Papadopoulos et al, 1971b). During the first week of life, erythrocyte 2, 3-DPG

increases 25 percent (Delivoria-Papadopoulos et al, '9710). Although this incre­

ment in 2, 3-DPG interacts only slightly with fetal hemoglobin, it lowers intra­

cellular pH, thereby decreasing blood O« affinity. During the first year of life,

hemoglobin F decreases from about 75 to about 2 percent of the total, while 2,3-

DPG concentrations remain fairly constant at 5 to 6 U mol per ml RBC, except from 8

to 1 1 months when it rises to about 7 f*M· ml and the Ргл value increases to about

30 torr (Oski and Gottlieb, 1971).

2. 3. Blood Oxygen Capacity

Although generally it is realized that the blood O- capacity of the near-term

fetus exceeds that of the mother, it is less commonly appreciated that during much

of fetal life the reverse situation holds. (Fig 5). During the course of gestation,

the concentration of hemoglobin in fetal blood (and therefore the O« capacity)

rises reaching a value of about 17 g ·dl at term. Elliott et al, (1934) first demonstrated this phenomenon in the goat, followed by similar demonstrations by

Wintrobe and Shumacker in man and the cat, dog, pig, rabbit, and rat (1935,

1936) and by Barcroft in sheep (1946). Figure 5 depicts the changes in fetal blood

32 г 20 30 40 GESTATIONAL AGE (weeks)

Fig. 5. Changes in maternal (·) and fetal (o) hemoglobin concentration, and fetal mean corpus­ cular volume (Û) during the course of gestation.

Oj capacities for humans (Dawes, 1968). It also shows the decrease in maternal blood

O2 capacity which proceeds simultaneously. Although the fetal hemoglobin con­

centration increases during the course of gestation, as does the erythrocyte count and packed cell volume, the mean corpuscular volume (the ratio of packed cell

volume to erythrocyte count) (Fig. 5), and erythrocyte hemoglobin content (ratio of hemoglobin concentration to erythrocyte count) decreases.

3. Interrelations of Maternal and Fetal Blood Oxygen Affinity and Capacity 3.1. General Considerations

33 100

90 FETUS. § 80 -ΊΝ VIVO « iα 70 ''MOTHER O 5 60

< 50 Ζ й 40 O Ü o 30 Ш ζ >- 20 δ 10

10 20 30 40 50 60 70 80 90 100 OXYGEN TENSION (torr)

Fig. 6. Oxyhemoglobin saturation curves for human maternal and near-term fetal blood. A and V: maternal arterial and venous values under standard conditions. a and v: fetal arflerial and venous values under standard conditions. V', a', and v': maternal venous, fetal arterial and venous values in vivo, respectively.

34 24

22

20 ^-^

18 FETUS/

^^ 16 -о. г_---/-____ А Τ -^^ "ε 14 •о •ч /л ; /^MOTHER z 12 ίϋ > * / ζ 10 8 f /^ d* 8 6

4 2 jy 0. Ю 20 30 40 50 60 70 80 90 100 OXYGEN TENSION (torr)

Fig. 7. Blood ox/gen contenf as a funcHon of oxygen tension for human maternal and near-term fetal blood.

35 Figure 6 depicts the oxyhemoglobin saturation curves for human maternal and near-term fetal blood, and Fig. 7 shows the blood oxygen content as a function of

O« tension for these bloods. These figures demonstrate that despite an oxygen tension in fetal blood that is only one-fifth to one-fourth that of the adult, fetal arterial blood O« content and oxyhemoglobin saturation are not much lower than the adult. This, of course, results from a combination of high O- capacity and O^

affinity of fetal blood. Most investigators have considered the greater 0? affinity of fetal blood an advantage in that it permits it to incorporate oxygen and nearly saturate its hemoglobin at a relatively low O- tension. However, this could be a disadvantage in delivering O« to fetal tissues, except that the fetal oxyhemoglobin saturation curve is rather steep, so that a small decrease in О- tension results in a ma¡or decrement in oxyhemoglobin saturation, offsetting the relative disadvantage of high Oo affinity in releasing O- to tissues.

An additional consideration is the actual oxyhemoglobin saturation curve in-vivo as blood flows through the placental exchange capillaries. In contrast to the saturation curves under standard conditions, the maternal arterial blood is slightly alkalotic (pH 7.42) and hypocarbic (Pf-Q =32 torr) while the fetal blood is slightly acidotic (pH 7. 34) and hypercarbic (Р/~/-ч = 45 torr). As fetal blood courses through the exchange vessels it gives up hydrogen ions and CO- with a rise in pH and fall in CO^ partial pressure. The opposite changes occur in the maternal exchange vessels. Thus, in-vivo the maternal and fetal oxyhemoglobin saturation curves may be almost superimposed, as indicated in Fig. 6. Although the "double

Bohr effect" in the placenta has been credited with significantly augmenting oxygen exchange, theoretical studies suggest that this accounts for only two percent of the total Oy transferred (Hill et al., 1975). In contrast the "double

Haldane effect" accounts for almost half of the effect of O- on CO- exchange.

The Bohr effect is expressed by the factor ДІод Prn/ Ò· pH, which has a value of about 0.48 for both fetal and maternal blood at pH values between 7.2 and 7.8. However, below a pH of 7. 2 the Bohr effect is greater for fetal than for maternal 36 CAP. r SPECIES WEIGHT Wt. O2 P5O 50 REFERENCE (gest. days) (kg) RATIO CAP, (torr) fet. mat. (%) fet. mat. fet. mat. HUMAN (267) a. nonpreg. female 20 26.1 b. pregnant ,, 3,5 65 5.4 22 17 39 22. 30.5 7.6 Вйиег et al., 1969 a. nonpreg. female 28 b. pregnant ,, 16 28 Lucius et al., 1970 pregnant female 22 15 37 22 26 4 Beeret al., 1955 LLAMA (330) 19 14 33 18 21 3 Meschia et al., 1960 CAMEL (400) 17 15 32 16 21 5 Riegei et al., 1967 ELEPHANT (624) 93 2600 3 17 20 37 17 23 6 Riegei et al., 1967 SEAL (270) 13. 6 169 8 28 32 60 21 29 8 Lenfant et al., 1968 GOAT (151) 3.5 45 7 12 13 25 19 30 11 Bartels et al., 1959 GUINEA PIG (68) 0. 08 0. 7 11 16 16 32 19 30 11 Bartels et al., 1967 DOG (63) 21 31 10 Metcalfe et al., 1972 COW (284) 41 430 9 12 15 27 22 31 9 Gahlenbeck et al., 1968 RABBIT (32) 0.06 1.9 3 14 15 29 27 31 4 Bartels et al., 1967 PIG (112) 2.5 100 2 23 33 10 Metcalfe et al., 1972 SHEEP (150) 5 62 8 17 34 17 Bartels et al., 1959 17 15 32 Barron & Meschia, 1954 RAT (22) 0.005 0.2 3 28 38 10 Metcalfe et al., 1972 CHICKEN (22) 0.03 1.1 3 12 14 26 34 49 15 Bartels et al., 1966 MACACA MULATTA (164) 0.45 8 18 15 33 Hellegerset al., 1964

Table I. Relation of fetal to maternal body weight, blood oxygen capacity and affinity, modified from Bartels (1970) 3 and Metcalfe et al. (1973). blood. An additional factor tending to shift the in-vivo fetal dissociation curve to the right is the temperature of the fetus which exceeds that of the mother by 0. 5 to 1.0oC.

3. 2. Species Differences

Among the various placental mammals, the relation of fetal to maternal blood

O^ capacity and affinity varies widely. An increase in O« capacity in either maternal or fetal blood will promote placental transfer (Bartels, 1970; Longo et al.,

1972). All other factors remaining the same, the larger the sum of maternal and fetal O- capacities, the more oxygen will be exchanged before equilibrium is reached (see below). Table 1 modified from Bartels (1970) and Metcalfe et al.,

(1973) summarizes the blood О« capacities of a number of species. In general, fetal blood O- capacity exceeds the maternal value by 10 to 20 per cent, but in some species (ije. guinea pigs) the values are about equal, and in others (te. cow, goat, pig, rat, and rabbit) the fetal O- capacity is less than in the mother.

The seal, and to a lesser extent, elephants and humons, take advantage of this phenomenon to effect placental O« transfer.

Among almost all mammalian species the O« affinity of blood of the fetus exceeds that of the mother. Table 1 demonstrates this relationship. Many workers have considered that the larger the difference between maternal and fetal Pen values, the more efficient is placental O- exchange. Although this concept is not completely valid (see below) it is of interest to contrast the transplacental "driving" pressure of only 3 torr in llamas at high altitude to the 17 torr value in sheep at sea level.

4. Relation of Maternal and Fetal Oxygen Partial Pressures to Placental Oxygen Exchange.

4. 1. Theoretical Considerations

38 Although it long has been recognized that placental 0~ exchange is a function of

maternal O- levels, this relation has not been well defined. In light of the above

background information one now may ask to what extent placental O- exchange

varies as a function of changes in maternal or fetal blood 0„ partial pressure.

Ideally, the effects of various factors on placental respiratory gas exchange should

be studied by sampling the inflowing and end-capillary blood within a single

exchange unit. Unfortunately, however, it is not technically possible to sample

end-capillary blood directly. Thus, although the placenta has measurable maternal

and fetal arterial inputs and mixed venous outputs it may be compared to a "black

box" in which the exchange process per se remains masked from view. Uterine and

umbilical venous outflow, rather than faithfully representing the composition of blood from a single placental exchange unit, consists of blood from numerous

exchange units with different values of oxygen tension and content. This results

from non-uniform distribution of maternal and fetal placental 'jlood flows, non­

uniform distribution of diffusing capacity to blood flow, and placental O-

consumption (see below). In addition, vascular shunts in both the maternal and

fetal placental circulations contribute to the uterine and umbilical veins containing

a mixture of blood of various O« tensions. And of course, an additional problem

for investigators is that when one experimentally changes a given factor various

physiologic compensations conceal the effect of this change.

Because of these problems one may develop equations to simulate the exchange

process mathematically. The mathematical algorithm may be used to explore the

effects of variations in individual factors on placental O« exchange and to calculate

their relative importance in comparison with other factors (Hill et al., 1972; Longo

et al., 1972). In turn, these predictions of the model may be tested, or more

refined values for certain functions may be derived by carefully designed experi­

ments (Longo and Power, 1976).

39 •

ш N30- ^^ ^* ___. ^— — "-"-" <о — — "• Fig. 8. Effects of changes er л s· in maternal arterial O- ^Е 0- / 2 / tension on transient maternal /

(m l / and fetal end-capillary

ЕХСН , 1 О у / PO- and mean rate of O* >- КОп / / tr <м exchange (dashed lines) and 0 ία . steady state end-capillary -1 ,-. у / о. σι У/ Oo tensions and umbilical υ< Х40- E ^^ ff·^^" с^—"* "" arterial PO- (solid lines). Q E МАТ. „^^к^ ^ ζ r, 1 -V ω fw*^ ^ 20J^ЛЖ ^ FETAL CE CM -i Ш О Η Q- к-^^-^ — 15 ^— "" 5^ - у _ι σι / < I 10- , О E f d Ε δ­ I ω w Σ 0- Iι э и I 1 ι I I ι ι ' Ι ι 40 60 100 200 400 1000 2000 MATERNAL ARTERIAL PQ (mmHg)

Figure 8 shows the dependence of placental oxygen transfer and end-capillary

O^ tensions on maternal arterial O- tension. Uncompensated decreases in maternal

arterial Oj tension to about 70 torr (from a normal value of 95 to 100 torr) have

relatively little effect. However, a further decrease to 50 torr reduces the O-

transfer rate about 20% (to 20 from 24.8 ml »min and fetal and-capillary O^

tension to 26 from 32 torr). The solid lines indicate the changes in umbilical arterial

O- tension necessary to maintain normal O- transfer in steady-state conditions

with variation in maternal arterial O- tension. For example, when maternal O^

tension decreases to 50 torr, placental end-capillary O- tension falls to 21 torr

while umbilical arterial O- tension decreases to about 10 torr. Although the rate

of fetal O- consumption would remain normal, its tissues would be perfused with

blood at a lower O- tension. With even more severe maternal hypoxemia (O-

40 tension of less than 50 torr) compensations by the umbilical arterial O- tension

(with a widened maternal to fetal 0~ tension difference) become inadequate, as

shown by the steep slope of the solid line. In addition, as umbilical venous О«

tension falls dangerously low, it may be inadequate to maintain normal fetal oxygen

consumption.

Raising maternal arterial O« tension to 600 torr by breathing 100% О« initially

increases fetal placental end-capillary O- tension only 6 torr (to 38 torr) and

increases the O^ transfer rate about 9% (to 27 ml · min ). During the steady-state

conditions of life, however, placental end-capillary O« tension would increase

to about 41 torr and umbilical O- tension would increase to 18 torr. Although

these increases may not seem significant, it should be emphasized that this results

in higher O^ tensions in peripheral vessels and capillaries, that can be of benefit,

especially to the fetus which is marginally oxygenated or overtly hypoxic. Of

course, the fetal end-capillary and umbilical arterial O^ tensions will not rise to

higher levels because even though the maternal arterial O- tension is relatively

high, the amount of extra O« in maternal blood is only 1. 8 ml · dl , as at relatively high oxyhemoglobin saturations the additional O- is physically dissolved rather

than being bound to hemoglobin (See Figure 2).

Figure 9 depicts the interrelation of placental О« transfer and umbilical arterial

O- tension. On one hand, umbilical arterial O- tension is a function of placental

0~ transfer, umbilical venous О« tension and fetal O- consumption. On the other hand, it is a major determinant of placental O« transfer (by its effect on the maternal to fetal O« tension gradient). Recalling that umbilical arterial and venous

O- tensions each are functions of the other, we first may examine the effect of

isolated changes in umbilical arterial O- tension on placental O« exchange.

As umbilical arterial O- tension decreases the end-capillary O« tension falls, but the placental O- exchange rate increases. Alternatively, as umbilical arterial

41 .ь tu- ε ^^ •ч ІÜJ χ χ Fig. 9. Effecte of changes in Сзо- 4 Ν к 4 umbilical artery PO« on end- ω ν О ч capillary PO« and mean rate of ч ^ 20- ч\ 0« exchange. Changes in umbilical

\ artery PO? from normal values EX C 4Ν >м X result in opposite changes in end- о 10-1 х χ capillary PO« or mean O- -£,60-1 σι I s transfer rate, but not both, which ε y may serve to restore umbilical wεr\ J y o û. 40- s' artery PO« to a normal value. y c>c- г < MATER Ν AL -β-*-^-- < _l -J о. 20- ""FETAL < υ о ζ Ili /Λ LU U ι ι 1 ' ι 10 20 30

FETAL ARTERIAL P0 (mmHg)

О« tension increases, fetal end-capillary O« tension rises and O« exchange rate

falls. As discussed below, placental O« transfer rate and end-capillary O« tension

are more senstive to a given percentage change in umbilical arterial O« tension

than to a similar percentage change in any other determinant of (placental) O-

transfer.

Several points of difference may be noted for the intact fetus with blood

recirculating between the placenta and peripheral tissues. If fetal O« consumption

exceeds the rate of placental O« transfer and umbilical arterial O« tension

decreases, the maternal-to-fetal O« tension difference widens and the rate of

placental O« transfer increases. Thus, the transplacental O« exchange rate rises to

match the rate of fetal O« consumption and the blood gases will attain new steady-

state values. However, as already noted, adequate compensation by this passive

mechanism could not occur in other cases and placental end-capillary O2 tensions 42 would fall below these levels required for optimal respir.ition by fetal cells.

Of course, this simplified paradigm, (formalized in this mathematical model),

does not accurately represent the complexity of oxygen exchange in vivo.

The model cannot substitute for experimental measurements, but until these can be

obtained from individual placental vessels, it may be one of the most valid

approaches to study the exchange process. Theoretical studies, while of great

interest for theirown sake, ideally should have some relation to reality and the

experimental situation. The predictions of a mathematical model should be subject

to testing and, insofar as the model fails, it should be revised.

4. 2. Experimental Studies

The results of the theoretical studies above suggest that placental oxygen transfer

should be particularly sensitive to changes in maternal or fetal arterial O« tension.

The effects of variations of these factors are difficult to explore experimentally

because when a given variable, such as O- tension, is altered, other factors

(vascular resistance, blood flow, et cetera) may change to obscure the consequences

of the variation under study. When this occurs the investigator misjudges the role of that factor in affecting placental O« transfer. In addition, uterine and umbilical

venous blood represent a mixture froTi many placental exchange units with varying

O« tensions due to placental O- consumption, vascular shunts, non-uniform distribution

of maternal and fetal placental blood flows, and probably non-uniform distribution

of placental diffusing capacity to blood flow (see below). Although changes of a

given factor will affect the placental end-capillary O- tensions, the variation

may not be reflected accurately by the mixed venous O- values.

Surprisingly, few studies have attempted to systemically explore the relation of

fetal to maternal O- tensions. Although several reports have purported to examine

these interrelations, the experimental situations varied widely and in only a few

43 instances was an at'empt made to control the numerous variables. Perhaps one of

the most rigorous attempts to test the theoretical predictions and circumvent some

of the attendant experimental problems was by isolation of a placental cotyledon

and perfusing its umbilical circulation in-situ at accurately controlled flow rates

with blood of known oxygen tensions (Power and Jenkins, 1975). Samples of blood

flowing to and from the cotyledon were collected in glass capillary tubes and analyzed

for oxygen and carbon dioxide partial pressure and pH. The net oxygen transfer rate

across the placenta was calculated as the product of fetal perfusion rate and the

arterio-venous O« content difference. Experimental results were compared with

the predicted results obtained using the mathematical model.

The upper panel of Figure 10 shows the effect of varying experimentally maternal

arterial O« tension on the O- transfer rate, while the lower panel shows the effect on umbilical venous О« tension. As maternal 0~ tension was increased from 90 to

300 torr, the oxygen transfer rate rose about 40% to 0. 056 from 0. 04 ml · min

This relatively slight change was not surprising in light of the shape of the

oxyhemoglobin saturation curve and emphasizes the small quantities of oxygen

carried by maternal blood in physical solution. Umbilical venous 0~ tension rose

to 34 from 29 torr for the same maternal O^ change (Power and Jenkins, 1975).

These authors also observed that small decreases in maternal arterial O« tension

from the normal level had little effect, but larger decreases became progressively

more significant. As maternal CU tension fell below about 70 torr both the umbilical

venous 07 tension and placental O- exchange rate declined sharply. For example, as the maternal O« tension approached 50 torr, the umbilical venous O« tension decreased to 23 from 29 torr and the 0„ exchange rate decreased to 0. 027 from

0. 04 ml· min

Because certain assumptions such as the value of uterine blood flow were unknown,

any agreement between experimental and theoretical curves is not meaningful in

44 •50 OUTFLOW Рог .40 (MMHG)

MATERNAL ARTERIAL P02 ГММНС)

Fig. lOA. The effect of var/ing experimentally maternal arterial oxygen tension on the oxygen transfer rate. Solid line indicates the theoretical curve. absolute terms. However, the trend of the plot is valid and it can be seen that, in general, the experimental data follow the theoretical curves reasonably well.

Power and Jenkins also tested the effects of changes in umbilical arterial oxygen tension on placental O- exchange while other factors (total flow, CO« tension, et cetera) were held constant. Figure 11 depicts the effects of varying umbilical arterial Oj tension on the placental O- transfer rate and outflowing O2 tension.

The results predicted by the mathematical model agree fairly well with the experi­ mental results. Both the experimental O^ transfer rate and outflowing O- tension show marked sensitivity to small changes in umbilical arterial O- tension resulted in a

20% increase in O« transfer rate. These results, in accord with the theoretical studies above, indicate that umbilical arterial O- tension should be the factor to which 0„

45 •IO 02 Ί2.0 EXCHANGE RATE -eML/MIN

• ^ -6 J^ V02 MEASURED V02 ΔΤ STANDARD 1.0 ^r MAT. P02 -4 : ·/ • • • 0.5

'•/!• 100 200 300 1 1 ι I MATERNAL ARTERIAL P02 (MM HGJ

Fig. 10В. The effect of varying experimentally maternal arterial oxygen tension on umbilical venous oxygen tension. Solid line indicates the theoretical curve.

46 •1.5

02 EXCHANGE RATE

V02 MEASURED

V02AT STANDARD INFLOW P02

•0.5

INFLOW P02 MM HG IO 20 30 АО 50

Fig. 11 A. The effect of varying umbilical arterial oxygen tension on placental oxygen transfer rate. Solid line indicates theoretical curve. delivery to the fetus is most sensitive. They also suggest that the small day-to-day changes in umbilical arterial and venous O- tensions measured in unanesthetized sheep (Comiine and Silver, 1970) are associated with sizeable variations in the rate of oxygen transfer.

