9 Encounters with the

Thomas C. Lloyd, Jr., M.D.

Professor, Departments of Medicine, Physiology and Biophysics, Indiana University School of Medicine, Indi­ anapolis, Indiana

"You don't really believe that hyp­ apply physical principles to gain an un­ oxia causes pulmonary vasoconstriction, derstanding of natural phenomena. She do you?" That question was put to me by recognized my curiosity and encouraged C. J. Lambertsen while I was a medical my interest with frequent, often demand­ student at the University of Pennsylvania . ing, challenges. Unanswered and forgotten at the time, it As a teenager I had a consuming inter ­ ultimately became the basis of my career est in electronics, and I read avidly about as an independent investigator. But before basic circuitry and radio communication, getting to that story and to a review of my often to the exclusion of school assign­ experiences with the lung circulation, I ments. Although I maintained an active will review critical points on the path that interest in electronics while in college, my led to my career and provided , in no or­ undergraduate education was in chemis­ ganized way, my training for research. try . My introduction to biomedical re­ An early interest in technology and search came during two summers of my science blossomed after being cultivated undergraduate years when I was able to by my high school physics teacher, who, combine interests in chemistry and elec ­ remarkably, was a young woman in her tronics through employment under the di­ first and only year of teaching. She en­ rection of Britton Chance at the Johnson abled me to see that mathematics could be Foundation for Biophysics at the Univer­ a useful investigative tool and that I could sity of Pennsylvania. This environment

From: Wagner WW, Jr, Weir EK (eds): The Pulmonary Circulation and . ©1994, Futura Publishing Co Inc , Armonk , NY. 167 168 • THE PULMONARY CIRCULATION AND GAS EXCHANGE provided many lessons on topics in sci­ risk. Consequently, experiments were un­ ence as well as on the process of scientific dertaken only after meticulous and de­ investigation . An early lesson about re­ tailed planning. I began to appreciate that search was that the investigator is obli ­ research must be organi zed from the start gated to know the characteristics, calibra­ and that inquiry requires an hypothesis tions, strengths, and weaknesses of his or and a detailed protocol to test it. Data her instrumentation. Another unforgetta­ reduction in Chris's lab was attended to ble lesson concerned intellectual honesty, with great care. One of Chris's rules was and it came about in a painful way. Brit that everything had to be calculated at and I were using a hand spectroscope, least twice, preferably by two different watching the appearance and disappear­ people. His rule made a lasting impression ance of absorption bands of various cyto­ of another fundamental requirement for chromes as oxygen was added to a circu - research : one must be confident in the lating suspension of yeast. With obvious data and their transformations. displeasure at my performance, he in­ The lung was not a subject of much sisted that I should be seeing a band at a interest in Cleveland while I was there as certain wavelength. I could not see the an intern and medical resident. However, change and persistently said so. Finally, one dedicated pulmono logist (David Gil­ after inducing considerable discomfort in lespie) at what is now Metropolitan Gen­ his employee, he said, "Good. I didn't see eral Hospital introduced me to clinical it either. Don't ever say you do when you pulmonary physiology. Through this ex­ don't." Then he walked away, leaving me perience I learned that the lung is an organ to reflect on the experience. whose physiology is susceptible to the I finished college unsure of whether I methods of physics, mathematics, and en­ wanted to pursue medicine, chemistry, or gineering , in all of which I had an interest biophysics, but an early admission to (if not an education), so I decided on a medical school decided the issue, at least career in pulmonary medicine . temporarily. At about that time George Wright had Chris Lambertsen's laboratory was a left the Trudeau Foundation at Saranac popular spot for medical students to expe­ Lake to assume a position in Cleveland as rience research, and it was my good for­ director of medical research at St. Luke's tune to be able to participate in it during Hospital, and in 1958 he agreed to serve as one free summer and several months of my mentor for a fellowship year. I pro­ senior elective time . My assignment was posed to study the pulmonary circulation, to build apparatus for continuous record ­ a choice that reflected my interest in both ing of pH and for controlling alveolar Pco 2 lung and cardiovascular physiology. at hypercapnic levels. I also assisted with George apparently believed in the sink -or­ teaching in the sophomore medical phar ­ swim training method, for he left me on macology laborator y exercises, an experi ­ my own to devise and perform my first ence that gave me a taste of what medical experiments. His lessons came during the school was like from the "other side." preparations for publication, and he was a Chris's guidance and personal warmth stringent editor. One memorable bit of were intangible assets that I recall fondly . advice he gave me was "to write it as if it I look on my experience in the pharmacol­ was to be published in the Boy Scout ogy department at Penn as the determin­ Handbook." Like Wiltz Wagner in recent ing factor in opting for a career in investi­ times , George was critical of my unneces­ gative medicine. Lambertsen 's experi­ sary obfuscation and my use of mathemat­ ments were complicated team efforts in ical expressions where words were a bet­ which human subjects were at personal ter choice. His tutoring certainly was help- Encounters with the Pulmonary Circulation • 169 ful: my first publication ,29 coauthored In the following description of the with George Wright , was accepted by the experiments that shaped my perception of Journal of Applied Physiology in its ini­ the pulmonary circulation I will unasham ­ tially submitted form. That publication edly stress my own publications, knowing concerned the shape of the pres ­ that in many instances my observations sure-flow curves of excised dog lobes and were not unique and that references to demonstrated the roles of venous and al­ antecedent and supporting information veolar pressures in determining transmu­ are provided in the reports that have been ral and longitudinal vascular pressure gra­ cited. dients. It was a satisfying piece of work that, in addition to presenting new mate­ rial, gave me a start toward understanding Effect of on the the physical properties of lung vessels and Pulmonary Circulation their perfusion. Jerome Klei nerman was associated In 1962 the phenomenon of hypoxic with the research laboratory at St. Luke's pulmonary vasoconstriction had been during my fellowship. Jerry became my known for at least 15 years, but it was not antagonist, protagonist, colleague, and universally accepted . Some investigators friend . He taught me to think beyond had been unable to demonstrate it , and physiology into the areas of anatomy, pa­ many were uneasy because constriction thology, and morphometry, and he caused was opposite to the effects of hypoxia in me to consider the relationships between other organs. Then as now the basic ques ­ structure and function. Jerry also taught tions included, is hypoxic pulmonary me a lot about the philosophy and politics vasoconstriction a direct response of vas ­ of science and about the funding of re­ cular smooth muscle or is it mediated search. indirectly, either humorally or reflexly? After fellowship I entered the Public does it occur in the arterial or in the Health Service to study the pulmonary venous bed , or both? and where is the effects of air pollutants. One year of ser­ sensor located? In hopes that I might con ­ vice was spent at St. Bartholemew's Hos ­ tribute to the confusion, I undertook my pital in London, where I came upon first experiments. Donald McDonald and his important book The goal of those first experiments Blood Flow in . I spent many hours was to determine if responses to hypoxia chewing on the material in that volume could be demonstrated in excised dog and learning more mathematics so that I lobes under more rigid control of condi ­ could understand what McDonald had tions than in prior investigations and, if written . so, to attempt to define the anatomical After two and a half years I left the location of the response, the relationship Public Health Service for my first faculty to alveolar and perfusate Poz, and the position . On returning to Cleveland in effects of certain pharmacologic blocking 1962 I realized that I had a job but that I agents. The plan was to excise the left had not yet established what I was going to lower lobe of an anesthetized dog, enclose do . I needed a project that would be attrac­ it in a humid, warm environment, venti­ tive to granting agencies, and I had to late it with chosen gas mixtures, and per­ develop my first grant. It was then that I fuse it at constant flow with systemic ve­ remembered Chris Lambertsen 's question nous blood drawn from, and returned to, and the problem of regulation of the pul­ the donor dog . Although I had had experi­ monary circulation in response to hyp ­ ence perfusing excised lobes under other oxia. conditions in th e experiments with 170 • THE PULMONARY CIRCULATION AND GAS EXCHANGE