As noted, changes in umbilical arterial О« tension resulted in changes in both

O- transfer rate and outflowing Oj tension with the responses occurring in opposite directions. Thus, decreases in umbilical arterial O- tension increased О- transfer, but only at the cost of lowering umbilical venous O- tension. Conversely, increases in umbilical arterial О« tension increased umbilical venous O- tension at the expence of lowering the O« transfer rate. A similar type of divergent response is noted with changes in umbilical flow rates (see below). Indeed, the only change in fetal hemodynamics which would seemable to increase both outflowing Oo tension

47 * -2 / •а OUTFLOW Ροζ • / • / •1.5 • Ρθ MEASURED 2 • • ·· PO2 AT STANDARD INFLOW P02 f· · · / • •

-1 • • :· . * ·· INFLOW Ρ02 MM HG IO 20 30 40 50 Ι ι

Fig. 11 В. The effect of varying umbilical arterial oxygen tension on the the outflowing umbilical oxygen tension. The solid line indicates the theoretical curve.

48 and О« transfer rate simulianeously would be a more uniform distribution of fetal placental blood flow, such that fetal flow distribution more closely parallels maternal flow distribution and/or the available surface for gas exchange (see below).

4.3. Clinical Implications

4.3. 1. Maternal Oxygen Inhalation

Apparently Waters and Harris (1931) first appreciated the possible benefit of maternal oxygen administration on the fetus. During obstetrical anesthesia, in cases where the fetal heart rate was abnormally low, they noted an increase in response to maternal inspired O^. Subsequently, Lund (1940) reported several instances in which a slow fetal heart rate during labor or anesthesia responded to maternal oxygen inhalation. Eastman (1955) reported that umbilical venous oxy­ hemoglobin saturation increased to 75% from 65% with O« inhalation prior to vaginal delivery or Cesarean section. Since these early studies, numerous authors have reported increases in fetal oxygen levels following maternal 0„ inhalation.

All of the various reports are somewhat difficult to compare because the blood was sampled from different vessels and the reported values were for oxygen content or oxySemoglobin saturation rather than 0„ tension. Figure 12 summarizes those studies which probably are most valid.

Because of the shape of the oxyhemoglobin saturation curve, under most conditions relatively large increases in maternal arterial oxygen tension result in only small increases in fetal Oy tension and content. For instance, raising maternal arterial

O- tension from 100 torr (breathing air) to 600 torr (breathing 100% O«) increases maternal blood O-content 1. 5 ml < dl to 17. 5 from 16 ml. dl . Rivardetal.

(1967) noted that in sheep, maternal arterial O^ tension had to be raised about 200 torr for umbilical venous O« tension to increase 4 torr. (Fig 12). Newman et al.

(1967) reported in humans undergoing vaginal delivery the empirical relation: fetal scalp blood O- tension = 17. 69 + 0. 06 maternal O- tension (torr). 49 Fetal 0„ tensions change relatively rapidly following a change in maternal

inspired Oj concentrations. For instance, following the administration of 100% O«

to pregnant ewes, maternal arterial Oj tension began to increase in about 6 sec, and

reached a new steady state level in about 4 min. Fetal umbilical venous O« tension started to rise in about 13 sec, increased 5 torr during the first 10 min, and continued to increase during the next 20 min (Parker and Purves, 1967; Comline and Silver,

1965).

Because relatively small changes in fetal oxygen tension follow large changes in maternal O« levels when maternal oxygen tension exceeds 80 torr, raises the question whether O- administration to the mother during labor or delivery improves fetal oxygenation. The answer ís definitively yes.

48

42

36

't 30 S гч 2 24 í LU "- 18

12

0 60 120 180 240 300 360 420 480 MATERNAL POj (torr)

Fig. 12. Effect of maternal oxygen inhalation on fetal umbilical venous tension (sheep:o+0; monkey: Δ ).

50 Although inhalation of 100% O« by sheep results in about an eight percent decrease in uterine hemoglobin flow, (Makowski, Hertz and Meschia, 1973) there is no evidence of impaired placental O- transfer.On the other hand, under conditions of impaired fetal oxygenation any increase in fetal oxygen levels decreases the chance of damage to the brain or other vital stri.ctures (Rivard et al. , 1967).

Thus, although one cannot be certain that oxygen will have beneficai effects, there is no justification for withholding it.

Of courseyOne may question whether O« administration may harm the fetus. As early as 1802 Lobstein, on observing the relatively dark blood of the fetus.speculated that overly arterialized blood might damage the immature tissues. Since then, other authors have postulated that excessive O« tensions might result in closure of the ductus arteriosus resulting In "hara-kiri" in-utero (Assali et al., 1968), or that alternatively the fetus might protect itself from excessive oxygen tensions by constriction of the placental capillary bed (Kirschbaum et al.,

1967). Indeed, in-vitro high O- tensions constrict maternal and fetal placental vessels (Nyberg and Westin, 1957) as well as the ductus arteriosus (Born et al.,

1956; Assali et al., 1963; Kovalcik, 1963). However, it does not follow that such changes, even if they occur in vivo (Assali et al., 1968) would be deleterious to fetal well-being. As the arterial O- tension equals about 90 torr within a few minutes of birth in the normal newborn infant, it is unreasonable that such levels would be harmful just before birth.

4. 3. 2. Hyperbaric Oxygenation

Therapy with oxygen at pressures greater than one atmosphere can be useful in conditions such as carbon monoxide poisoning and gas bacillus infections. It might be asked how hyperbaric oxygenation affects fetal oxygenation and what potential this modality has in the treatment of fetal hypoxia. Longo, Power and Forster (1967;

51 1969) first investigated the effects of hyperbaric pressures on placental oxygen

exchange. Above 3.5 atmospheres absolute the uterine arterial to venous Oj

tension difference was almost constant at 1200 torr, or 2.6 ml 0„· dl and the

entire amount of oxygen exchanged was derived from that O« physically dissolved

in maternal blood. The umbilical venous O- tension increased as a function of

increasing pressure, to 700 torr at 5 atmosphere while umbilical arterial О« tension rose only slightly, to about 40 torr at that pressure. Subsequently, Kirschbaum et al.(1969) demonstrated small, but probably insignificant decreases in uterine and umbilical blood flows at 3 atm pressure, with no change in fetal O^ consumption.

Although hyperbaric oxygenation might be considered of value in treating fetal hypoxia, it presently has no clinical role because of the danger of maternal oxygen toxicity. During maternal breathing of 100% oxygen at 3 atm pressure, coma, convulsions and death may occur within 10 to 15 minutes (Lambertson, 1965).

In near term rabbits maternal hyperoxia resulted in an increased incidence of prematurity, stillbirths and neonatal deaths, and eye abnormalities (Fujikura, 1964).

4. 3. 3. Acute Decrease in Inspired Oxygen Tension

Probably the earliest study of the effects of hypoxia on the developing mammal was Robert Boyle's demonstration in 1670 that one day old "Kitlings continued three times longer in the exhausted receiver than other animals of that bigness would probably have done" (Boyle, 1670). Since then many studies have verified that fetal and newborn animals are more resistant to the effects of hypoxia than adults. Numerous other studies have explored different aspects of the effects of hypoxia on the developing fetus in utero. We will review only those significant contributions of fetal hypoxia produced by acute decreases in maternal inspired O- concentrations.

In a study of the role of anesthesia in the production of asphyxia neonatorum,

52 Eastman (1936) reported a rather startling human experiment. Inspired oxygen

concentrations as low as 5 per cent (with the balance nitrous oxide) were given for

5 to 21 minutes to pregnant mothers for vaginal delivery or Cesarean section. When

mothers breathed a 80:20 nitrous oxide-oxygen mixture oxyhemoglobin saturation

averaged 41 and 22% in umbilical venous and arterial bloods, respectively.

Assuming umbilical venous and arterial pH values of 7. 35 and 7. 32, these satura­

tions correspond to O- tensions of about 18 and '2 torr, respectively. In nine

subjects who breathed 15% O^ the umbilical venous and arterial oxyhemoglobin

saturations were 32 and 16%, respectively., corresponding to O« tensions of 16

and 11 torr, while the maternal arterial oxyhemoglobin saturation averaged 79%,

corresponding to an O- tension or 43 torr.

In three subjects who breathed 10% O- maternal oxyhemoglobin saturation was

67% (0„ tension =33 torr), and umbilical venous oxyhemoglobin saturation and

О« tension were 12% and 9 torr, respectively. Two of these mothers were

"slightly cyanotic" and the newborn infants showed evidence of asphyxia. In seven subject's who breathed only 5% O- for up to 20 min arterial oxyhemoglobin

saturation equaled 59% (0„ tension = 29 torr), the umbilical venous oxyhemo­ globin saturation was 8,6% (O« tension =7 torr) and the umbilical arteries of most of these infants were collapsed, presumably due to inadequate systemic perfusion pressure. All of the newborns were asphyxiated, five were apneic for 15 to 25 min., and two died despite recusitative efforts. Eastman observed that the hypoxic fetuses had shown decreased respiratory movements in utero. He concluded by warning physicans of the extreme danger of using less than 15% oxygen concentra­ tion in conjunction with nitrous oxide anesthesia. Obviously such experiments cannot be condoned, yet one must recall that this study occurred before the inception of modem obstetrical anesthesia.

Numerous reports since Eastman's have shown a drop in fetal О- tensions as maternal O- levels decrease. Figure 12 demonstrates this relationship and the

53 relative stability of umbilical venous 0„ tension despite large changes in maternal

О« tensions. Part of this constancy results from the shape of the oxyhemoglobin curves. It also follows from the compensatory adjuslrnents of maternal and fetal circulations to maintain placental oxygen exchange as near normal as possible.

During mild maternal hypoxia, uterine blood flow remains constant (Eisner et al.,

1969/1970) or decreases slighMy (Makowski et al. 1973), but the blood hemoglobin concentration increases so that uterine hemoglobin flow (see below) increases slightly (Makowski et al. 1973).

The fetal adjustments to hypoxia also are of interest In response to relatively mild to moderate hypoxia the fetal heart rate may initially increase in lambs (Born et al.,1956; Α-sali et al ,1962; Mann, 1970/and swine (Harris and Cummings, 1973), fetal blood pressure increases (Born et al. 1956, Reynolds and Paul, 1958, Assali et al., 1962; Parker and Purves,1967; Col-n et al., 1974), as does blood O- capacity

(hemoglobin concentration) (Born et al , 1956; Adamson et al , 1970; Harris and

Gumming, 1973). As the fetal hypoxia becomes increasingly severe these compensatory mechanisms fail; fetal heart rate decreases (Greenfield and Shepherd,

1953; Born et al., 1956; Reynolds and Paul, 1958, Assali et al., 1962; Mann et al.,

1970) as does blood pressure (Reynolds and Paul, 1958; Harris and Cummings,

1973) and umbilical blood flow (Born et al., 1956; Dilts et al., 1969; Mann, 1970).

The levels at which these changes occur have not been defined. Using the radio­ active microsphere techniques several groups have shown that during hypoxia fetal cardiac output remains relatively constant, while blood flow to the brain, heart and adrenal glands increases significantly (Cohn et al. , 1974; Peeters et ol,, 1979;

Longo et al., 1978).

4.3.4. Chronic Decrease in Maternal Inspired Oxygen Tension- High Altitude

The barometric pressure, and thus the ambient O« tension, dscreases exponen­ tially with distance above sea level. For instance, the pressure at 1525 meters

54 (5, 000 ft. ) is 633 torr, while that at 3, 050 m (10, 000 ft. ) is 523 torr and that at

4,575 m (15,000 ft. ) is 430 torr. In healthy conditioned individuals the effects of altitude are not evident below 1,500 m. Between 1, 500 and 3, 000 m relatively minor compensatory adjustments serve to maintain normal tissue oxygenation.

Immediate changes include hyperventilation and increased cardiac output, while long term effects include increases in erythrocyte mass, hemoglobin concentration, intraerythrocyte 2, 3-DPG concentration, muscle concentration, tissue capillary density, as well as other compensations. However, no evidence of tissue hypoxia presents at these altitudes. Above 3, 000 m hypoxic effects become more apparent, particularly with the added stresses of exercise or cold exposure. At

4,500 m an unacclimatized healthy individual may show signs of hypoxia and

"altitude sickness". At 5,490 m (18CD0ft) atmospheric pressure equals only one- half that at sea level (380torr), and a healthy unacclimatized individual may show signs of inadequate oxygenation within half an hour or so.

Despite the hypoxia associated with high altitude, about 16 million individuals live at elevations over 3,000 m, and in the Andes permanent residen's live higher than 4,600 m. Because the arterial O^ tensions of the fetus at sea level approximate that of an adult at about 5,000 m, one would anticipate that the O- tension of a fetus at high altitude would be much lower than at sea level. Surprisingly, such is not the case (Figure 13), and it is instructive to examine the blood gas values reported at various altitudes. Near 1,525 m (5,000 ft. ) maternal arterial O« tensions equal about 70 (Makowski et al., 1968) to 75 (Blechner et al., 1968) torr in sheep. Umbilical venous and arterial О, tensions at these altitudes range from 20 to 29 and 16 to 21 torr, respectively. The higher values are from acclimatized sheep (Makowski et al, 1968), while the lower values were obtained in sheep acutely exposed to simulated altitudes (Blechner et aL, 1968).

At about 3, 050 m (10, 000 ft. ) Howard et al. (1957) reported that in infants at

55 150

120 SEA ί- Ο ARTERIAL LEVEL 1 BLOOD 90- ~\ HIGH ALTITUDE

Fig. 13. The cascadal course from ambient air to fetal tissues of oxygen at sea level and at high altitude. birth before the onset of respiration umbilical arterial and venous oxyhemoglobin saturations averaged about 50% as compared with 58% at 1,525 m. The O- tensions of sheep and goats acutely exposed to a simulated altitude of 3,050 m were: maternal artery =47 torr, umbilical vein = 15 torr, and umbilical artery = 1 1 torr

(Blechner et al., 1968).

At elevations near 4,575 m (15,000 ft. ) hypoxia may impose severe stresses on the organism. Undoubtedly our greatest understanding of high altitude effects on placental oxygen transfer has resulted from the classic studies of Barron and his colleagues in the Peruvian Andes. These workers explored numerous aspects of the physiologic adjustments of the mother and fetus in sheep and in a few humans that had been born and reared at high elevations. Maternal arterial Oj tensions in

56 acclimatized sheep were 38 to 40 torr, depending upon whether the ewes had hemo­ globin type A or B, respectively (Metcalfe et al.,1962). Umbilical venous and

arterial 0? tensions averaged 22 and 12 torr, respectively (Metcalfe et al., 1962).

In acutely anesthetized and surgically operated llamas at these elevations, O« tensions and saturations averaged 38 torr and 85% in the maternal artery, 20 torr and 49% in the umbilical vein, and 7 torr and 5% in the umbilical artery

(Meschia et alT 1960) however, these values probably are less than truly physiologic.

In chronically catheterized sheep acutely exposed to hypobaric conditions,

Blechner et al. (1968) subsequently reported lower O- tensions in the various vessels, the O« tension in the maternal artery was 34 torr and in the umbilical vein and artery equaled 19 and 1 3 torr, respectively. The differences between results in acclimatized and non-acclimatized animals are more readily under­ standable in light of observations by Makowski et al. (1968). These workers measured blood gases of sheep following a change in altitude from about 1, 525 to

4,300 m. Initially the Oj tension and oxyhemoglobin saturation in uterine and umbilical vessels fell dramatically. For instance, umbilical venous and arterial oxyhemoglobin saturations decreased from normal values of 74 and 61 percent, respectively, to 45 and 28 percent on the fi rst few days at the new elevation.

Then, over a period of 6 to 18 days the oxyhemoglobin saturations increcsed to values similar to those at lower altitudes (Makowski et al., 1968).

Apparently the only study of human fetal blood gas values at high elevations is that of Sobrevilla et al. (1971) from Cerro de Pasco, Peru (4,200 m). During labor these workers reported that fetal scalp О« tension averaged 19 torr, a value not significantly different from the sea level value of 22 torr.

Although humans do not subsist for prolonged periods much above 4,500 m, studies of placental oxygen exchange have been carried out at simulated altitudes up to 6, 100 m (20,300 ft. ). Kaiser et al. (1958) exposed pregnant sheep to these

57 hypobaric pressures for 10 days at about days 95 to 100 of gestation. At a simulated altitude of 5,490 m (barometric pressure = 385 torr), the O- tensions and oxy­ hemoglobin saturations in 4 animals averaged: maternal artery, 35 torr and 50%; umbilical vein 21 torr and 60%; and umbilical artery 8 torr and 30%. At a simulated altitude of 6, 100 m (pressure = 345 torr), maternal arterial O- tension and oxyhemoglobin saturation in 3 other animals averaged 30 torr and 43%, respectively.

In a single fetus, umbilical venous and arterial O- tensions saturation were 20 torr and 60%, and 12 torr calculated from his data and 18%, respectively. One ewe as well as the fetuses of two other ewes died at this diminished pressure.

Taken together, these results indicate that at high altitude various physiologic mechanisms operate to maintain placental O- exchange at a rate surprisingly close to that of sea level. Of course, the question arises as to the maternal and fetal compensatory mechanisms which serve to maintain O« exchange under these hypobaric conditions. Although some of the "acclimatization" responses are known, their specific details lie outside the domain of this review.

4.3.5. Chronic Maternal Hypoxemia - Cyanotic Congenital Heart Disease

The incidence of cardiovascular disease among childbearing women in the

United States is 1 to 2% (Burwell and Metcalfe, 1958). Those women with cyanotic congenital heart disease may account for 1 out of 15,000 deliveries (Reid, 1963)

(this number probably is not exact). Women with cyanotic heart disease are chronically hypoxic and develop compensctory adjustments such as increased blood

О™ capacity and decreased О« affinity to augment their ¡issue oxygen needs.

The reproductive performance in these patients is inversely proportional to their hematocrit and hemoglobin concentrations, undoubtedly reflecting compromised fetal oxygenation.

The larges series of pregnant women with cyanotic congenital heart disease

58 originated from the Johns Hopkins University. Unfortuntately, the results from these

54 pregnancies have been presented only as an abstract (Neill and Swanson, 1961)

and a brief discussion (Barnes, 1963) of another report (Cannell and Vernon, 1963).

Of 12 pregnancies in the severly cyanotic group (hematocrit^) 60; hemoglobin

concentration^ 19 g>dl ) ten patients aborted (83%) and two delivered prema­

ture infants, only one of which survived (Barnes, 1963). Of 16 pregnancies in the

moderately cyanotic group (hematocrit =48 to 60; hemoglobin = 15 to 19 g. dl ) six aborted, nine delivered infants weighing less than 2500 g and one was only slightly greater than this weight. Seven of these liveborn infants survived (70%).

Of 26 pregnancies in the mildly cyanotic group(hematocrit^48%, hemoglobin

S 15 g· dl ) four aborted,! 1 delivered premature and 11 term infants. Twenty of these 22 liveborn infants survived (91%). The percent of abortions and perinatal loss in these three groups were 92%, 56% and 23%, respectively (Barnes, 1963).

Eights patients with cyanotic heart disease without pulmonary hypertension were reported by Copeland et al. (1963). The fetal loss was 50% in 18 pregnancies, but these were not categorized by degree of hypoxemia, hematocrit, or other index. Novy, Peterson and Metcalfe (1968) reported five pregnancies in three women with cyanotic heart disease. One patient, who would be classified as severely cyanotic by Barnes (1963) (hematocrit ^60), had two pregnancies, one of which terminated in abortion and the other in premature delivery with neonatal death. Three pregnancies in two patients that would be classified as moderately cyanotic (hematocrit 48 to ^0) resulted in infants weighing less than 2,500 g which survived. These workers noted that not only the maternal blood O« capacity was increased, but the O« affinity was decreased such that Ρ,-Λ averaged 28 to 31 torr. That the fetus partially compensated for this impending oxygen dfeficîency was indicated by a hematocrit of 80% in one infant the day following delivery (Novy et al., 1968). Finally, a group of ten pregnancies in six patients with cyanotic heart disease reported by Schaefer et al. (1968) experienced a 50%

59 fetal loss with two abortions, seven premature deliveries (one of which was

stillborn) and two neonatal deaths. No other data were given on these patients.

Metcalfe (1969 - 1970) has recommended the need of a s/stematic study of the

role of maternal and fetal physiologic compensatory mechanisms in patients with

cyanotic congenital heart disease. In addition to maternal and fetal increased

CU content and decreased O- capacity, other compensations may come into play.

Apparently these compensations may be successful, to a point, as there was no

evidence of acute hypoxia or asphyxia in 2 infants studied carefully by Navy et al.

(1968). Nonetheless, chronic hypoxia in these infants may lead to intrauterine

growth retardation. Figure 14 shows the weights of the infants reported by Barnes

(1963) and Novy et al. (1968) compared with normal percentile curves of

Lubchenco et аЦ1963). In general, newborn weight varied inversely with maternal

hematocrit, and intrauterine growth retardation is known to be associated with

elevated maternal hematocrit (Battaglia, 1970).

5. Relotion of Maternal and Fetal Oxygen Affinity to Placental Oxygen Exchange

5.1. Theoretical considerations

As noted above, blood О- affinity may differ from normal in a number of disease

states and in the presence of abnormal hemoglobins. Unfortunately, however,

few studies have explored rigorously the relations of either maternal or fetal

blood О- affinity to placental О« exchange or fetal oxygenation.

Using the mathematical model of placental О« exchange described above one

may examine the effect of an isolated change in either maternal or fetal blood O«

affinity on the transfer process. Figure 15 illustrates the manner in which such

changes in maternal O- affinity would be expected to act (Longo et al.,1972). As

maternal Ргл increases from its normal value both the rate of O^ exchange (ml· min )

and the O« tension in maternal and fetal end-capillary vessels increases, particularly

if no compensatory adjustments occur. A decrease in maternal Ρ,Λ displays the

60 4000-

E о L -Ξ1 зооо ι υ ω 2000- < ω

1000-

ι τ- 10 Ι 5 20 25 30 GESTETIONAL AGE (weeks)

Fig. 14. Relation between fetal weight and gestational age among fetuses born to mothers with cyanotic heart disease (Normal percentile curves after Lubchenco et al., 1963). Ρ 40-1 < a Fig. 15. Effects of changes in ш 35- O O- affinity of mater­ ¡,30- nal blood on transient 5^25- -» **" "" —·· On transfer rate and 111 E s' 1 placental end-capil­ ό ^ 20- s eg y lary O- tensions (dash­ oP 15-1 / к 40-, . _^— — ed lines) and on steady- ^ ^ ^ ^^ ^^--i^-- state end-capillary Oj =! ^30- MATERNAL ^— β^" v-^. ^ «f--*^^ ζ;1 ^ tensions and umbilical ,* Ε 0 Ε 20- S-'^JSP**^ •^^-FETAL arterial P,-, (solid lines) U 5 10-1 2

op 20-1 I > 5 ^ 15- —

< Ε υ - 5- _| 00 η- 1 él Σ υ1 1 I 1 Ι ι ι 1 1 => 10 20 30 40 50 6 P -nmHg) 5oM<< opposite effect. Even if one allows a certain amount of compensatory adjustment

(in this latter instance, an increase in the rate of placental O2 tension gradient) a similar trend is noted.

Figure 16 depicts the results for variations in fetal Ρ,-Q (Longo et al., 1972).

Raising fetal Ρ,-г. , such as might occur by transfusing the fetus with maternal blood, lowers the O- transfer rate only slightly. For example, the uncompensated transfer

rate would fall less than 10% if fetal P50 were raised to 30 torr, and the decrease in fetal end-capillary 0~ tension would be less than 1 torr. In contrast decreasing

fetal Prn below its normal value would significantly decrease the C^ exchange rate, while the end-capillary O- tension would rise significantly. Thus, normal maternal and fetal Pen values are not greatly different from those optimal for Oj transfer. This is clearly shown in the case of fetal blood by the maximum which appears in the plot relating transfer rate and fetal P^Q (Figure 16). The explanation

Fig. 16. Effects of changes in CU affinity of fetal blood on transient O« transfer rate and placental end-capillary O- tensions (dashed lines) and on steady-state end- capillary O« tensions and umbilical arterial P^ 2 (solid lines).

Psopi mmHg )

62 for the unusual shape of the plot concerns the dual and opposing effects of changing

P.- when umbilical arterial O« tension is assumed to remain constant at 20 torr.

Lowering the Ρ,-л increases the 0„ content of umbilical arterial blood, thereby

decreasing the amount of total exchange. Raising fetal Ρ,-Λ, on the other hand, also decreases total exchange because final equilibrium is reached at a higher oxygen tension, thus curtailing oxygen unloading from maternal hemoglobin. During steady state conditions with ohysiologic comoensations fetal Р^л can vary over a wide

range with end-caoillary O« tensions remaining normal by changes in umbilical arterial О« tensions. Beyond these limits other compensations probably maintain normal 0~ tensions.

5. 2. Decreasing Fetal Blood Oxygen Affinity by Intrauterine Transfusion

During the late ^óO'sand ^970's, intrauterine transfusion for erythroblastosis fetalis developed as a life saving procedure in instances of Rh incompatability

(Liley, 1963). Undoubtedly stimulated by the implications of this mode of therapy, several investigators have examined the physiologic effects of transfusion of blood with differing O« affinities in experimental animals. Battaglia et al. (19é9) first explored this question by exchange transfusing 6 chronically catheterized singleton fetal lombs with type В maternal sheep blood so that the fetal blood O« affinity decreased (Pen values not given) and the blood O- capacity remained constant.

As predicted from theoretical considerations, umbilical venous O« tension increased about 4 torr (to 31 from 27 torr) and in fact in seven fetuses this O« tension reached

36 torr. Despite this increased O« tension, the oxyhemoglobin saturation decreased

28% (to 51 from 79%). Although these authors (Battaglia et al., 1969) had no adequate explanation for this rise in umbilical venous 0~ tension, it is entirely reasonable on the basis of the theoretical considerations detailed above (Longo et al.,

'972). That this decrease in blood O« affinity had no deleterious effect on fetal survival is attested by the continuance of pregnancy for one to five weeks in 5 of the 6 animals;

63 Of cojrse during this time fetal erythrocytes gradually replaced the transfused maternal

blood. Other aspects of this same study were reported by Meschia, Battaglia et al.

(1969). With the increase in umbilical venous O- tension, the uterine vein to

umbilical vein O- tension difference decreased. In addition, there was no

significant change in umbilical blood flow or the placental O- transfer (fetal O«

consumption ) rate, results which again would be predicted. In acutely anesthetized

sheep, Kirschbaum et al. (1971) confirmed the findings of Battaglia, Meschia and

co-workers that exchange transfusion with maternal blood resulted in an increased

umbilical venous O« tension, but no significant change in placental 0„ transfer

or umbilical flow. Mathers et al. (1970) first examined the effects of intrauterine

transfusion of human fetuses with hemolytic disease. The average concentration of

hemoglobin F decreased to 5. 9 from a normal value of 12. 8 g. dl , and in several

instances no fetal erythrocytes were detected. The fetal blood Pj-л value increased

to about 26 from a control value of 21. 6 torr. The fetuses survived for up to six

weeks, but the authors did not present data on how this decrease in blood O-

affinity affected blood gas values. In another group of 15 fetuses which were

transfused by Novy et al. (1972, 1971) the mean P^n value increased to 27. 1 from

a control of 20. 8 torr and the mean 2, 3-DPG concentration increased to 7. 1 from a

control value of 4. 8 mM· I erythrocytes. At delivery, weight, head circumference,

and crown-heel length of these infants were normal for gestational age and they

showed no evidence of acidosis or asphyxia.

The manner in which greater than normal fetal blood O- affinity affects

placental O- exchange may be clarified by an illustration. Because of the dogma

that it is the difference in positions of the maternal and fetal oxyhemoglobin

saturation curves that results in placental O- diffusion it may not be readily apparent

how the fetus survives if its ox/hemoglobin saturation curve superimposes that of

the mother. Consider Fig. 17. Here oxygen content is plotted as a function of O-

tension for maternal and fetal blood both of which have a Pen of 26. 5 torr.

64 24 10 22 9 20

8 18

7 1Λ -о s 14 F 6 E 7 Ζ 12 ш •- S Ζ Ζ 10 η (J 4 υ о Ο 8 3 6

2 4

1 2 . 0 0 Ю 20 30 40 50 60 70 80 90 100 OXYGEN TENSION (torr)

Fig, 17. Oxygen content as a function of oxygen tension for maternal and fetal blood both of which have a Pj-л of 26.5 torr.

Under these circumstances the maternal and fetal end-capillary О« tensions would

equilibrate at 30 torr, and the umbilical venous and arterial O- tensions would

equal about 28 and 20 torr, respectively. These latter values with the arterio-venous

О у content difference of 5 ml · dl are indicated. (Maternal and fetal placental

blood flows are considered equal, and the blood O- capacities are assumed to equal

15 and 22 ml«dl , respectively). The mean maternal—to-fetal O^ partial pressure

difference under these circumstances would equal about 9 torr. Fig. 17 also depicts

the role of the greater fetal blood O« capacity in helping to insure that the fetal

blood contains additional O« despite its lowered O» tension. This "protective"

role of fetal blood 0~ capacity will be discussed below.

65 5. 3. Changes in Oxygen Affinity due to Hemoglobinopathies

Human hemoglobin presentsan almost bewildering diversity; to date the structure

of over 250 varients has been described (Bunn et al.,1977). By amino acid substitu­

tion or deletion the genetic mutation responsible for these variants may affects

the alpha, or beta, gamma, or other chain. As might be expected, the first

abnormal hemoglobins to be reco0nized were those which attracted clinical attention, such as one of the M hemoglobins in individuals with congenital cyanosis (Hörlein and Weber, 1948), and hemoglobin S in patients with sickle cell anemia (Pauling et al.,1949). The first abnormal fetal hemoglobin discovered, Hb Barts (Ager and

Lehman, 1958) consists of four gamma chains (Hunt and Lehman, 1959). Because of a lack of heme-heme interaction has a high O- affinity

(Horton et al., 1962), without the usual sigmoid shaped dissociation curve.

tt also lacks a Bohr effect (Weatherall, 1963). Bart's hemoglobin comprises almost 100% of the hemoglobin in homozygous alpha-thalassemia and occurs in trace amounts in the cord blood of many normal infants. Hemoglobin variants affect blood O- affinity by changes in the normal loci of contact between the hemoglobin subunits. These alterations result in Ргл values ranging from 10 torr in hemoglobin

McKees Rocks and 12 torr in hemoglobins Rainier and Yakima, to about 70 torr in hemoglobin Kansas (Fig. 18). Apparently in an attempt to optimize tissue oxygen delivery, blood with a high O- affinity (low Pen) generally develops a high O^ capacity (i.e. relative polycythemia) as shown in Fig. 19.

Numerous case reports attest to the ability of mothers with altered O« affinity due to a to bear a normal infant. Likewise, fetuses with altered O- affinity due to abnormal hemoglobins can develop within the uterus of a normal woman. For instance, Moore et al. (1967) reported a case in which a woman with 30 per cent hemoglobin Zürich (Pen =21.6 torr) bore four normal infants successfully. Subsequently, Parer (1970) reported a case of a woman with hemoglobin Rdnier (Peg = 12. 8 torr) who delivered an infant with hemoglobin F and

66 A юо г ^__ •— «A

90 • "A RAINIER/^" ^ ^^ I 80 / NORMAL· /^ ν/ SEATTLE/ A I Л, У ti / KANSAS/^ • I / / / /v

/ / / S ë è 8 OXYHEMOGLOBI N SATURAT I 10

η ι ι 1 . 1 10 20 30 40 50 60 70 80 90 100 OXYGEN TENSION (rorr) Fig. 18. Oxyhemoglobin saturation curve for different hemoglobin variants. 24 A

10- • 22 RAINIER/" 9- A 20 ν/ ^___— 8- 18 ƒ ^ NORMAL/^ 7 I £ ' ^ 16 I / S 14 у A J 6- i ζ I 12 : / ΐϋ 5- UJ Z О o io / / SEATTLE^^-^^/^ V ^^ VjS W 4 / / <У 8 ч/ / ó* / / 3- 6 ' /KANSAS • / / 2- 4 - / / /

1 · 2 о jyL^ 10 20 30 40 50 60 70 80 90 Ю0 OXYGEN TENSION (torr) Fig. 19. Oxygen content as a function of oxygen tension for different hemoglobin variants . 67 α normal blood О- affinity. A normal fetus also has been reported in a mother

with hemoglobin McKees Rocks (Winslow et al., 1976) (Ρ,-Q = 10 torr, the highest

O^ affinity yet reported ^however actual details of the newborn's condition were

not presented. In contrast, an increased incidence of fetal mortality has been

reported in a family with hemoglobin Yakima (Pen = 12 torr) (Jones et al, 1967), but

the significance of this is not clear. The manner in which altered maternal or fetal

blood O- affinity due to a hemoglobinopathy affects placental O« exchange may

be illustrated by two contrasting situations. In one normal mother with hemoglobin

A who carries a fetus in which hemoglobin Kansas rather than hemoglobin A replaces

its hemoglobin F near term. In the other situation a mother with hemoglobin Rainier

bears a normal fetus with hemoglobin F.

10

9

8

7

6

5

4

3

2

1

0 io 20 30 40 50 60 70 80 90 100 OXYGEN TENSION (torr)

Fig. 20. Oxygen content as a function of oxygen tension for normal maternal blood and a fetus with hemoglobin Kansas. 68 Figure 20 depicts the first instance, that in which a pregnant woman with hemo­ globin A carries a fetus in which hemoglobin Kansas (Pen = 70 torr) rather than hemoglobin A replaces hemoglobin F. Because under normal conditions near term,

hemoglobin A constitutes about 25% per cent of the total hemoglobin, we assume that hemoglobin Kansas would comprise the same fraction. Thus, the fetal P^-n under these conditions would equal about 35 torr, while the values for other factors would be the same as those detailed above for Figure 17. In this case the maternal a nd fetal end-capillary O« tension would equilibrate at 31 torr, and the umbilical venous and arterial O- tensions would equal about 30 and 18 torr, respectively.

The mean maternal to fetal O- tension gradient would equal 9 torr, and the placental О« exchange rate would be normal.

Our other example is illustrated by Figure 21. Here a mother with hemoglobin

Rainier carries an infant with normal hemoglobin F. The hemoglobin concentration of such patients usually increases such that blood O« capacity equals about 25 ml · dl

Under these conditions the maternal and fetal end-capillary O« tensions would equilibrate at 19 torr, and the umbilical venous and arterial O- tensions would equal about 17 and 9 torr, respectively. The mean maternal to fetal 0„ tension gradient would equal 9.5 torr, and again the placental O- exchange rate would be normal.

Thus, one can appreciate the mechanisms by which normal placental O« exchange can proceed in the face of altered maternal or fetal blood O« affinity from whatever cause. In addition, these examples help emphasize that the normal positions of the maternal and fetal dissociation curves are not sacrosanct and absolutely required essential for placental O- transfer.

On the other hand, placental O- exchange and normal fetal oxygenation may not be maintained under all circumstances of altered O- affinity. For instance, most instances in which th? fetus has more than a 50 per cent concentration of hemoglobin Barts result in grossly abnormal development with generalized hydrops,

69 24 MATERNAL Ρ5ο=12, [Hgb]-15 10 22 Α 9 20 - ^ MOTHER^^/7 8 18 -^ Τ / / Ό 7 16 / / / /FETUS -о /ν / - 6 "ε 14

ζ 12 ò ш ζ оζ ο 10 - 4 • υ Ο 8 ' / 7ν 3 ΐ ƒ / 6 "ΊΞ / / 2 •ο / / 4 - ч» / / 1 2 -77° 0 0 _*¿k_ 1ι0 2ι0 310 410 510 610 710 80 90 100 OXYGEN TENSION (torr)

Fig. 21. Oxygen content· as a function of oxygen tension for a mother with hemoglobin Rainier and a normal fetus. ascites, and hepatic and spleenic enlargement. These infants either are stillborn

or deliver early in the third trimester and die (Lie-lnjo, 1962). As noted above,

in the fetus hemoglobin Bart's high O« affinity probably favors placental O-

transfer, but impedes О« unloading from its blood. Thus, in fetal capillaries, the

blood O« tension would have to decrease to an abnormally low level before 0„

с ould be unloaded to the cells. That occasionally a fetus with 100% hemoglobin

Bart's survives is attested by the case of Gray et al. (1972) in which no evidence

of fetal hypoxia or acidosis developed during labor or delivery. Even in this

instance, however, the fetus had hydrops and marked cardiac hypertrophy.

Recently hemoglobin A.,-, a minor variant of hemoglobin A distinguished by

the presence of a glucose molecule covalently bound to the N-terminal /o-chainoC,

(J «-glue has attracted interest as an index of control in diabetic patients. (Bunn

70 et al..1976; 1978). This glycohemoglobin comprises about 5 per cent of the total hemoglobin of normal individuals, but may be elevated two or three fold in diabetics. (Widness et al.,1978). Although A.^- concentrations in pregnant women tend to decrease during the second and third trimester (Widness et al, 1980) the concentration is correlated with mean blood glucose concentration. Some inves­ tigators have reported that Hb Α.,- concentration correlated with fetal weight, but others have denied this relation (Miller et al., 1979; O'Shaughnessy et al.,

1979). Although the О« affinity of this variant is only slightly greater than

normal (Pen =24 torr) Widness, Schwartz and Schwartz (1979) suggest that in thepresence of thickened placental membranes and abnormal placental function in association with diabetes, maternal to fetal Oj exchange may be compromised.

5.4. Maternal Hyperventilation and Other Conditions

Maternal blood O- affinity also is affected by changes in blood acid-base balance. For instance, with hyperventilation during labor maternal arterial CO- tension can decrease to 26 to 30 torr with increases in pH to 7. 6.

During the ^óO's rather extreme hyperventilation during Cesarean section was popularized for "hypocapnic enhancement" of the anesthestic agent (Crawford,

1962; Coleman, 1967). Soon, several papers cautioned of problems associated with the respiratory alkalosis of this excessive hyperventilation. Morishima et al.(1964) reported that fetal pH varied inversely with maternal arterial pH in guinea pigs, and that maternal pH values greater than 7. 5 resulted in fetal hypoxia, acidosis and increased CO- partial pressure. At maternal CO- tensions below 20 torr, these authors also observed maternal hypotension and lower Apgar scores in the newborn piglets. Moya et al. (1965) then reported similar findings in humans. Two of 61 patients undergoing forced hyperventilation during Cesarean section developed severe respiratory alkalosis with arterial CO- tensions below 17

71 torr and pH values of 7. 66 or more. There was no correlation between maternal CO-

tension and blood pressure. The newborn infants of these patients had severe metabolic

acidosis (i. e. umbilical venous pH = 7. 10 and CO- tension = 73 torr) with abnormal­

ly low oxyhemoglobin saturations and Apgar scores.

A spate of experiments and editorials either supporting or denying this concept then followed thete reports (Anonymous, Lancet, 1966; Battaglia, 1967; Coleman, 1967;

James, 1967). Motoyama et al. (1966) attempted to resolve this controversy observing

in hyperventilated ewes that the carotid arterial and umbilical venous O- tensions

of exteriorized lambs were proportional to maternal arterial CO- tensions and

hydrogen ion concentrations (Motoyama etal.,1967). Further, 'hese workers

demonstrated that the fetal changes were associated with decreases in uterine blood

flow, and maternal hydrogen ion concentration, rather than changes in CO- tension

per se (Motoyama et al., 1967). Hyperventilation without changes in pH or CO-

tensions produced no significant effect (Motoyama et alT 1966, 1967).

Several groups have attempted to uncover the mechanisms responsible for these

changes. Significant associated factors were: decreased umbilical blood flow,

lessened placental O- transfer (Motoyama et al.,1967), and a diminished coefficient

of oxygen utilization (arterial-venous O- content/arterial O- content) (Peng et alv

1972). Uterine blood flow decreased as much as 25% from the mechanical effects

of severe hyperventilation alone (presumably by impeding venous return) rather

than from changes in blood gas values (Levinson et al.,1974). Administration of

sodium bicarbonate to produce maternal metabolic alkalosis (pH =7.7) in ewes

also resulted in markedly reduced fetal O- tensions and oxyhemoglobin saturation

(Ralston et al.,1974).

Data on the interrelations of human fetal O^ and maternal CO- tensions are

limited. A positive relation has been shown by several groups (Rorke et al.,1968;

Peng et al.,1972; Miller et al.,1974) including a re-analysis (Motoyama et al.,1966)

72 of Wulfs report (1964). On the other hand, others have reported only minor changes

in fetal 02 tensions (Fisher and Prys-Roberts, 1968; Lumley et al., 1969). But this

may have been related to the duration of hyperventilation and the degree of alkalosis attained [Coleman (1967) reported no adverse effect on the fetus, but in light to errors in his report (Acheson et al., 1967; Little, 1968) the opposite conclusion should be drawn from his data]. Lumley et al. (1969) first measured fetal scalp blood gases during voluntary hyperventilation in humans, reporting no significant decrease in fetal O- tensions or pH. Assessing a larger number of subjects. Low et al. (1970) and Miller et al. (1974) reported small but signifcant decreases in scalp blood CD- tension during hyperventilation unaccompanied by oxygen debt, fetal acidosis, or other evidence of abnormality of the newborn.

Several animal studies also have reported only small decrements in fetal umbilical venous O- tension following maternal hyperventilation, with no significant decrease in transplacental oxygen exchange (Behrman et al., 1967;Parer et al.,1970), acidosis

(Baillie et 01,1971^ other deterioration of the fetus.

What, then, can be deduced from these conflicting reports? First, hyperventi­ lation is common during pregnancy, particularly during labor and delivery. Evidence does not exist that it either improves fetal oxygenation or is to its wellbeing.

During anesthesia, forced ventilation may lower maternal arterial CO-tension and hydrogen ion concentration sufficiently to interfere with uterine blood flow and transplacental O« exchange. Despite several reports of minimal alterations in fetal O- tensions and pH, the overwhelming evidence is to the contrary. Thus, one cannot conclude that these changes may not affect fetal wellbeing, especially if the fetus already is compromised. Unfortunately, there are apparently no well controlled studies on the effects of O« or CO„ tensions, or pH on uterine or umbilical blood flow. Obviously, knowledge of these effects is necessary to an understanding and quantification of the relation of fetal to maternal blood gas

73 values.

Further, it might be thought that limited maternal hypoventilation either might protect the fetus from these effects and therefore be beneficial. This idea cannot be entertained, however. Hypoventilation during anesthesia, even with supplemental oxygen, is dangerous, predisposing the mother to increased blood pressure, cardiac arrhythmias an^ increasing the risk of post-operative atelectasis and pneumonia. In addition, hypoventilation with respiratory acidosis leads to respiratory acidosis in the fetus (Blechner et al.,1960; Bruns et al., 1963) and probably fetal hypoxia, due to lowered blood oxygen affinity.

6. Relation of Maternal and Fetal Hemoglobin Flow to Placental Oxygen Exchange

6. 1. General Considerations

Another question of both physiologic and clinical importance is the extent to which changes in maternal and fetal blood O- capacity or placental blood flow rates affect placental O- transfer. Although it may not readily be apparent why blood flow and O« capacity can be considered together, this is because these two factors exert a similar influence on oxygen transport. For instance, consider the

Fick principle Vj-, = 0·(Δ 0„ content) where VQ is the amount of O- trans- -1 · . -T ported (ml » min ), Q is the rate of flow (ml · min ) and ΔΟ« content is the arterio-venous content difference (ml O-· ml blood · min ). In this equation both blood flow and О- content (or capacity, its dependent function) determine the amount of O« transported. Because the product of blood flow and O- content defines the amount of oxygen transported, it is useful to consider them together as hemoglobin flow (Longo et al.,1972; Makowskï et aL, 1973). Although the equivalency of О- capacity (hemoglobin concentration χ 1. 368) and blood flow is a convenient approximation, it is only an approximation for several reasons.

First, changes in hemoglobin concentration affect the blood buffer capacity, which in turn influences O^ transfer by the Bohr effect. In addition, hemoglobin concen-

74 tration affects the placental diffusing capacity slightly (Longo et al., 1967).

Finally, alterations in hemoglobin concentration probably affect the distribution of flow within the placental vessels due to viscosity change. However, these effects probably are relatively minor and should not detract from the usefulness of "hemo­ globin flow" as a concept.

Beginning as endometrial capillaries eroded by the advancing mass of trophoblastic cells, the maternal placental circulation develops rapidly during the first weeks of gestation to become the vital supplier of fetal nutrients. Despi te innumerable studies in many species, little information is available on the explicit relations of maternal placental hemoglobin (or blood) flow and oxygen exchange.

Numerous studies in many species using a wide variety of techniques have examined the rate of blood flow per se (particularly near term), its changes during gestatior\ the relation of flow to blood pressure, and the effects of a myriad of chemical compounds of physiologic and pharmacologic interest. Early studies in anesthetized, acutely operated, or even exteriorized preparations were hampered by interference in the measurement one was attempting to make. Recently, however, the use of relatively non-invasive techniques in chronically catheter!zed awake animals has obviated this problem.

In 1933 B^rcroft et al. (1933) using a venous outflow technique in sheep first measured an increase in uterine flow and O« consumption proportional to the growth of the fetus. More recent studies in sheep indicate that maternal placental flow -1 . -1 increases from about 30 ml· min at 40 days gestation to 1500 ml· min at term

(Rosenfeld et al, 1974). Calculated per kg fetal weight, during this period flow decreased from 9000 to 250 ml· min .kg , a finding that agrees with the results of other investigators (Metcalfe et al., 1959; Huckabee et al.,1961). During this same time flow per g of maternal placental cotyledons increased from 0. 4 ml · min ·

• g at 90 days to 3 ml-min . g at term. Power et al. (1967) first determined that about 90% of maternal placental flow perfused the placenta while the balance

75 supplied *Ье endometrium and myomerrium. Similar f indinas have ^ееп reported oy

others (A/akowski et al., 1968; Duncan and Lewis, 1969).

Because of its relative inaccessibility, fetal placental blood flow under

physiologic conditions has been even more difficult to measure than maternal

flow. As with the determinations of maternal placental blood flow, more recent

studies in chronically catheterized unanesthetized preparations indicate that the

magnitude of flow is much greater than previously appreciated. For instance, in the

near-term fetal lamb placental flow equals about 200 ml min kg (Rudolph

and Heyman, 1967; Crenshaw et al., 1968; Clapp et al., 1974; Faber et al., 1973;

Longo et al., 1978). Only a few studies have explored in any rigorous manner the

relation of flow to respiratory gas exchange.

Fig. 22. Effects of changes in -— 30- UJ У maternal placental blood flow 1- y < ~ / (Q ) on transient maternal and er .E / m ω £ 20- / / fetal placental end-capillary О« / tensions and average rate of O- и / и s Χ 10- y exchange (dashed lines) and on ш y s /, Ν y /s ~ J ^ Ss steady state end-capillary O« О y y бОі /s' tensions and umbilical arterial (M χ y QP / y Ρ0 (solid lines). /y U >- ~ /y 2 cc σ» /y < ι 40- /y -1 _ /y =! E s Q 20- --'/ LzU —' "" /

JJ QP 20-1 >- ^^ — S _ 15- / < I 10- / 31* / W 1 -J 1 СО О l'I 1 ι ' • ι 1 40 60 100 200 400 1000 2000 4000 QM ( ml/min ) 76 6. 2. Theoretical Considerations

Figure 22 depicts the effects of changes in maternal flow on oxygen exchange

and end-capillary Oj tensions. At high flow rates the O- tension of end-capillary

blood approaches that of the maternal artery. At lower flow rates the equilibrated

O2 tensions decreases because less 0„ is available for exchange. The transient О™

transfer rate follows a trend similar to that of end-capillary O- tension except

that flow rates greater than 300 to 400 ml «min result in little additional oxygen

transfer because the rise in mean maternal O- tension adds little О™ content on the

flat part of the oxyhemoglobin saturation curve (Longo et al., 1972; Guillbeau et al.,

1970), With reduced flow in steady state conditions the umbilical arterial O«

tension would fall to low levels (Fig. 22) and below flow rates of 200 ml · min

changes in umbilical arterial O- tension alone could no longer maintain normal

oxygen transfer. Uncompensated decreases In maternal hemoglobin concentration

result in transient decreases in both 0? transfer rate and placental end-capillary

O« tension (Fig. 23). For example, if maternal hemoglobin were to fall to 6

g»dl , the O^ transfer rate would decrease momentarily by 23% (to 19 from 25

ml · min ) and end-capillary O- tension would fall by 21% (to 25 from 32 torr).

If fetal oxygen consumption was maintained this would soon result in a lowered

umbilical arterial O« tension, an increased maternal-fetal O« partial pressure

difference and a normal O« transfer rate during the steady state. A fall in umbilical arterial 0„ tension to 2 from 15 torr would be required to maintain oxygen transfer with this degree of anemia.

The effects of и ncompensated changes in fetal flow are shown in Fig. 24.

Increases in flow allow additional oxygen transport, and the end-capillary О«

tension will decrease to approach umbilical arterial O« tension. A maternal-to- fetal end-capillary O^ tension difference becomes apparent at high hemoglobin

77 5 10 15 20 25 НВм (gm/l00ml) Fig. 23. Effects of changes in maternal hemoglobin concentration (Hb m) on transient О- transfer rate and placental end-capillary 0„ tensions (dashed lines) and on steady-state end-capillary O« tensions and umbilical arterial PO- (solid lines). Effects are similar to those of maternal placental flow rate.

78 ш ___— 1- У*' < ^30- ОЕ / z\ / г 5 ч %„ ' ч ^ σ>40- ч ч ч MATERNAL u< ^ Ч — Р о ê >ΤΓ? ζ 20- ω FETALE—~-

оР 25- ^^ ^ 20- ^S^ ш ^Г^ 1- ^ 15 ^ ^? " / < ¡ίο­ / υ; ε / d ~ 5- // ш / 2 1 0- ' • I 1 Ι ι ι | 1 э ο-4ι0 60 100 200 400 1000 2000 4000 Qp ( ml/min)

Fig. 24. Effects of changes in fetal placental blood flow (GL) on transient maternal and fetal placental end-capillary 0~ tensions and average rate of О« exchange (dashed lines) and on steady-state end-capillary 0_ tensions and

umbilical arterial Ρη (solid lines). U2

79 flow values (Fig. 23) because oF a diffusional limitation i. e., a lower ratio of

placental diffusing capacity to flow (Power et al, 1972). The transient O« transfer

rate varies linearly with fetal flow over the range from 150 to 450 ml · min

Above about 500 ml »min the exchange rate no longer rises so steeply because

additional O^ is not available from naternal blood. During the steady state decreases

in flow are accompanied by a fall in umbilical arterial 0„ tension to low levels to

maintain normal O- transfer (Fig. 24). However, if fetal placental flow fell below

170 ml· min additional compensations would become necessary to maintain normal

oxygen exchange. Uncompensated changes in fetal hemoglobin concentration have

several effects on oxygen exchange (Fig. 25). The rate of O- transfer varies almost

linearly with fetal hemoglobin concentration as it does with umbilical flow. Fetal

end-capillar/ O- tension vcries inversely with fetal hemoglobin concentration, as a

higher concentration of fetal hemoglobin can combine with more O- from the

maternal blood. Thus, in fetal anemia end-capillary Oj tension increases and the rate

of placental О« exchange decreases. A fetal hemoglobin content of 6 g· dl , for

example, increases end-capillary O- tension to 44 torr and decreases O« transfer

rate to 15 ml· min , resultinresulting in a 37% decrease in the O- content of fetal placental end-capillary blood.

6. 3. Experimental Studies

The results of the theoretical studies above suggest that placental oxygen

transfer should be particularly sensitive to changes in maternal or fetal hemoglobin

flow. As previously emphasized, the effects of variations of these factors are difficult

to explore experimentally because when a variable such as blood flow is altered,

other variables may change to mask the consequences of the permutation under

study. When this occurs the investigator misjudges the role of that factor in affecting O« transfer.

80 ω 30- ^''~~ < ^*' о: / y ш^ у s S| 20- / / / Ο E / Fig. 25. Effects of changes in 10- / fetal hemoglobin concentration oN / J / (НЬ ) on transient Oj transfer ÍM - р

«^ 60Ί1 Ч I rate and placental end-capillary V \ ir Ν <^ 50- Ν O- tensions (dashed lines) and _i σι Ν on steady-state end-capillary EE40- "Ν Ι ^ MATERNAI O- tensions and umbilical û-20- arterial PO„ (solid lines), 2 ^>*===== ш 20-1 FETAL Effects are similar to those of Oí . oP fetal placental flow rate. >- го­ ^ ст Ul ^^^-^*1^ ¿^ 15- < « ^ <І ю- У E f di 5- ι m I Σ n0- 3 ^ ι ι ι • ι с) 5 10 15 20 2! HBF (gm/lOOml) In an effort to circumvent these problems several investigators have studied

placental exchange using isolated preparations, in which blood perfuses the uterine

or umbilical vessels at accurately controlled Oj contents and flow rates. In what

must stand as a landmark technical achievement. Fuller et al. (1978) developed a

method to perfuse the isolated maternal vessels in the innervated and in-vivo

sheep uterus. Fig. 26 exhibits oxygen uptake by the pregnant uterus as a function

of uterine flow. The closed circles represents values obtained in near-term (132-

145 days) sheep, while the open circles indicate values earlier in gestation (SO­

US days). The O« uptake near-term averaged 8. 1 (+ 0.6) ml. »min · kg uterine

and fetal weight , while where as in the mid-gestation group this value averaged

6. 3 ( + 0. 3) ml« min" · kg" . In the near-term group O- uptake increased pari passu

with uterine flow, suggesting as indicated above that oxygen transfer is limited by

81 14-

12-

|io- σι 1, ^ ? 8- E '.'·% • · /Χ 0 ο ο ..0ο^0 ο» 0ο ··*· ί&Ι ο0» οο ο ο ш 6 - 0 χ. ί 4- οΝ 2-

J 0 1 1¿¿ 1 1 1 1— 1 1 ' 200 400 600 θ 00 UTERINE BLOOD FLOW (ml.min1.kg"1) Fig, 26. Oxygen uptake as a function of uterine perfusion flow in 5 ewes between 50-118 days gestation (open circles) and 5 ewes between 132-145 days gestation (solid circles). Shaded area represents physiological range of uterine blood flow. (Fuller et al., 1978). blood flow rather than diffusion. On the other hand, during mid-gestation this relation was not apparent above flow rates of 200-250 ml «min · kg , although the relatively sonali size of the mid-gestation conceptus (and its maximal O« consumption rate) in relation to total blood flow may have made such an observation difficult,

In the first study of the effects of variations of fetal placental flow an 0~ transfer, Dawes and Mott (1964) varied flow by constriction of the umbilical vein or aorta, or by phlebotomy in immature (79-86 days) fetal lambs. Fetal oxygen consumption varied as a function of umbilical flow between values of about 40 to

200 ml t min »kg , but was relatively constant above that highest value. The change in oxygen consumption was greater when flow was altered by hemorrhage than when it was reduced by vessel constriction perhaps because vessel manipulation

82 resulted in release of catecholamines with increased oxygen consumption tending

to offset the decrease (Lorijn and Longo, 1980).

In an effort to more scrupulously control the variables. Power and Jenkins (1975) perfused isolated placental cotyledons in sheep and rabbits. Figure 27 A illustrates the dependence of oxygen transfer on flow rate while Fig. 27 В shows the relation of venous outflow O- tension to cotyledonary flow. As predicted from theoretical considerations (see above) the oxygen transfer rate (normalized as measured О« exchange/O- exchange at standard flow) rose as the rate of flow increased (Fig. 27

A). Fig. 27 В demonstrates that outflowing O- tension tended to increase at lowered flow rates. Again, this finding was anticipated as fetal end-capillary O- tension should increase at low flow rates where excess oxygen is supplied in relation to the blood's capacity to transport it away from the exchange area. In fact, ignoring

•1.5

02 EXCHANGE RATE

Vo2 MEASURED V02 AT ··/ STANDARD FLOW .0.5 У

/

FETAL COTYLEDONARY FLOW ML/MIN

Fig. 27. A. Relationship between transplacental oxygen transfer and fetal cotyledonary flow in perfused isolated sheep and rabbit placenta.

83 OUTFLOW

P02

P02 MEASURED PO2 AT STANDARD FLOW

-0

FETAL COTYLEDONARY BLOOD FLOW ML/MIN 12 3 4 1 ' 1 1 ,

Fig. 27. B. Relationship between venous outflow oxygen tension and cotyledonary flow in perfused isolated sheep and rabbit placenta. placental O- consumption, the O- tension of fetal outflowing blood theoretically should approach that of the maternal arterial blood at low umbilical flow rates.

At high flow rates blood remains for a relatively short time interval in the exchange vessels; and a diffusional limitation to oxygen exchange, if present, should become apparent by an abrupt decrease in fetal effluent O« tension above a certain critical flow rate (Longo et al,, 1972). These data give no hint of such a change and supports the concept that oxygen exchange normally is limited by blood flow.

An additional consideration is the effects of variation in hemoglobin concen­ tration. Although certain investigators have experimentally decreased maternal (or less often fetal) hemoglobin concentration by phlebotomy, we know of no systematic studies of placental О- transfer under these conditions. Actually, the hemoglobin concentration in placental exchange vessels is not necessarily that concentration

84 in the major vessels. For example, Newcomb and Power (1975) demonstrated that

the hematocrit in the fetal rabbit placental vessels is about 25% as compared with

the large vessel hematocrit of 37%; and the ratio of placental to large vessel

hematocrit equaled 0. 76. This plasma skimming has implications for placental

O- exchange. During any steady-state, the volume of erythrocytes and plasma

which flow into the placenta must equal these volumes which leave it. Because

the hematocrit of blood in the larger vessels is greater than that in smaller vessels

within the placenta, erythrocytes must traverse the placental vessels more rapidly

than the plasma, i. e. the erythrocytes have a shorter mean transit time than plasma.

This abbreviated erythrocyte transit time implies a truncated period for oxygen

exchange to proceed. For instance. Longo et al. (1969) calculated that the mean

transit time in sheep placental capillaries equals about 1.7 sec. Whereas maternal and

fetal oxygen tensions are predicted to equilibrate during a 1.7 sec capillary transit

if the transit times of erythrocytes and plasma were 1. 26 and 1.84 sec, respectively

(values that would accord with present results), O« tensions may not equilibrate.

In other words, the placental diffusion reserves may not be as great as previously

considered, and a small maternal to fetal 0„ tension difference could result from

diffusional limitation.

6.4. Clinical Implications

Maternal blood 0„ capacity can vary from polycythemia to profound anemia

under a wide variety of circumstances, however, blood 0~ capacity probably cannot

change without some alteration of О« affinity with anemia oxyhemoglobin saturation

curve shift to the right, as erythocyte 2, 3-DPG concentration varies inversely with

hemoglobin concentration (Hjelm, 1969; Valeri et al, 1969; Torrance et al, 1970;

Koch and Schroter, 1973). Kloeke (1972) has analyzed the physiologic advantage of this relationship. In patients with polycythemia vera the 2, 3-DPG concentration

increases from 55 (Hakin et alv1972) to 145 (Hjelm and Wadman, 1972; Koch and

85 Schröter, 1973) per cent with concomitant decrease« in O^ affinity. Intracellular

2, 3-DPG concentrations increase in a wide variety of physiologic adaptations such

as at high altitude, and in pathologic states, such as pulmonary disease, cyanotic

congenital heart disease, renal failure, and hepatic cirrhosis.

In humans, maternal placental flow is notoriously difficult to measure, and none

of the various techniques that have been employed (nitrous oxide equilibration, rate

of disappearance of radioactive sodium injected into the uterus, electromagnetic

flow probes, etc. ) are satisfactory. Recent studies using a radioactive inert

gas such as Krypton or Xenon may prove useful, but the validity of this approach

has yet to be established. Most of our knowledge of the changes in uterine flow

under various circumstances derive from studies in experimental animals. In several

species uterine flow decreases in a wide variety of circumstance including: hypo­

tension with hypovolemia, circulatory shock, spinal anesthesia; hypertension;

hypoxia; and with the administration of a host of pharmacologic agents. For instance,

following spinal anesthesia in sheep uterine flow fell 65%, but placental O«

transfer decreased only 14% and umbilical O^ tension declined 9% to 21 from 23 torr (Lucas et al, 1965). These changes are relatively small for such marked

reductions in flow. Theoretical considerations (see above) predict that the normal placental O- rate exchange cannot be maintained in the face of flow decreases without compensatory changes.

Fig. 28 illustrates some consequences of reduced maternal hemoglobin flow on placental oxygen exchange. We assume a maternal hemoglobin concentration of

5.75 (rather than 11) g«dl" , i.e. a 50% anemia, and an associated slight increase in blood Oy affinity (Рсл = 28). In addition, maternal cardiac output and uterine

flow experience a compensatory increase such that the flow rate doubles. Under these conditions (other variables remaining constant), the uterinearterîo-venous

0~ difference would equal about 2. 5 ml · dl (rather than a normal value of about '2

5 ml»dl ). The mean maternal to fetal O« tension gradient under these conditions

86 24 MATERNAL Р5о=28, [Hgb] = 5 75 10 22 9 20

8 18

7 16 -о 14 E 6 1 ι— 12 LU Ь IΖ— ζ Ζ 10 О 8 4 υ О 8 3 6 2 4

1 2

0 0 10 20 30 40 50 60 70 80 90 100 OXYGEN TENSION (torr)

Fig. 28. Oxygen content as a function of oxygen tensions during maternal anemia, and an associated increase in blood O« affinity,

would equal about 9torr; and the end-capillary O« tensions would equilibrate at

about 25torr. This illustrates how placental O^ exchange can be maintained with

this degree of anemia. Of course with a more marked decrease in placental

hemoglobin flow, O- delivery would be compromised. Although no reports of fetal

blood gas values in maternal anemia are available, they may be reduced in severe

anemia, as suggested by the finding of lowered O- tensions in the amniotic fluid

of such patients (Johnson and Ojo, 1967). Although the observation that placental

weight increases in anemic mothers (hemoglobin ζ 8 g · dl ) (Beischer et aL, 1970)

suggests that placental diffusing capacity increases under these circumstances, this

is not established.

Lowered fetal O- capacity commonly occurs in association with hemolytic

disease. Although fetal primary polycythemia may not occur, its hemoglobin

87 concentration may be slightly higher than normal in residents at high altitude (Prys-

towski et al., 1960). As in the mother, a number of conditions can decrease the rate

of umbilical blood flow. Hypoxia and asphyxia probably most commonly lower fetal

placental hemoglobin flow (Dawes, 1962), although relatively normal flows can be

maintained even with severe hypoxia (Peeters et al. Longo et aL, 1978).

Figure 29 portrays the effects on placental oxygen exchange of uncorrected fetal anemia (as in erythroblastosis fetalis) with decreased placental hemoglobin flow.

In this example the fetal hemoglobin concentration is assumed to equal 8. 25,

rather than a normal value of 16 g-dl . Under these conditions there is no evidence of either a shift in the oxyhemoglobin saturation curve or an increase in umbilical flow. Thus, the mean maternal to fetal 0„ tension gradient would equal

10 torr (arrow), the end-capillary Or, tensions would equilibrate at 32 torr (some­ what higher than normal) and the umbilical arterio-venous 0„ difference would

24 r FETAL [Hgb] = 8 25 10 22 9 20

8 18

7 16 -о 14 E 6 1

ζ 12 Ь LU z Ζ о 10 4 υ 8 (Ν 8 О 3 6 2 4

1 2

0 0 40 50 60 70 OXYGEN TENSION (torr) Fig. 29. The effects on placental oxygen exchange of uncorrected fetal anemia

with decreased placental blood flow. 88 remain at 5 ml · dl . This suggests that despite this degree of hemolysis, fetal oxygenation could be maintained. Of course with more profound anemia, oxygen

exchange would be compromised or fall completely. Adamsons et al. (1969) demon­

strated that in monkeys fetal hemoglobin concentration could be reduced 50% and

hematocrit to 10% before fetal heart rate, ECG, or arterial blood pressure and acid-

base status were affected. The human fetus with severe hemolytic anemia can demonstrate normal blood gases and acid-base status at the onset of labor, although

metabolic acidosis may develop during the second stage of labor perhaps as a result of hypoxia (Hobel, 1970).

7. Interrelations of Maternal and Fetal Placental Flows and Oxygen Exchange

7. 1. Distribution of Maternal and Fetal Placental Flows

7. 1. 1. Non-uniform Distribution of Maternal and Fetal Blood Flows

To this point, the discussion of the effects of changes in flow on oxygen exchange has assumed a uniform distribution of maternal to fetal placental flow. When such flows are evenly matched with respects to one another, O« exchange is optimized.

On the other hand, several studies suggest that the distribution of these flows is not necessarily uniform. For instance, the opening of arteriolar orifices into the human intervillous space is inhomogeneous (Brosens and Dixon, 19¿6), and cineradioangiography demonstrates intermittant circulation in maternal and fetal placental vessels (BoreiI et al., 1958; Ramsey et al., 1963; Martin et al, 1964).

Further evidence derives indirectly from comparison of placental oxygen exchange, which is relatively sensitive to flow and carbon monoxide which is not (Longo et aL,

1967). The most completing evidence, however, originates from studies of the flow distribution per se. Using radioactive labeled macroaggregates of albumin in the sheep placenta. Power et al. (1967) demonstrated that in 1 g placental samples both the maternal and fetal blood flows were distributed nonuniformly. Moreover the ratio of maternal to fetal flow was markedly uneven, i. e. only about 30% of the placenta

89 hcH α flow ratio of 50% of the mean value; the remainder of the placenta displayed

ratios of maternal to fetal flow above or below these limits. A subsequent study by

Rankin et al. (1970) using radioactive microspheres in chronically catheterized sheep

reported that when analyzing 5 g portions of placenta, the flow distributions appeared much more uniform than reported by Power's group. However, a recent

study by Power (1978), demonstrated that the degree of nonuniformity observed

critically depends upon the size of the placental sample taken so that in small

(40 mg) individual portions a great deal of nonuniformity is present. As an inves­

tigator uses larger tissue samples, and therefore counts greater numbers of radioactive

microspheres the inhomogeneties evident in the smaller portions tend to be nullified, obscuring the true variation.

Although seemingly of littl e importance, the distribution of maternal and fetal flows has profound implications for placental exchanje in general and oxygen exchange

in particular. If both flows were distributed perfectly uniformly, the end-capillary

O« tensions in all exchange units would be equal (Longo and Power, 1969). On the other hand, nonuniform distribution of these flows results in equilibration of end-

capillaries at various oxygen tensions, which vary as a function of the degree of

nonuniformity. For instance, for ratios of maternal to fetal flow of 3 and 0. 3 respectively, end-capillary 0~ tensions would equal about 51 and 21 torr, respecti­

vely (Power et al., 1972). Under these conditions maternal blood would leave the first compartment with an О« content of 12.0 ml · dl and exit from the other with 0. 34 ml· dl . The compartment with the higher ratio of maternal to fetal flow, and the higher O- values would contribute a disproportionately large share to the uterine venous blood, raising its O- tension and content to 41 torr and 9. 9 ml- dl ,

respectively. In the fetal vessels the compartment with the low flow ratio and the

low О« values would contribute a disproportionately large fraction to the umbilical

venous blood, decreasing its O- tension and content to 23 torr and 11. 7 ml · dl ,

respectively. Thus, despite equilibration of oxygen tensions in individual units, a

90 uterine vein to umbilical vein (V-v) О« tension difference of 18 torr would result

from the nonuniform placental flow. Using this type of analysis one can calculate

the overall O^ tension and content differences for any given pattern of dis­

tribution of placental blood flows (Power et al, 1967; Power and Longo, 1969; Longo and Power, 1969; Power et al., 1972).

7. 1. 2. Distribution of Placental Diffusing Capacity to Blood Flows

Not only may the distribution of blood flows vary, but there may be a non- uniformity in the distribution of placental diffusing capacity to blood flow. This is suggested by such morphologic findings as wide variation in the thickness of placental tissue layers which separate the maternal and fetal circulations (Ludwig, 1962; Wynn,

1971). Placental diffusing capacity quantifies the respiratory function of that organ and is a function of surface area, thickness, and volumes of the exchange vessels.

Although it will be a certain value for the placenta as a whole, the value may vary in different portions of the placenta, and be adequate or inadequate in relation to the ability of fetal blood to transport Oj. It is the relation of diffusing capacity to blood transport capacity that determines whether oxygen exchange proceeds to equilibrium rather than the value of diffusing capacity per se. Power et al. (1972) presented a detailed analysis of the theoretical aspects of distribution of diffusing capacity to blood flow on placental O- exchange. However, at present there is no method to experimentally assess the importance of this phenomenon in limiting transplacental O« exchange.

7.2. "Sluice" or "Waterfall" Flow in Placental Vessels

Because of the juxtaposed nature of the maternal and fetal placental circulations it is not unreasonable that changes in spatial relations in one vascular bed might affect the vascular resistance in the other, with resultant effects on flow through

91 the involved exchange vessels. Although in most organs blood flow is a function of the hydrostatic driving pressure (the inflow outflow pressure difference), in the lung and some other organs, blood flow varies as a function of the inflow minus the surrounding pressure difference. It has recently been demonstrated that such a phenomenon might exist in the placental circulation, with flow through the fetal capillaries affected by the surrounding pressure in maternal vessels or intervillous space, or flow through the maternal placental vessels affected by the surrounding pressure in the fetal vessels. Using an isolated perfused sheep cotyledon preparation. Power and Longo (1973) first showed that the surrounding pressure in maternal placental vessels affects fetal placental flow. This was shown with several techniques. For instance, at a constant perfusion rate, fetal arterial

(inflow) pressure rose when the surrounding pressure in maternal vessels was raised by clamping the inferior vena cava. Umbilical inflow pressure (at constant flow rate) was independent of changes in outflow pressure until the outflow pressure excee the surrounding pressure, about 15 mm Hg. Further experiments showed that umbilical vascular resistance not only increased with increar-es· in placental surrounding pressure, but decreased with a decrement in surrounding pressure, as by lowered maternal arterial pressure.

In an extension of these studies Bissonnette and Farrell (1973) also exhibited a

"sluice" flow phenomenon in the vascular bed of the sheep placenta. In addition, these authors showed that increases in i_rmbilical flow resulted in distension of the fetal placental bed, but there was no evidence of recruitment of previously unper- fused channels under these circumstances. Additional evidence for maternal-fetal vcscular interaction derives from the demonstration that fetal placental vascular compliance (change in volume per given change in pressure) increases with decreases in maternal vascular pressure (Power and Gilbert, 1977). Schröder et al.(personal communication) also evinced sluice flow in the perfused guinea pig placenta. Finally, Fuller et al. (1979) has shown a sluice effect in the opposite di­

rection, i.e. increased fetal placental vascular pressure affects placental flow

through the perfused sheep uterine vessels.

The question arises whether sluice flow affects placental oxygen exchange.

In some women who assume the supine position the pregnant uterus can impinge

upon the inferior vena cava, thereby impeding venous return (McRoberts, 1951).

This results in decreased cardiac output and arterial hypotension. During cavai

occlusion uterine venous outflow is impeded and presumably pressure in the inter­

villous space surrounding fetal vessels rises. This could cause increased umbilical

vascular resistance, with a momentary drop in umbilical venous return, and fetal

bradycardia. Within a few moments the fetal blood pressure would increase to

maintain umbilical flow, and the heart rate would slow further due to a barore-

ceptor response. Fetal bradycardia occurring in association with the supine

hypotension syndrome (Reed et al., 1970), in fact may result from this mechanism.

Undoubtedly, as cavai compression continues placental oxygen transfer would be

impaired with resultant myocardial depression. Indeed, the experimental demon­ stration of sluice flow in the sheep was associated with patterns of fetal brady­ cardia usually associated with cord compression or "placental insufficiency"

(Power and Longo, 1973).

An additional factor to consider is the effect of decreses in maternal surroun­ ding pressure as might result from anesthesia or blood loss. These conditions would

result in lessened umbilical vascular resistance with an increase in flow. It remains to be determined, however, whether these alterations quantitatively affect 0~ transfer in humans. As an aside, the changes in maternal and fetal placental capillary hydrostatic pressure under these conditions probably affect transplacental water movements with resultant effects on amniotic fluid and fetal blood volumes

(Power and Longo, 1973).

93 7. 3. Uterine Contractions and Oxygen Exchange

Up to now we have considered the virtually continuous oxygen exchange during

fetal life. However, during uterine contractions O- transfer may decrease or become

discontinuous, and fetal distress may become evident. Although the changes associated

with labor generally are ascribed to placental "insufficiency", these changes un­

doubtedly have a physiologic basis. Studies in primates indicate that uterine

contraction results in decreased uterine venous outflow (Ramsey et al., 1966) and in

some instances depressed arterial inflow (Ramsey et al., 1963; Ramsey 1968). Although

relatively little is known about the dynamics of the uterine circulation during

contractions, less is known about umbilical flow. In an effort to explore these

interrelations in a systematic manner, Butler et al, (1976; 1978) developed a

mathematical model to examine the effects of changes in intrauterine pressure on

maternal and fetal placental flows and О« exchange. This formulation predicted that during a normal contraction (50 mm Hg peak intensity, lasting for 1 min), uterine flow and maternal intervillous space blood volume decreased about 50% and 20%, respectively. Umbilical venous O- tension decreased about 36% (to 18 from 28 torr) and the rate of placental oxygen transfer dropped 40% (to 12 from

20 ml · min ). Umbilical venous О« tension was most sensitive to the intensity of contraction and maternal blood pressure, and less sensitive to maternal arterial

O- tension. The contraction duration had essentially no effect on umbilical

Venous 0„ tension. These workers (Butler et al., 1976) derived an empirical relation for the minimal umbilical vein O« tension reached during a uterine contraction:

Pf. minimum (torr) = 26. 7 - 0. 219 intensity (mm Hg). U2 This model further predicted that if two or more factors change simultaneously

their combined effect would not be the same as the sum of their individual effects.

For example, the interaction between contraction intensity and duration could be

expressed by the relation:

94 О- deficit (ml) = 0. 33 + Duration (sec)'[Ìntensity (mm Hg) ·

2. 04 χ IO-3- 3x IO"2]

This relationship applied in the range of 20 to 60 mm Hg intensity and from

0. 5 to '.· min duration. For instance, a 50 mm Hg contraction lasting for 60 sec

would produce an oxygen deficit of 4. 6 ml, this debt constitutes about 20% of the

normal fetal O« consumption during one minute. With more intense, longer

contractions oxygen exchange would be compromised even more severely.

Although of theoretical interest and probably of vital clinical importance, these predictions require experimental validation.

During the normal labor, maternal placental flow briefly and repeatedly decreases, but the fetal blood is reoxygenated during the recovery period. Using electro- mggnetic flowmeters, Greiss and his colleagues (1965; 1968) showed that uterine flow in the ewe decreased with either raised uterine resting tonus or with increased frequency and duration of contractions. The direction of these changes agree with the mathematical prediction. Dawes et al. (1960) demonstrated 1 . 5 to 9% decrease in rhesus monkey fetal femoral arterial oxyhemoglobin saturation during a contrac­ tion. However, as the intrauterine pressure was not reported, no quantitative rela­ tion between pressure and fetal oxygen level can be established. Also in the fetal monkey, Myers et al (1973) demonstrated decrements in fetal arterial O- tension in association with fetal bradycardia (late decelerations).

From the above considerations, it can be appreciated that vaginal delivery poses a special problem. On one hand, uterine contractions must be of such

intensity and duration as to effect delivery of the conceptus. On the other hand, the contractions must not be ofvsuch magnitude to overly curtail placental O« exchange,

inducing the development of fetal hypoxia and central nervous system damage.

Fig. 30 illustrates how the seemingly conflicting requirements of mechanical efficiency for fetal expulsion and minimal interference with O- exchange might

95 interact. Under optimal conditions, indicated by the intersection of these two curves,

the efficiency of uterine contractions in such that transplacental oxygen flux

continues in a relatively uninterrupted fashion.

EFFECTIVENESS IN OXYGEN ADVANCEING LABOR TRANSFER

INTRAUTERINE PRESSURE

Fig. 30. The effect of increased intrauterine pressure during labor upon transplacental oxygen transfer and efficiency for fetal expulsion.

96 TRANSFER OF OXYGEN DURING INCREASED FETAL OXYGEN CONSUMPTION

I. Introduction

During fetal development oxygen continually diffuses through the placenta to meet fetal needs, and the rate of O« exchange increases pari passu as the fetus grows. Although under normal circumstances the placenta meets the fetal oxygen demands, it is unclear how much "reserve" or excess capacity for diffusion exists.

Several investigators have calculated the placental diffusing capacity (a measure of respiratory exchange) in volume (ml) per unit time (min) per unit of partial pressure difference (torr). Unfortunately the term "diffusing capacity" may be confusing, as the maximal exchange usually is not measured Rather, diffusing capacity is determined as discussed earlier, by many factors such as the matsrnal and fetal capillary blood volumes, hemoglobin chemical reaction rates, and the diffusion characteristics of the placental membranes per se.

In an attempt to stress the placental diffusion "reserves" and determine whether more than normal amounts of oxygen can diffuse across the placenta, we artificially increased the rate of fetal oxygen consumption by infusing norepinephrine or triiodothyronine into the chronically catheterized fetal lamb. Additionally, we examined the effects of these hormones on fetal blood gas values, cardiac output, and blood flow to various organs. This subject is of clinical as well as physiologi­ cal interest in understanding the problems involved during norepinephrine release during fetal hypoxia (Comune and Silver, 1972; Jones, 1977; Jones and Robinson,

1975) and labor (Lagercrantz, and Bistoletti, 1977), the events occurring during fetal thyrotoxicosis (Dirmikis and Munro, 1975; Petersen and Serup, 1977; Ramsay,

1976; Robinson et al., 1979;Samuel et al., 1971) and the intrauterine treatment of hypothyroid fetuses with thyroid hormones(Rsher, 1975; Lightner et al. , 1977;

Riddick et al., 1979; Van Merle et al., 1975).

97 2 Methods and Materials

The principle of the method was to infuse norepinephrine or triiodothyronine into the chronically catheterized fetal lamb. We measured the effects of these hormones upon placental gas exchange and the cardiovascular system of the fetus.

2. 1. Surgical Procedure

We studied 24 fetuses, 1 1 in the norepinephrine (NE) group and 13 in the triiodothyronine (To) group, whose gestational ages on the day of surgery, checked radiographically, ranged between 125 and 135 days (term = 147 to 150 days).

Fort y eight hours before surgery, the ewes were taken off food, and 12 hours prior to surgery they were taken off water. The ewe was given a spinal anesthesia (2 ml of 0. 2% tetracaine HCl in 6% dextrose and 0. 2 ml of epinephrine 1/1000) and

400 mg of pentobarbital sodium as a sedative, which was supplemented during the surgery as required into the intravenous catheter, situated in the maternal jugular vein and kept open by a drip infusion (5% dextrose in lactated Ringer's solution). The ewe was placed in a supine position on the operating table, her legs restrained and eyes blindfolded. We then placed a polyvinyl catheter (1. 8 mm

OD) in a maternal pedal artery Then we opened the abdomen through a midline incision, exposed the pregnant uterine horn, and placed a polyvinyl catheter into the middle uterine vein. Through a hysterotomy incision we withdrew a fetal fore- limb and after anesthetizing the skin with 0.5 cc lidocaine (1 5 ml subcutaneously) we placed catheters into the radial artery and cephalic vein and passed the tips into the brachiocephalic artery and superior vena cava, respectively. Through a separate hysterotomy incision we withdrew the hindlimb and after administering local anesthesia we placed catheters from a pedal artery and vein into the descending aorta and inferior vena cava, respectively. We also placed an amniotic fluid catheter at the fetal cardiac level to measure the intra-amniotic pressure,

98 and electrocardiography electrodes on the fetal forelimb and hindlimb Fetal skin was closed with 4-0 chromic suture, the amniotic membranes with 2-0 silk suture, and the uterine incision was closed in two layers using 2-0 chromic suture Through a

small hysterotomy incision, carefully avoiding the rupture of the amniotic membranes, we obtained access to a tributary of the umbilical vein, in which we placed a silastic catheter (1. 5 mm OD), which was then guided into one of the main umbilical veins. Every catheter was lubricated with a polydime thylsiloxane fluid

(360 Medical Fluid, Do w Corning Corp , Midland, Ml) in order to facilitate its insertion and positioning. After closing the uterus and abdominal wall in layers, we led the catheters through a subcutaneous tunnel into a pouch attached to the ewe's flank

In the triiodothyronine group of fetuses we inserted an osmotic minipump

(Alza, Palo Alto, CA) filled with a solution of triiodothyronine in polyethelene glycol subcutaneously in the fetal axilla The osmotic mini pump has a reservoir volume of 170ul arc/ delivers solutions continuously at an average rate of 1 ul· hr for a period up to one week without the need for external interference In our study the triiodothyronine infusion rate equalled 5 9 (ig.hr . In order to establish a high initial triiodothyronine concentration, which then could be maintained by the minipump's continuous infusion, we administered a loading dose of 300 Jig of triiodothyronine by slow infusion into the fetal inferior vena cava during the surgery.

While the ewe and fetus recovered during four to six days, we kept the catheters patent by flushing them daily with a heparin solution. In addition, we administered

600,000 units of procaine penicillin G and streptomycin to the ewe and 55 mg of sodium ampicillin (Polycillin-M, Bristol Meyers Company, Syracuse, NY) into the amniotic fluid to minimize maternal and/or fetal infection. During the postopera­ tive period in the T^ group we sampled daily maternal and fetal blood to monitor

99 the changes in thyroid hormones, Cortisol and catecholamine concentrations

2 2 Experimental Procedure

2 2 1. Norepinephrine

During the experiment the ewe stood in a metabolism cart We collected arterial blood samples for baseline maternal and fetal blood gas values, and the concentrations of epinephrine, norepinephrine and dopamine We measured the Pj-jo , Ppo? anc' pH using blood gas microelectrodes (Radiometer, model BMS ЗА, The London Co ,

Westlake, OH) and replaced the sampled fetal blood with maternal blood In addition we measured the hemoglobin, oxyhemoglobin, and concentrations (CO-oximeter, Model 182, Instrumentation Laboratories, Lexington,

MA) Duplicate carboxyhemoglobin measurements typically agreed within 0 5% to

1% We calculated blood O- content from the oxyhemoglobin saturation Both pulsatile and mean pressures in the fetal descending aorta (corrected for amniotic fluid pressure) and maternal pedal artery, as well as maternal and fetal heart rates were recorded using pressure transducers (Statham Model P23M, Oxnard, CA) and a polygraph recorder (Clevite Brush Recorder, Mark 200, Clevite Co., Cleveland,

OH), during a 30-to-60 min control period In the middle of this period we measured fetal cardiac output and distribution of blood flows with two sets of radioactive label­ led microspheres

After the control period we infused norepinephrine into the fetal inferior vena cavai catheter with an infusion pump (Harvard Apparatus Co , Ine , Millis, MA) at constant rates which ranged from 1 6 to 3 4 ug of norepinephrine per min (0.4

-1.1 jjg-min · kg of fetal wt ) for 20 to 50 min This approximates the endo­ genous discharge rate from fetal adrenal glands during asphyxia (Comiine et al ,

1965) For each experiment we prepared fresh norepinephrine solutions (3 2 pg of

NE per ml in 2 5% dextrose and 0 45% NaCI) in order to minimize oxidation

100 (Barton et al. , 1974) At 5, 10, 30, and 50 min we sampled fetal and maternal

blood for respiratory gas and catecholamine concentrations We used two additional

sets of microspheres to calculate cardiac output and blood flows 15 min after the

start of norepinephrine infusion After completing the norepinephrine infusion, we

allowed the fetus to recover for 60 min or more

In 5 fetuses (which did not undergo a placental carbon monoxide diffusing capacity

measurement) we determined the norepinephrine dose-response relations. Norepine­

phrine was infused for 5 min at rates of 3.2, 5.7, 9.2, and 11. 5 ug. min . After

each infusion we waited untili all parameters had returned to control values

(usually 15 to 20 min) before we increased the infusion dose.

2.2.2. 3, 5, 3'-L-Triiodothyronine

On the day of the experiment with the ewe standing in the metabolism cart we

recorded the above-mentioned fetal and maternal cardiovascular variables. Blood

was collected from the maternal artery and uterine vein (2 ml), as well as from the

umbilical vein, fetal descending aorta and inferior vena cava (2 ml) for determi­

nation of blood gases and the concentrations of the different hormones. We replaced

the sampled fetal blood by maternal blood To measure fetal cardiac output and

distribution of blood flows associated with elevated triiodothyronine concentrations

we simultaneously injected two different labelled microspheres.

2 3. Blood Flows and Cardiac Output

We used microspheres 15 (z 1) microns in diameter and labeled with Sr, Sc,

Cr, or Ce (Nuclear Products Div., Minnesota Mining and Manufacturing Co.,

St. Paul, MN) to determine cardiac output and the distribution of blood flows

(Rudolph and Heymann, 1967; Longo et al., 1978) Each arrived in 20 ml vials containing also 10% dextran solutions and 0.05% polyoxyethylene sorbitan mono­ oleate (Tween 80) which helps to avoid aggregation and to provide for even

101 distribuHon. The total activity of each vial, according to the statement of the manufacturer, is 1 mCi. Specific activities range from 7 to 15 mCi • g . W'-.en first received, the vials are counted with appropriate geometry and lead filters to give counts near 100, 000*min . Then approximately 0.5 ml of the well mixed suspension is drawn into a 3 ml plastic syringe and the needle broken off and sealed.

This syringe is counted in the same position as the vial was counted. From the ratio of counts the activity in the syringe is determined to the nearest 0. 1 uCi.

This syringe is then the reference standard which is compared with all doses sub­ sequently drawn from the vial Small drops of suspension are then added to test tubes to obtain about 100,000 counts·min" in the well counter A large cotton ball is pushed to the bottom of the tubes, and one tube is compared to the syringe reference standard in an appropriate position above the well counter. The ratio of counts is the basis for determining the activity in that test tube. In order to obtain a good approximation to the actual number of spheres in each standard, a drop of suspension is diluted in water so that a small drop of this dilution will contain 200 to 400 spheres. A small drop is placed on a narrow plastic cover slip and the spheres are counted under a microscope. This cover slip is then placed in a test tube and counted along with the other test tube reference standard in the automatic gamma counter. For better accuracy three such cover slips are prepared and counted. The average number of counts per sphere thus obtained allows cal­ culation of the number of spheres in each reference standard. This method was devised because counts done in a hemocytometer proved to be grossly inaccurate.

After dispersing the spheres we withdrew 0. 2 ml in a 3 ml syringe to be injected into the superior vena cava, and 0. 4 ml in a 3 ml syringe to be injected into the inferior vena cava in order to inject at least 10 microspheres per fetus. We mixed the microspheres in the syringes with fresh, heparinized maternal blood (up to 3 ml) and kept those solutions mixed

102 We injected \he microspheres (wUh continuous mixing) over α 15 sec period while we simultaneously withdrew blood from the fetal ascending (AA) and descending aorta (DA) over a period of i. 25 min at the rate of 4. 8 ml· min . After injecting the microspheres, we rapidly flushed the catheters with 3 ml of heparinized maternal blood and 3 ml of heparinized physiologic saline.

At the end of each experiment we sacrificed ewe and fetus by injecting 14 ml of T-61 (N- [2-(m-methoxyphenyl-2-ethyl-butyl-(l )] -gamma-hydroxy-butyramide

80%, 4,4' -methylene-bis (cyclohyxyl-trimethyl-ammonium iode 20%, tetracaine

HCl 2% and dimenthylformamide 0. 24% in distilled water; National Laboratories

Corporation, Somerville, NJ) We towel dried and weighed the fetus and dissected carefully the organs (brain, heart, lungs, liver, gastro-intestinal tract, kidneys, adrenals, spleen and thyroid gland), as well as the placental cotyledons and amniotic membranes Each organ was weighed, ashed at 325 С for 72 hours, prepared for radioactive counting (Longo et al. , 1978), and counted in a gamma counter (Packard Auto Spectrometer, Packard Instrument Co., Inc., Downers Grove,

IL). These data weieanalyzed on a digital computer in order to calculate the quantity of microspheres per organ, which then was expressed as blood flow to that organ.

Blood flow to the fetal brain, heart, thyroid gland, and upper body was C'j!culafed as A _ spheres in organ χ 4. 8 ml. min ,,. organ sphere*, withdrawn in AA аатірі-е v ' whereas the flow to the other organs and lower body was calculated as

spheres in organ χ 4. 8 ml min ,-ч Q = spheres withdrawn in DA sample organ r r

We obtained cardiac output from the equation

С U organs \.-i)

103 and relative flow (Longo et al. , 1978) to a given organ from

Д . organ weight Q = organ ...

organre| « (4) Q Q Q ' fetal weight

Relative flow represents the fraction of cardiac output to an organ normalized to the fraction of total body weight represented by that organ. This quantity is dimension- less and tends to eliminate scatter due to weight and cardiac output variai ¡or s, providing a normalized index of distribution.

2.4. Placental Carbon Monoxide Diffusing Capacity

Following the injection of microspheres and after a period of one hour or more to allow the animal to return to control conditions, we measured placental diffusing capacity for carbon monoxide (Dp ) both during a control period in the norepine- rco phrine group and during the infusion of norepinephrine or triiodothyronine (Longo et al. , 1967, 1977) The best way of investigating the physiological placental exchange of oxygen would be sampling the inflowing and end-capillary blood within a single exchange unit Because that is still impossible, we can only measure the maternal and fetal arterial input and mixed venous outputs. These values do not represent strictly those of a single unit. Venous outflow for example consists of blood from numerous exchange units w ith different values of Ρζ-,ο, because of non-uniform distribution of maternal and fetal flows (Power et al. , 1967), non­ uniform distribution of diffusing capacity to flow, and placental oxygen consump­ tion (Longo et al., 1972). There are also vascular shunts in the maternal and fetal circulations that contribute to the uterine and umbilical veins containing a mixture of blood of various oxygen tensions. For these reasons, if one wants to study the placental diffusing capacity it is preferable to measure the carbon mono­ xide diffusing capacity instead of the oxygen diffusing capacity. For CO is not consumed by the uterine or placental tissues in appreciable amount^, and in

104 contrast to the maternal and fetal oxygen partial pressures, uneven distribution of flows does not significantly influence the amount of CO exchanged or the mean partial pressure gradient, which finds its physiological explanation in the Haldane relation: _ [НЬСО] xP Ρ u?

Μ χ [ньо2] in which P/~/~> represents the carbon monoxide partial pressure, Ρ,-,-ρ the oxygen partial pressure, HbCO the concentration of carboxyhemoglobin, HbO- the concentration of oxyhemoglobin, and M the Haldane constant (2 18 for maternal blood, and 2 16 for fetal blood). Thus P/-/-) depends upon the ratio Ρ'r\j/ HbO- which is relatively constant over the physiological range in which we perform the experiments (see fig. 1 ) so that mixing of end-capillary blood with shunted blood will have only minimal effects on the mean value of the above mentioned ratio and therefore on the capillary Pf-z-y Another reason is that the arterio-venous HbCO difference is negligible, therefore mixing of end-capillary blood with shunted blood will not change the HbCO

We administered CO to the ewe by injecting 80 to 100 ml into a 5 I plastic bag, filled with air, which was then placed around the nose and mouth of the ewe. She was allowed to breathe this mixture for two min, during which time we carefully controlled that no leaking occurred. Thereafter we placed her in a special helmet fitted around the head and neck Then a mixture of 100 parts per milion CO in air was flushed continuously through the hood at a rate of 30 bmin in an effort to maintain a steady maternal carboxyhemoglobin concentration. We monitored inspired CO concentrations with a CO analyzer (Ecolyzer, Energetics Science, Inc. ,

Elmsford, NY). Beginning at 10 min after the administration of CO and sub­ sequently at 30, 50, and 70 min, we took 2 ml of maternal arterial and uterine venous blood and 1 ml of umbilical venous, fetal arterial and venous blood. As

105 noted above we rapidly analyzed those samples for hemoglobin, oxyhemoglobin and carboxyhemoglobin concentrations

After obtaining the 70 min blood samples for the control Dp measurement in the norepinephrine group, we commenced infusion of norepinephrine into the fetal inferior vena cava and kept the ewe on the same inspired CO gas mixture for the diffusing capacity measurement during norepinephrine infusion. Again we sampled maternal and fetal blood at 10, 30, 50, and 70 min after starting the infusion.

Those samples were analyzed as detailed above. We then replacsd the vol urne of fetal blood withdrawn by an equal volume of maternal blood (taken from the ewe before starting the D- ) and determined the fetal carbon monoxide space (Longo 'co et al., 1967). We calculated Dp as (Longo et al., 1967, 1977) •co

Vco (ml) =Д[НЬС0] χ blood CO capacity χ CO "space" (5)

Dp (ml· min • torr • kg fetal weight ) = rco m| YCO ( ) (6)

time (min) χ (?(-(-> "Pf-о )(torr) х fetal wt (kg)

where V^f-. denotes the amount of CO crossing the placenta , Δ [HbCQ the change in fetal carboxyhemoglobin concentration during the period under conside­ ration expressed as a function of fetal blood CO capacity CO capacity (ml· ml ) -1 -2 equaled Hb concentration (g · 100 ml χ 1. 36 χ 10 ), CO "space" equaled the CO content of blood injected at the end of the experiment (ml) divided by the chang:hange in fetal CO content (ml· ml ), and P/-/-P^ » ""^"m squaled the mean maternal- m f to-fetal CO partial pressure difference (torr).

2.5. Hormones Determination

Serum Тд, Tg and rT-, were measured by radioimmunoassay (Golditch, 1977;

106 Parslow, 1977a _ b). The measurement of tne plasma catecholamine concentrations

was accomplished by use of a single isotope enzyme assay modified from Passon and

Peuler (Passon and Peuler, 1973), while the plasma Cortisol concentrations were

determined by the specific radioimmunoassay described by Magyar et al. (Magyar

et al., 1979).

2. 6. Statistical Anal) sis

For each variable under consideration in the norepinephrine group we performed

statistical calculations on the control measurements, the experimental measurements

and the paired differences between experimental and control values. We computed

mean values, standard deviations and standard errors of the mean (SEM), and

used the paired t-test to determine the significance of changes from control.

Because we introduced triiodothyronine into the fetus before we measured any

cardiovascular functions, we could not use a given animal as its own control.

For this reason we accumulated necessary control data from animals of the same

breed and gestational age which were treated in an identical fashion as the

experimental animals with respect to surgical preparations and post-operative care.

Again we calculated mean values, standard deviations and standard errors of the

means, and in these cases used the non-paired t-test to determine the significance of changes from control.

3 Results

3 1. Norepinephrine

3 1 1 Plasma Catecholamine Concentrations

During norepinephrine infusion (about 1 ug· min · kg ) fetal norepinephrine

concentration increased 613% from a control value of 631 (+ 91 SEM) to a plateau

of about 4500 (+ 800) pg· ml" (p< 0. 001) (see fig.3land tableE ). Incontrasi, the

107 5000л

4000-

3000·

E к

2000

Ю00- 50'

500

0J

Fïg. 31. Fetal plasma catecholamine concen­ trations during fetal norepinephrine infusion.

108 CATECHOLAMINE CONCENTRATIONS BEFORE AND DURING FETAL NOREPINEPHRINE INFUSION*

maternal fetal norepinephrine epinephrine dopamine norepinephrine epinephrine dopamine

CONTROL 1310 + 420 240 + 89 13 + 11 631 + 91 24.8 + 5.4 194 ±68

10 MIN 190 + 71 90 + 44 28 + 27 4290 + 790 16.8 + 6.9 62 + 33

30 MIN 690 + 430 168 + 66 50 + 49 4850 + 700 27 + 10 69 + 29

50 MIN 620 + 510 145 + 71 < 1 4500 + 820 34 + 14 74 + 40

values are given as mean + SEM (pg-ml ), N = 11.

Table II. pH pCOj pOj (torr) (t-огг) umb. vein umb. artery umb. vein umb. artery umb. vein umb. artery

CONTROL 7.34 + 0. ΟΓ 7.32 + 0.01 43.8 + 1.7 47.1+1.2 33.4+1.5 24.1+1.4

30 - 50 min of NE 7.35 + 0.01 7.32 + 0.01 43.3 + 0.9 47.5 + 1.8 32.9+1.8 21.3+1.5 infusion

4-5 days of 7.32 + 0.01 7.32 + 0.01 40.4+1.3 47. Í + 0.8 32.7 + 2.1 22.0 + 0.9 T3 infusion —

Table III A.

о 00 content Q , V„ 2 umb О 2 (ml.df1) umb. vein umb. artery (ml-min -kg ) (ml.min -kg" )

CONTROL 10.0 + 0.6* 5.4+1.0 177+13 8.2 + 1.1

30 - 50 min of 9.6 + 0.5 5.1+0.7 226+18 10.2+1.2 NE infusion

4-5 days of 10.0 + 0.9 5.4 + 0.5 231+14 10.5 + 0.5 Tg infusion —

Table III В. Fetal blood gas values, umbilical blood flow and fetal oxygen consumption before and during the infusion of norepinephrine or triiodothyronine into the fetal lamb. * Means + S Ε M the fetal epinephrine concentration remained essentially constant, while the dopamine concentration decreased significantly (fig.31).

During fetal norepinephrine infusion maternal plasma NE comcentration showed no increase and, in fact, decreased during the study (table Œ). For instance, 50 min after the start of the fetal norepinephrine infusion, maternal norepinephrine concentration had decreased 38 (+ 27) percent, while epinephrine and dopamine concentrations had decreased 57 (f 6) and 52 (+ 48) percent, respectively.

In two fetuses, excluded from our evaluation of norepinephrine effects because of low values of pH and oxygen tension at the start of the experiment, the control plasma norepinephrine, epinephrine, and dopamine concentrations were 2660, 750, and 390, (total = 3800 -'- 1321 ) pg-ml , values about six times greater than those in the other fetuses (p^ 0.02) During the NE infusion the norepinephrine concentrations in these latter fetuses rose to a 50 min plateau value of 10,610

(+ 70) pg-ml , twice the concentration of the other experimental animals ( p<

0.01).

3. 1.2. Fetal Oxygen Consumption

Oxygen consumption, calculated from the product of the oxygen content difference between the umbilical artery and vein, and the umbilical blood flow, rose 25%, or 2.0 (+0.79) ml-min" · kg fetal wt~ to 10. 2 (+ 1. 2) from a control value of 8. 2 (+ 1. 1 ) ml· min · kg fetal wt after 50 min of norepinephrine infusion.

This was statistically significant on the basis of the paired 't'-test (pi.0. 05). Although

Vf-ч- increased, fetal arterial as well as umbilical venous blood gas values remained essentially unchanged (table III)

3. 1.3 Placental Diffusing Capacity

In five animals the mean Dp during the control period equaled 0.49 (+ 0.05) Kco

112 FETAL BLOOD PRESSURE AND HEART RATE BEFORE AND DURING FETAL NOREPINEPHRINE INFUSION*

mean blood pressure change from control heart rate change from control (mmHg) (beats . min )

CONTROL 45 + 3. 5 184 + 8.2

5 MIN 58 + 4.8 13 + 3.7 155 + 10. 6 -29 + 8. 9

10 MIN 57 + 6.7 11 i 4. 2 166 + 10.9 -19 + 8.4

15 MIN 52 + 6.9 10 + 4. 9 167 + 11. 0 -21 i 9.4

* values are given as mean + SEM, N = 11

• p<0. 02 paired "t" test о p<0.05

Table IV ω ml · min , torr «kg fetal wt . During the NE infusion the Dp increased rco slightly but not significantly to 0. 59 (+0. 13) ml «min »torr .kg .

A paired 't'-test on all data failed to demonstrate a significant change in diffusing capacity.

3.1.4. Cardiovascular Responses

3. 1. 4. 1, Maternal

During none of the infusions of norepinephrine into the fetal inferior vena cava did we detect ony significant changes in maternal heart rate or blood pressure.

3.1.4.2. Fetal

In an effort to evaluate only the effects of norepinephrine upon the fetal cardio- vasculra system, rather than the effects of hypoxia, acidosis, or other factors, we excluded all fetuses which did not have a control descending aortic oxygen tension greater tha )( torr and a control pH of greater than 7. 32, With these requirements we observed the following changes during the first 15 min of NE infusion (table IV).

After the first 5 min the fetal mean blood pressure rose 13 (+ 4) mmHg (p<0. 02)

from 45 to 58 mmHg/an increase of about 29%. After 10 min of onfusion blood pressure decreased slightly, remaining at 11 (+4) mmHg (ρ <0, 05) above control values. By 15 min mean blood pressure although still 24% higher than the control value was not statistically different from control. After the first 5 min fetal heart rate decreased 29 (£ 9) beats «min , or 16% (p <0. 02). Thereafter the alteration in fetal heart rate was statistically insignificant.

3.1.4.3. Norepinephrine Dose-Response Relations

Figure 32 shows that mean fetal blood pressure increased with the augmented NE dosages at infusion rates less than 1 лдд^тіп «kg . Above this rate blood pressure failed to rise further. The fetal heart rate (fig. 33) changed in an inconsistent manner 114 1.0 2.0 3.0 4.0

NOREPINEPHRINE DOSE (¿ig · min"' · kg')

Fig, 32. Absolute change in fetal blood pressure as a function of increasing doses of norepinephrine administered to the fetus.

115 NOREPINEPHRINE DOSE (^g. min'.kg') 0 U8 1.6 24 12 4.0 4.! о

Fig. 33. Change in fetal heart rate with increasing doses of norepinephrine to the fetus. Closed circles (·) indicate fetuses which weighed less than 3000 g. Open circles (o) indicate fetuses which weighed 3000 g or more.

116 with the increase in norepinephrine dosage. In general, the fetal heart rate decreased 10 to 30 beats «min at all infusion rates, but in several instances the heart rate increased slightly. As noted in fig. 33, whereas fetal heart rate decreased about 31 (+1) beats, min in fetuses weighing 3 kg or more, it decreased only about 9 (+ 2) beats · min in those fetuses weighing less than 3 kg.

3. 1.4.4. Cardiac Output and Blood Flow Changes

In 7 fetuses we measured the effect of norepinephrine infusions upon cardiac output and regional blood flow. Cardiac output remained essentially constant at its control values of 538 (+ 23) ml «min »kg at all rates of norepinephrine infusion.

Norepinephrine infusion resulted in a redistribution of organ blood flows as indicated in fig. 34. Coronary blood flow increased 40% to 28. 4 from 20, 3 ml «min · kg fetal wt (p^O. 05). Pulmonary flow increased lé0% to 77.9 from 29.6 ml «min . kg (piO. 005). Furthermore, during NE infusion umbilical flow increased 27% to 225.7 from 177.4 ml. min"1, kg"1 (p <0. 05).

117 PULMONARY Q CORONARY Ó PIACEHTAL CARDIAC JIUL Q NS. N.S. p<0.025 p<0.05 ρ < O.Ol INTESTINAL

Fig. 34. Percent change in fetal cardiac output and regional blood flows following 15 min of fetus.

118 Total blood flow to the upper and lower body and gastrointestinal tract decreased

25%, 30%, and 34%, respectively. Although statistically insignificant at the p(0. 05 level, this may indicate a compensatory decrease in blood flow to non-vital tissue. No significant changes occurred in blood flow to the brain, spleen, liver

(hepatic artery), kidneys and adrenal glands.

3. 2. 3,5, 3' - L - Triiodothyronine

3. 2. 1. Changes in Thyroid Hormones

Fig. 35 shows the changes in fetal Τ,, Τ ,, and rT- concentrations over the course of the five days postoperatively. Fetal serum T_ concentration increased tenfold from a control value of 18. 4 (+ 0, 4) to a plateau of about 200 ng-dl

This was accompanied by a decrease in T. concentration from a normal value of

10. 1 (+ 0. 5) to 3. 9 (+ 1. 0) yg-dr by the fifth postoperative day.

Reverse T„ concentration decreased from 524 (+ 22. 5) to 90 (+ 1. 4) ng-dl by the fifth day.

3. 2. 2. Fetal Plasma Cortisol and Catecholamine Concentrations

Both fetal plasma Cortisol, epinephrine, norepinephrine and dopamine concentrations during the five postoperative days remained constant at values of

12. ó (+ 3. 6) ng-ml"1 , 23. 8 (+ 5. 7), 421 (+ 99), and 99 (+ 25. 8) pg-ml-1 , respectively.

3. 2. 3. Fetal Oxygen Consumption

After five days of triiodothyronine infusion fetal oxygen consumption was

28% greater than in control animals [10. 5 (+ 0. 5) vs 8. 2 (+ 1. 1 ) ml· min-1· kg"1}.

119 500 ,.

400

IO 300

8 _

200 - 6 .

100

2 . 50

Fig, 35. Fetal serum fhyroîd hormone concentrations during fetal triiodothyronine infusion.

120 Blood gas values in the umbilical vein and artery, as well as in the maternal artery and uterine vein remained the same as in the control group (table Щ).

The increased oxygen consumption was not associated with significant changes in blood hemoglobin concentrations, hematocrit, or hemoglobin oxygen affinity.

3 2. 4 Placental Carbon Monoxide Diffusing Capacity

In five animals placental CO diffusing capacity equalled 0. 53 (+ 0 02) ml" min · torr -kg , essentially the same value as reported for normal sheep fetuses (Longo and Ching, 1977).

3. 2. 5. Cardiovascular Responses

3. 2. 5. 1 Maternal

There was no significant difference in maternal blood pressure and heart rate as compared with the control animals .

3.2.5.2. Fetal

On the other hand, fetal heart rate was 29% higher [210 (+ 7)vs 163 (+ 5) -1 -ι beats- min ; p<0. 01). The mean blood pressure (47 +1.7 mmHg) was not statis­ tically different from control values.

3. 2. 5 3 Blood Flow and Cardiac Output

As shown in fig. 36 fetal cardiac output was 22% greater than control values [678

(+ 33) vs 554 (+ 16) ml· min -kg J Umbilical blood flow also was 22% higher

[231 (+ 14) vs 191 (V 8)ml· min" · kg fetal wt~ ] Blood flow was redistributed to several organs. Coronary blood flow was up 35% [27 (+ 3) vs 20 (+ η ml' min kg J and pulmonary blood flow increased 90% [84^+ 1 Π vs 44 Ç+ 7)ml· min • kg J

In contrast, blood flow to the thyroid gland was 52% less than control [0. 3(+ 0. 04) vs 0.6 ЛЬ 0. l) ml· min · kg J. The redistribution of the cardiac output to various organs was similar whether calculated as asbolute or relative flowUable V).

121 niLMONAIT Q

COIONAIY UtDIAC PLUENUl Q OUTPUT 1 Q P

THYHOID Q

Fîg. 3ί. Percent change in fetal cardiac output and regional blood flows following the administration of triiodothyronine to the fetus during four to five days.

122 Table V. Fetal Cardiac Output and Blood Flows

• -i Q.kg fetal wt Q - 100 g organ wt % Cardiac Output Relative Flew

Control 554 + 1бх CARDIAC OLTPUT T 678 + 33* 3

4 Control 191 + 8 64. t 6.1 33.4 + 1.4 1.03 + 0.10 UMBILICAL FLOW T + + 1.14 3 213 14* 77.7 + 4.4* 34.5 4.5 + 0.15

Control 44 + 7 137 + 24 7.9 + 1.0 2.41 + 0.37 PUUBNAEY FI/CW T 84 + 11* 265 + 36* 16.2 + 2.5* 3.96 + 0.64* 3

Control 0.5 6 + 0.10 219 + 27 0.11 + 0.01 4.34 + 0.32 1HYROIDAL FLOW m 0.2 7 + 0.04* 129 + 32* 0.06 + 0.02 2.7 + 1.2* хз

Control 20 + 1.3 266 + 20 3.5 + 0.2 4.69 + 0.32 CDRTNARy FLOW τ, 27 + 2.6* 344 + 25* 4.3 + 0.3* 5.12 + 0.29 t all values reported are means +SH4 • -1 Q blood flow expressed in шітпіп

* significant change frem control using non-paired 't'-test Ρ < 0.05 3.2.5.4. Histological Examination of the Fetal Lung

Using light microscopy we examined the lungs of fetuses treated with T^ and compared them with lungs fron non-treated animals. The lungs from the control animals showed thick alveolar septa with prominent alveolar epithelial cells encroaching sightly on the alveolar space; i. e. the alveolar space was relatively small. Lungs from the animals treated with Tg showed further maturation, their alveolar septa and epithelial cells were inconspicuous, and air spaces were wider

124 IV Discussion

1. Fetal Oxygen Consumption

1.1. Previous Estimates of Fetal Oxygen Consumption

During the past four decades several different techniques and species have been used to measure fetal oxygen consumption (Table VI). Following the first reports by Barcroft and coworkers (1934, 1939, 1946), the values of fetal oxygen consumption measured in acutely anesthetized and operated animals averaged 4 ml min kg of fetal wt . With the introduction of the chronically catheterized sheep preparation by Meschia, Barron and their colleagues (1965), the reported values for fetal oxygen consumption increases sharply, plateauing at the present figure of about 8 ml min kg (Fig. 37).

1.2. Physiologic Implications

In an attempt to increase fetal oxygen consumption and to determine the change, if any, in oxygen transfer across the placenta, we administered norepi­ nephrine and triiodothyronine to the fetus. In other words, we attempted to alter placental oxygen transfer from being limited by blood flow, as is normal, to being limited by diffusion (Longo and Power, 1969; Longo et al., 1967) in order to explore wether the placenta possesses a "reserve" respiratory function. This is somewhat analogous to the adult exercising at high altitude, a condition under which pulmonary oxygen exchange becomes more limited by diffusion. Our results indicate that overall felal oxygen consumption increased 25% to 30% in response to norepinephrine or triiodothyronine. However, this bears further examination in the T^ group. At the end of the experiment we determined the oxyhemoglobin saturation curve for fetal blood. This revealed a Prr. of 19 torr, essentially the same value as reported for normal sheep fetuses of the same age. That T« did not change the fetal Pen could have been expected. The influence upon the blood oxygen affinity of T, is mediated through its effect upon the red cell 2, 3-diphosphoglycerate

125 IN Ч-^IN: SPECIES GEST. AGE METHOD PREPARATION umb '· REFERENCE uterus + fetus content

GOAT 1 cardiometer acute 150* 2.5* Barcroft et al., 1934 SHEEP 1 ligation of acute 110 4.3 Barcroft et al., 1939 umb. cord SHEEP 1 assumption: acute 151 7.7 Barcroft & Eisden, 1946 Qumb=î card· outP· HUMAN 1 nitrous oxide acute 5.0 Romney et al., 1955 SHEEP 0. 5-0. 6 venous occlusion acute 129 4.0 Acheson et al., 1957 0. 8-0. 9 Plethysmograph + acute 92 3.7 flowmeter 1 Plethysmograph + acute 76 3.8 flowmeter HUMAN 0. 2-0. 7 electromagnetic acute 110 4.0 Assali et al., 1960 flowmeter GOAT 0. 5-0. 9 acute 10.1 Huckabee et al., 1961 SHEEP 0. 5-0. 6 occlusion of the acute 217 5.4 Dawes & Mott, 1 964 1 umb. cord acute 170 4.6 SHEEP 0. 5-0. 9 antipyrine chronic 199 6.9 Meschia et al., 1965 SHEEP 0. 7-0. 9 antipyrine chronic 233 7.1 Meschia et al., 1966 GOAT MONKEY 0.9 tritiated H2O / acute 115 5.4 Parer & Behrman, 1 967 antipyrine MONKEY 0.9 antipyrine acute 7.0 Parer et al., 1968 SHEEP 0.6-1 antipyrine chronic 154 5.7 Battaglia et al., 1969 SHEEP 0.6-1 antipyrine acute 231 8.4 Chrenshaw et al., 1968 0.7-1 antipyrine chronic 302 8.5 SHEEP 1 electromognetic acute 169 4.5 Kirschbaum et al., 1969 flowmeter GOAT <0.4 antipyrine acute 4.5 Cotter et al., 1969 >0.4 antipyrine acute 8.1 MONKEY 1 antipyrine acute 212 7.7 Behrman et al., 1969 GUINEA PIG 1 occlusion uterine acute vessels + matern. 9 Moll et al., 1970 oxygen consumpt. SHEEP occlusion uterine acute vessels + matern. 9.4 Clapp et al., 1971 oxygen consumpt. GUINEA PIG occlusion uterine acute 8.0 vessels + matern. Kunzel & Moli, 1972 oxygen consumpt. SHEEP Q . · O^content chronic 9.2 uterus ¿ Clapp et al., 1972 SHEEP 0. 8-0. 9 antipyrine chronic 175 6.0 James et al., 1972 SHEEP <0.4 antipyrine chronic 5.4 Huckabee et al., 1972 >0. 6 antipyrine chronic 10.4 SHEEP 0. 8-0. 9 electromagnetic chronic 217 7.9 Parer, 1978 flowmeter SHEEP 0. 8-0. 9 microspheres chronic 177 8.2 Lorijn (thesis, 1980)

Table VI. Historical review of measurements of uterine blood flow and/or oxygen consumption. Gestational age is expressed as fraction of total gestation. * ml-min · kg of fetal weight 6

5-

3-

2-

Ol— 1930 то 1950 1960 1970 1980 YEARS

Fig. 37. Values of fetal oxygen consumption reported during the past several decades. Triangles (A) indicate sheep and goat values, open circles (o) human, closed (·) circles monkey, and squares (•) guinea pig data.

128 100 FRACTION TRANSIT TIME

Fig. 38. Time course of change ri maternal and fetal О- tensions during a single transit in placental exchange vessels during fetal norepinephrine infusion. The arrow indicates the mean maternal-to-fetal O« tension difference.

129 (Gahlenbeck and Bartels, 1968; Gahlenbeck et al , 1968/1969; Miller et al. ,

1970; Mills et al. , 1966; Snijder and Reddy, 1970 a + b). Fetal hemoglobin does

not bind significant amounts of 2, 3-DPG and cannot increase its level in response

to hypoxia (Bauer et al., 1968; De Verdier and Garby, 1969; Tyuma and Shimizu,

1969). Thus, even if fetal 2, 3-DPG concentrations incrsased with the Tq infusion,

this would not be expected to alter fetal hemoglobin O« affinity.

In an attempt, to explore the physiologic effects of the increased fetal oxygen

consumption and increased blood flows observed in these studies on placental O«

exchange we employed a mathematical model (Hill et al. , 1972). Using values

for placental oxygen flux, umbilical blood flows, and maternal and fetal arterial

blood gases observed in this study, we calculated the extent of equilibration of oxygen tensions in maternal and fetal blood during the course of a transit in placental exchange vessels. These calculations (ПдзЯ) predicted that maternal and fetal end-capillary oxygen tensions equilibrated at about 34 torr and the mean maternal-to-fetal oxygen tension difference equaled 8 torr, essentially the same as under normal control conditions (Longo et al., 1972). That this occurred despite a 25% increase in transplacental O- flux and a 20% decrease in capillary transit time (to 1.3 from 1.7 sec assuming no change in capillary volume) suggests that at this level of increased oxygen consumption in the healthy fetal-placental unit, the placental reserve for oxygen diffusion exceeded the oxygen requirements. Of course this does not determine the extent to which fetal oxygen consumption would have to increase before the placental reserve is exceeded We have calculated that the oxygen consumption probably must increase 80% to 100% before placental reserves would be exceeded, but this must be tested experimentally.

1.3. Clinical Implications

During fetal hypoxia or asphyxia a particularly paradoxical situation could arise where oxygen consumption is actually increased. This is because norepinephrine

130 release (Comiine and Silver, 1972; Jones, 1977; Jones and Robinson, 1975) would sHmulate metabolism, resulting in a higher demand for oxygen, thus aggravating the hypoxic state and stressing placental oxygen transfer. Even during labor catecho­ lamine release (Lagercrantz and Bistoletti, 1977) could increase the rate of fetal oxygen consumption, stressing the placenta's capability for oxygen diffusion.

Our results indicate, however that in the healthy fetal placental unit additional oxygen can diffuse without an alteration of fetal blood gas values.

Fetal oxygen consumption also can be increased secondary to a rise in fetal serum thyroid hormone concentrations as may occur during fetal thyrotoxicosis

(Dirmikis and Munro, 1975; Petersen and Serup, 1977; Ramsay, 1976; Robinson et al., 1979; Samuel et al. , 1971) or during the intrauterine treatment of hypo­ thyroid fetuses (Fisher, 1975; Lightner et al., 1977; Riddick et al., 1979; Van

Herle et al. , 1975). ^gain, under these circumstancesenough oxygen can diffuse across the placenta to meet the fetal demands without altering fetal blood gas values.

2 Norepinephrine Experiments

2 1 Catecholamine Concentration in the Mother and Fetus

Our finding that catecholamine concentrations failed to rise in the ewes agrees with the data of Cession (1971) and Jones and Robinson (1975) who respectively infused radioactive labeled norepinephrine into the mother and fetus. In fact, in our studies maternal catecholamine concentration decreased This may indicate that despite precautions taken, the ewes were excited or stressed and not in a truly basal state when we started the experiment, due to the experimental procedure, but became so as the experiment proceeded. Although some catecholamine may exchange between mother and fetus, they probably would be rapidly taken up by the maternal erythrocytes (Bain et al., 1937; Roston, 1967). This would be compa­ tible with Rankin and Phernetfon's (1976) finding that subsequeni to a single injection of norepinephrine into the fetal lamb (27 to 120 ug-kg fetal wt" )r . uterine blood flow decreased, although blood flow to other maternal organs remained constant. Because we did not measure uterine venous catecholamine concentrations we cannot confirm this statement.

The catecholamine concentrations in our fetuses with the norepinephrine infusions are in the range reported by Jones and Robinson (1975) and Jones (1977) during fetal hypoxia, suggesting that we did not exceed physiologic concentrations, and, in fact, we stimulated the fetus as may occur during intrauterine life.

Jones and Robinson (1975) and Jones (1977) reported that total plasma catecho­ lamine concentrations in the chronically catheter!zed sheep, determined by using a modified trihydroxyindol method (Haggendal, 1963), equalled less than 50 pg-ml in the ewe and less than 100 pg· ml in the fetus. Using a radioimmunoassay technique we noted substantially higher catecholamine concentrations in both fetus and adult (table II ). In all fetuses the norepinephrine concentrations exceeded that of epinephrine ten-to twenty-fold, while in the ewes this difference was only three- to four - fold. These concentration differences agree with the findings of

Jones and Robinson (1975), who also reported four "stressed" fetuses with low values of arterial pH and oxygen tension, two of which had resting total plasma catechola­ mine concentrations about 1500 pg. ml , similar to concentrations noted in our two stressed fetuses.

2. 2. Cardiova^cjlar Responses and Results of Previous Studies in the Fetus and

Newborn.

Following Barcroft and Barron (1945) many authors have infused norepinephrine in sheep fetuses (Dawes et al., 1956; Pardi et al., 1969; Barrett et al., 1972;

Comline and Silver, 1972; Joelsson et al., 1972; Novy et al., 1974; Rankin and

Phernetton, 1976; Assali et al., 1977) and lamb and human newborns (Adams et al.,

1958; Scopes and Tizard, 1963; Karlberg et al. , 1965). All noticed an increase in arterial blood pressure ranging between 4% and 75%. Although in some

132 instances the heart rate increased (Dawes et al. , 1956; Assali et al. , 1977), in others it decreased (Dawes et al. , 1956; Adams et al., 1958; Karlberg et al., 1965;

Pardi et al., 1969; Comline and Silver, 1972) or did not change at all (Pardi et al. ,

1969; Barret et al., 1972, Novy et al., 1974).

Comparison of our data with those of the above mentioned authors may course confusion. Comline and Silver (1972), Joelsson et al. , (1972) and Jones and

Robinson (1975), for example, are the only workers who studied chronically catheterized sheep preparations. The others exployed acute experiments or dealt with newborn mammals. Several workers have reported significant alterations in cardiovascular hemodynamics in the acutely operated anesthetized and perhaps exteriorized sheep fetus (Heymann and Rudolph, 1967), and this well may account for the variations in response by different authors. Another major difference is the dose of norepinephrine used; these concentrations ranged from 0. 1 (Assali et al. ,

1977) to 120 Ugikg (Rankin and Phernetton, 1976) In the low dosage range

(Dawes et al., 1956; Novy et al., 1974; Assali et al., 1977) fetal heart rate did not change or increased When high doses were used (Rankin and Phernetton, 1976;

Barrett et al. , 1972) the heart rate decreased and peripheral vascular resistance increased significantk. Finally, the manner of drug administration varied. Earlier studies tended to use single dose intravenous injections while later studies employed intravenous infusions.

Our dose response data indicate that the influence of norepinephrine upon the heart rate probably is age dependent. In fetuses weighing more than 3000 g the heart rate decreased about 20%, while in less mature fetuses it decreased only about

7% (Fig. 33).

In all fetuses blood pressure at dosages greater than 1 jig· min · kg increased about 52%, suggesting that the alpha-receptor response does not change over the maturational range represented by these animals. This finding agrees with the results of Pardi et al. (1969) and Barrett et al (1972)who did not detect an age-dependent

133 response to the fetal cardiovascular system to NE

In contrast to the effect in the adult, norepinephrine infusion resulted in increased blood flow to the fetal lungs, a finding also reported by others (Assali et al. , 1977; Barrett et al., 1972). Our observation that umbilical blood flow increased 27% agrees with that of Novy et al. , (1974) and Dawes et al. (1956) but contrasts with the decrease in umbilical and renal flows reported by Rankin and

Phernetton (1976) and Barrett et al. (1972). Our finding that brain blood flow was not altered by norepinephrine infusion agrees with the results of O'Neill and

Traystman (1976) in their experiments in adult dogs, and as described by McCalden

(1977) (due to the existence of a protective mechanism in the brain which reduces the concentration of norepinephrine in the tunica media below the threshold for alpha adrenoreceptor stimulation). Our finding that blood flow to the lower extremities remained essentially unchanged contrasts with the studies which show that it increased (Assali et al. , 1962) or decreased (Dawes et al. , 1968).

Our finding that cardiac output remained unchanged contrasts with that of

Barrett et al. (1972), who reported a 22. 5% decrease of the combined ventricular output in acute studies. As indicated by Gilbert (1979) fetal cardiac output is, during the normal, non-stressed situation, already almost maximal. Decreases in fetal aiterial 0? tensions and pH as during anesthesia and surgery, can stress fetal responses, whereafter an additional stress, such as catecholamine infusion decreases cardiac output. That even low norepinephrine dosages influence the fetal heart was established by Friedman (1972), who measured the dose-response curves of the fetal and adult ventricular myocardium to NE in vitro. The fetal myocardium has a much lower threshold to the inotropic effects of norepinephrine and is three­ fold more sensitive to norepinephrine as compared to the adult.

3. 3,5,3' -L-Triiodothyronine Experiments

134 3. 1 General Considerations

There is accumulating evidence that in a quantitative sense triiodothyronine plays a more important role in the adult, than its relatively low blood concentration would suggest (Naumam et al , 1967; Sterling et al , 1969; Oppenheimer, 1979).

Following the administration of 300 llg of T^ orally to normal human subjects for two weeks the metabolic rate remained constant for 6 to 12 hours, then increased progressively between 17 and 35% (Mills et al. , 1966) The metabolic rate in rats increased about 75% after 10 days of daily injections of 20 ))g of T^ (Gahlenbeck et al., 1968). The increase in oxygen comsumption occurs in almost every tissue except the brain, spleen, and testes (Barker and Klitgaard, 1952). Triiodothyronine also failed to increase the oxygen consumption in the neonatal rat's brain, despite the use of multiple dosage regimens (Schwartz and Oppenheimer, 1978). Because many of the well established T^ induced changes in rat brain (such as the increase in To receptor sites (Schwartz and Oppenheimer, 1978), and the rise in nerve growth factor concentration (Walker et al. , 1979) occur within the first weeks after birth, it is reasonable to assume that the neonatal brain is sensitive to T-j, and that oxygei consumption is not an appropriate indicator for measuring thyroid action in this tissue (Oppenheimer, 1979).

The molecular mechanisms responsible for the cellular effects of the thyroid hormones are unknown (Tata, 1974; Oppenheimer, 1979) The known actions include direct stimulation of the mitochondrial respiration as well as stimulation of nucleic acid and synthesis (Sterling et al. , 1977, 1978). Further possible routes of thyroid hormones action include effects upon the plasma membrane (Gold- fine et al. , 1975 a and b), and incorporation of the hormone molecule into tyrosine sites in (Dratman, 1974). The activation of mitochondrial energy metabolism would seem a reasonable candidate for the first effects observable, such as increased oxygen consumption soon after the administration of T^. However, a large portion of this induced rise in oxygen consumption results from stimulation of the

135 sodium pump, which in turn is caused by the hormonal induction of the membrane-

linked sodium and potassium-dependent adenosinetriphosphatase.

3. 2 Thyroid Development and Effects upon the Fetus

The ontogenesis of the fetal thyroid gland and the maturation of the fetal

hypothalamic-pituitary control of the fetal thyroid gland are well established.

The human fetal thyroid appears as a midline outpouching of the ventral buccal entoderm, visible by 16 to 17 days of gestation. The ultimobranchial portions of the fourth pharyngeal pouches will form the calcitonin secreting parafollicular

С cells (a plasma calcium lowering hormone). The anterior pituitary gland develops in parallel and is capable of synthesizing thyroid stimulating hormone (TSH) by 10 to 12 weeks. However maturation of the hypothalamic-pituitary control of the fetal thyroid gland begins at midgestation (Fisher and Dussault, 1974).

Kinetic studies reveal that although the fetal T, production exceeds maternal production 6 to 8 fold, the fetus develops in a state of relative T^ deficiency with high levels of rTo (Dussault et al. , 1972; Fisher et al. , 1972; Eren' erg and Fisher,

1973; Fisher, 1975). This rT« is derived from extrathyroidal deiodination of L· at the 5 positions on the tyrosyl ring (3,3' ,5' -T^), rather than as in the healthy adult where monodeiodination generally occurs at the 5' position on the phenolic ring

to yield 3, 5, 3' -T3 (Chopra et al. , 1975 a and b).

The sequelae of an insufficient amount of thyroid hormones during development includes prolonged gestation, and retarded growth and maturation of the whole fetus, its central nervous system, lungs, and other organ systems (Hopkins and Thornburn,

1972; Thornburn and Hopkins, 1973; Nathanielsz, 1976)

3. 3. Changes in Maternal and Fetal Hormone Concentrations

Fetal triiodothyronine concentrations increased dramatically following five days

136 MATERNAL FETAL

χ ± χ 1 T4(pg.dl ) T3(ng.dl "•) rT3(ng.dl T^C/ig.dl ) T3(ng.dl ) rT3(ng.dl" ) CONTROL 4.6 + 0.5# 89.9 + 13.0 103 +9.3 10.1 + 0.5 18.4 + 0.4 5~2"4.1 + 2 2.5

DAY 1 3.5 + 0.6 42.4 + 13.2 19 6.3+4.) 5.9 + 1.1 155.2 + 19.1 486.0 +24.7

DAY 2 5.2 + 0. 13.3 + 11.0 93.2 +13.5 4.7 + 0.6 200.0 + 28.1; 273.6 + 21.9

DAY 3 6.6 + 0.7 * 105.0+ 14.4 92.5 +15.6 4.2 + 0.7 197.6 + 20.4* 175.0 + 27.2

DAY 4 7.1 + 0.6 ' 116.8+ 18.9 81.6 +8.1 3.7 + 0.7 190.0 + 20.1 167.7 + 19.3

DAY 5 6.8 + 0.9* 142.3+ 11.1 75.3 +13.3 3.9 + 1.0 205.0 + 25.1 90.0 + 1.4'

Table VII. Serum fhyroid concentrations before and after fetal Τ- infusion. •7^ mean + s. e. m. о ρ 0.05 * ρ 0.001

ω vi of To infusion to levels similar to those of the newborn infant 24 hrs after birth

(Ehrenberg et al. , 1974). Because of the concomitant decrease in both J. and

rTo (fig.iS) we assume that the fetal TSH levels decreased, in agreement with

the concepts of the autonomic control of the fetal hypothalamic-pituitary-thyroid

axis. (Fisher and Dussault, 1974).

As shown in table 7,maternal Tq decreased 53%, and rTo increased 90% by the

first day postoperatively. M aternal T, also decreased, but this was not statis­

tically significant. These maternal hormone changes agree with the report by Burr

and co-workers (1975) on the alterations in thyroid hormone concentrations after

surgery in humans The maternal concentrations returned to normal by the second

postoperative day; and thereafter, To increased above and rT« decreased below

control (p^ 0. 05), perhaps indicating that T, crossed the placenta from the fetus

to the mother

Cortisol, which is known to affect fetal lung maturation (Buckingham et al. ,

1968; De Lemos et al., 1970; Kotas and Avery, 1971; Liggins and Howie, 1973),

was also measured during the recovery period. The finding that during the To

infusion fetal plasma Cortisol did not change, but that the maturation of the fetal

lungs increased supports the findings of others that thyroid hormones affect pulmonary

development and that this process is not mediated through the corticosteroids

(Abbassi et al., 1977; Gross et al., 1979; Lindenberg et al., 1978; Mashiach et al.,

1978; Redding et al., 1972; Wu et al., 1971, 1973).

Wu et al. (1971, 1973) first reported an accelerated lung maturation in fetal

rabbits injected directly and intraamniotically witH to 2 ))g of Ti, while Redding

and co-workers (1972) reported augmented storage, production, and secretion of

lung surfactant in adult rats following thyroid hormone administration. Similar findings are noted in prematurely delivered infants who received thyroid hormone

(Abbassi et al. , 1977; Mashiach et al., 1978). In agreement with this concept

is the finding of saturable, high affinity binding of To by nuclei from rabbit lung

138 tissue and by cell lines originating from type II alveolar cells (Lindenberg et al.,

1978).

3.4. Fetal Cardiovascular Responses

In these studies we noted only a 22% increase in fetal cardiac output as compared to a 100% rise during the first week of extrauterine life, despite the fact that we increased fetal plasma T., concentrations tenfold. This seems to suggest that besides the low T« concentrations in the fetus, other factors (perhaps the low

O« tension) counteract a greater increase in fetal cardiac output. The 22% greater umbilical blood flow and 29% higher heart rate with an unchanged arterial blood pressure indicates a decreased umbilical vascular resistance and agrees with

Rudolph's (1976) finding that there is a linear relationship between heart rate and umbilical blood flow. Perhaps the most striking change was the markedly increased pulmonary blood flow, as compared with control, and one could speculate that increased pulmonary flow is a factor in accelerating lung maturation.

The only significant decrease in organ blood flow noted in these studies occured in the thyroid gland. This suggests a depression of its function due to the elevated

Tq levels, in agreement with depressed T, and reverse T^ concentrations.

4. Comparison of Norepinephrine and 3, 5, 3'-L-Triiodothyronine Results

One of the questions to be raised is whether the effects noticed during the TQ infusion might, in fact, have stimulated fetal responsiveness to catecholamines. In the adult some of the manifestations of clinical thyrotoxicosis resemble excessive adrenal medullary activity. Moreover, the beta adrenergic blocking agent proprano­ lol, has been succesfully employed in the management of some manifestations of thyrotoxicosis (Sterling and Hoffenberg, 1971). Recently several authors have reported increased numbers of beta adrenergic receptor sites in the cardiac

139 plasma membranes of experimental hyperthyroid rafs, as well as in vitro cultured myocardial cells from neonatal rats (Banerjee and Kung, 1977; Ciaraldi and

Marinetti, 1977; Williams et al., 1977; Tsai and Chen, 1977) However, as indicated by Sterling (1979) it is far from certain that an increase in number of beta receptors would actually account for the sympathomimetic effects associated with clinical thyrotoxicosis in view of the close relation between presynaptic and postsynaptic neurons in their physiologic function.

A comparison of the cardiovascular responses in the To animals with those in the norepinephrine fetuses reveals striking similarities. Coronary, and umbilical blood flow increased equally in both groups, while the augmentation in pulmonary flow was greater in the norepinephrine group (160%) than in the T« group (90%).

However, the higher cardiac output seen in the T-, group did not occur during the norepinephrine infusion. This may indicate that thyroid hormones are one of the few factors which can increase fetal cardiac output and may be one of the factors causing the increase in cardiac output during the neonatal period.

That the increase in oxygen consumption was similar in both groups is of interest.

In both groups of animals this increase was achieved by an increase in umbilical blood flow rather than a change in umbilical vein to artery oxygen content difference.

Fisher (1976) noted in the newborn's first week of life a correlation of the neonatal hyperthyroidism with augmented secretion and an increased sensitivity to exogenous norepinephrine thermogenesis. Malbon et al. (1978) recently proposed an alternative explanation that involves beta adrenergic receptors. These authors inferred that thyroid hormones exert their influence on fat cells by regulating the transduction of information between hormone receptors and adenylate cyclase. In other words, thyroid hormones may modulate the amplification of the adrenergic signal at the level of the cell membrane. In this context, fat cells from hypothyroid individuals show a decreased lipolytic response to norepinephrine in vitro, an effect due to exchanged alpha adrenergic responsiveness (Rosenqvist, 1972; Grill and Rosenqvist, 1973).

140 V SUMMARY

In order to understand the physiologic factors which affect placental oxygen exchange, their interactions, dynamics, and specific control mechanisms the first part of the thesis attempted to synthesize into a coherent whole many of the isolated findings reported during the past decade or so. Despite about a dozen studies from reputable laboratories, the precise alteration of a pregnant woman's blood oxygen affinity, if any, remain to be determined. Only recently has it been appreciated how many factors influence blood oxygen affinity and most of these have been examined only superficially, if at all, during gestation. During the course of gestation the maternal hemoglobin concentration progressi­ vely decreases, due to the hemodilution of a relative greater increase in plasma volume as compared with red cell mass, to about 11.5 g«dl , with an oxygen capacity of 15. 7 ml-dl It is well established that fetal blood has a higher oxygen affinity than mater­ nal blood in humans as well as many other species. The progressive increase in Prn during the course of gestation presumably results from an increase in the con­ centrations of HgA relative to that of HgF. The mechanism responsible for the increased oxygen affinity of fetal blood in humans is the weak interaction of 2, 3-DPG and fetal hemoglobin, while in other species (e.g. goats and rhesus monkeys) the fetal hemoglobin possesses a high oxygen affinity per se. Despite the attention given to the importance of fetal hemoglobin, this hemo­ globin type is not required to survive intrauterine life in all species (e.g. beagle dog, horse, and pig). Although generally it is realized that the blood oxygen capacity of the near- term fetus exceeds that of the mother, it is less commonly appreciated that during much of fetal life the reverse situation holds. As a result from the combination of high oxygen capacity and oxygen affinity of fetal blood, fetal arterial blood oxygen content and oxyhemoglobin saturation are not much lower than the adult, despite an oxygen tension in fetal blood that is only one-fifth to one-fourth that of the adult. In-vivo as blood flows through the placental exchange capillaries the maternal and fetal oxyhemoglobin satura­ tion curves may be almost superimposed. The results of theoretical studies using a mathematical model suggest that placental oxygen transfer should be particularly sensitive to changes in maternal or fetal arterial oxygen tension. These predictions agree fairly well with the

141 experimental results. Both the oxygen transfer rate and outflowing oxygen tension show marked sensitivity to small changes in umbilical arterial oxygen tension with the responses occurring in opposite direction. At high altitude various physiologic mechanisms operate to maintain placental oxygen exchange at a rate surprisingly close to that of sea level. The reproductive performance of women with cyanotic heart disease is inversely proportional to their hematocrit and hemoglobin concentrations, undoubtedly reflecting compromised fetal oxygenation. A decrease in maternal blood oxygen affinity calculated with the mathematical model lowers the rate of oxygen exchange and the oxygen tension in maternal and fetal end-capillary vessels. A decrease in Peg displays the opposite effect. However, raising fetal Pen lowers the oxygen transfer rate only slightly. In contrast decreasing fetal Ρ,-η would significantly decrease the oxygen exchange rate, while the end-capillary oxygen tension would rise significantly. Decreasing fetal blood oxygen affinity by intrauterine transfusion confirmed these predictions. The dogma that it is the difference in position of the maternal and fetal oxyhemoglobin saturation curves that results in placental oxygen diffusion has been refuted both by theoretical consideration as well as by clinical evidence. Following Longo et al. (1972) and Makowski (1973) the term "hemoglobin flow" was introduced, because it is the product of flow and oxygen content which defines the amount of oxygen transported. At high maternal hemoglobin flow rates the mathematical model showed that the oxygen tension of end-capillary blood approaches that of the maternal artery. At lower flow rates the equilibrated oxygen tension decreases because less oxygen is available for exchange. Increases in fetal flow allow additional oxygen transport and the end-capillary oxygen tension will decrease to approach umbilical arterial oxygen tension and decreasing fetal flow will increase the end-capillary oxygen tension. These theoretical considerations predict that the normal placental oxygen exchange rate cannot be maintained in the face of flow decreases without compensatory changes.

Several studies suggest that the distribution of maternal and fetal placental flows is nonuniform, like the distribution of placental diffusing capacity to blood flows. Nonuniform distribution of those flows results in equilibration of end-capillaries at various oxygen tensions, which vary as a function of the degree of nonuniformity. At present there is no method to experimentally assess the importance of the nonuniformity in placental diffusing capacity to blood

142 flows distribution in limiting transplacental oxygen exchange. The effect of "sluice" flow in the placental vessels on oxygen exchange was also discussed. Finally the seemingly conflicting requirements of mechanical efficiency for fetal expulsion and minimal interference with oxygen exchange was analyzed. In the experimental part of this study the effects of norepinephrine and triiodothyronine upon the chronically catheterized fetal lamb were measured. Norepinephrine infusion into the fetus, reaching levels seen during stress or asphyxia, can be handled by the fetus without changing its pH, or oxygen or carbon dioxide tensions. Although fetal oxygen consumption increases 25% the fetus does not increase its cardiac output, but redistributes only the flow to the placenta in order to meet the demands. The effects upon fetal heart rate depend not only upon the experimental preparation and the way of administration, but also upon the fetal maturity.

Continuous infusion of triiodol'hyronine into the fetal lamb over a five day period induces a state comparable to hyperthyroidism with an increase in lung maturation, but without changing fetal blood pH, or oxygen or carbon dioxide tensions. Triiodothyronine infusion resulted in fetal cardiac output being 22% greater than in control animals, a small rise in comparison to the doubling which occurs during the neonatal period. Blood pressure was unchanged and the umbilical vascular resistance was lowered, causing the umbilical flow to rise. A redistribution of blood flow occurred to the fetal lung and heart and from the thyroid gland. Although oxygen consumption was somewhat greater than in control animals, the placental reserve for oxygen diffusion exceeded the oxygen require­ ments and no alterations occured in the diffusing characteristics of the placenta. This may also be of clinical importance in understanding the problem occurring during fetal thyrotoxicosis in women with hyperthyroidism or mothers who have been treated for this disease and during the intrauterine treatment of hypothyroid fetuses.

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171

Curriculum vitae

3 - 10 - 1951 Geboren te 's Gravenhage. 1969 Eindexamen HBS-b aan het Thomas More College te Oudenbosch 1974 - 1975 Onderzoek naar de mogelijkheid tot quantitatieve standarisatie van intra-uteriene drukmeting bij niet zwangere vrouwen, uitge­ voerd in de Universiteitskliniek voor Obstetrie en Gynaecologie van het St. Radboudziekenhuis te Nijmegen (hoofden: prof. dr T. K. A. B. Eskes en prof. dr J. L. Mastboom). 1975 Doctoraalexamen geneeskunde aan de Katholieke Universiteit Nijmegen. 1977 Artsexamen 1977 - 1979 Research fellowship aan de afd. Perinatale Biologie van Lorna Linda University, Medical School, Loma Linda, Californie, U.S.A., о. I.v. prof. L. D. Longo, Prof. G.G. Power en dr R.D. Gilbert. 1979 Aanvang opleiding tot vrouwenarts in de Universiteitskliniek voor Obstetrie en Gynaecologie van het St. Radboudziekenhuis te Nijmegen (hoofden: prof. dr T. K. A. B. Eskes en prof. dr J. L. Mastboom. 173

Stellingen behorend bij

FETAL OXYGEN CONSUMPTION

AND

PLACENTAL OXYGEN EXCHANGE

Nijmegen, 9 mei 1980 R.H.W. Lorijn STELLINGEN

I

De verandering in de zuurstofaffiniteit van het bloed bij zwangeren, zo deze al mocht optreden, is nog onbekend (dit proefschrift).

II

De aanwezigheid van haemoglobine F is niet absoluut noodzal

Ml

Het geven van extra zuurstof aan een gezonde vrouw tijdens de baring, teneinde een betere oxygenatie van de foetus te bewerkstelligen, is niet gebaseerd op kennis omtrent de fysiologie van het zuurstof trans­ port over de placenta (dit proefschrift).

IV

Indien de foetale zuurstofconsumptie is verhoogd, zoals bijvoorbeeld tijdens de baring, kan in de gezonde foeto-placentaire eenheid additioneel zuurstof worden uitgewisseld, teneinde aan de vraag te voldoen, zonder dat hierdoor de foetale bloedgaswaarden veranderen

(dit proefschrift).

V

Schildklierhormoon stimuleert de longrijping van de foetus, hetgeen onafhankelijk van corticosteroiden geschiedt (dit proefschrift). VI

Een van de gevolgen der onbeperkte, vrije verkoop van een groot aantal analgetica is, dat de omzetstijging van de farmaceutische industrie evenredig is aan de onkosten van de medische consumptie.

VII

De vermelding "D. C. P. " op een recept impliceert niet alleen dat bij gebruik van het medicijn de rijvaardigheid ten nadele beïn­ vloed wordt, doch evenzeer de besluitvaardigheid tijdens de uit­ oefening van het beroep.

VIM

Alleen door middel van periodieke examens kan er meer zekerheid worden verkregen omtrent de actuele kennis en het "up-to-date" zijn van medici.

IX

Tenzij duidelijk geïndiceerd, dient uiterste terughoudendheid betoont te worden bij het voorschrijven tijdens de graviditeit van benzodiazepines en phenothiazines, m. n. diazepam en chloor- diazepoxide.

X

Experimenten dienen reproduceerbaar te zijn. Alle behoren op dezelfde wijze te mislukken.

XI

De circulus vitiosus, het perpetuum mobile en het axioma zijn erkenningen van de beperkfheid van het menselijk kenvermogen

(Willem Loeb).

XII

Het met de helm op geboren worden biedt geen bescherming tegen

verkeerstrauma's.

XIII

Research: "We dance 'round in a ring and suppose

The secret sits in the middle and knows"

(after R. Frost: The Secret).