'.0;02 Wright, I was not prepared for the prob­ 14 6 0 14 6 14 lems that arose when perfusing ventilated ~ .,,.. f\ -. P. lobes with blood for extended periods. It S HT I10cm H70 -----'""llJs ~ was amazing how quickly lobes would become edematous, and for many months we obtained greater flow out the airway BLOOD DEXTRAN than out the vein. Finally, we learned how ~02 94 6 94 2S to minimize the formation of edema and ..... ~--- ~1Scm -- H70 art. 295 62 4-4 produce preparations that were usable for 002 several hours. We also were able to dis­ pense with the donor dog and use an external reservoir for perfusion with ei­ ther blood or a physiological salt-colloid P 0 f 20 . solution. TOc~ ~o KCN Some of our findings with those lobes are shown in Figure 1. The percentage of Figure 1. Arterial pressure changes in per­ 0 2 in ventilating gas is indicated above fused lung lobes during hypoxia and serotonin each tracing, but in each case the CO2 challenges. Further details for this and subse ­ composition was 6.5%. The top left trac­ quent figures are given in the text. From refer­ ing illustrates the pressor response of ence 2, by permission. blood-perfused lobes to a bolus of sero­ tonin (5HT) and to reduction of the in­ 3o0 c except for brief periods at 37oc, spired %0 2 from the normal control 14 to during which the response to hypoxia was 6% and then to 0%. Responses to hypoxia tested. This further supported the idea were immediately reversed by raising in­ that there was a relatively fragile active , spired %0 2 as seen in the middle left metabolic process involved. panel. Responses to 6% 0 2 would be sus­ The relation between response and 0 2 tained, but responses to anoxia would composition of arterial blood and alveolar spontaneously recede to baseline within gas is illustrated in the middle left panel

5- 7 min. Thereafter, there would be no (Fig. 1). Here, the initial ventilating %0 2 response to varying inspired 0 2, although has been switched from 94 to 6% and then the response to 5HT was unchanged (Fig. back to 94%. Note that a pressor response

1, top right panel). Sometimes the re­ began while arterial Po 2 was 295 mmHg sponse to hypoxia would return after but that it subsided while arterial Po 2 (i.e., many minutes of ventilation with the Po 2 of blood leaving the reservoir) was normoxic mixture. A further finding in all still falling. This suggested that the re­ preparations was that responses to hyp­ sponse was sensed at either the alveolar or oxia would disappear after 50- 60 min of venous level, but not in the arteries. Later, perfusion, whereas responses to 5HT re­ this was followed up in other lobes that mained unchanged for several hours. The were perfused from vein to ,3 and we evanescence of the hypoxic response and found that the response to hypoxia was , its depression by severe hypoxia, in con­ again dissociated from input blood Po 2 trast to the sustained responses to 5HT, suggesting that the sensory site was not in suggested that the hypoxic response might the veins either. Our conclusion, then, involve some easily impaired process ex­ was that the sensor lay somewhere in the ternal to the smooth muscle. Several years alveolar gas environment. later we were to find 5 that preparations When salt solution was substituted would respond to hypoxia for a much for blood in our earliest experiments, hyp­ longer interval if they were cooled to 25- oxia usually caused a small depressor ef- Encounters with the Pulmonary Circulation • 171 feet and never caused constriction , al­ ine, however, significantly reduced the though the responses to 5HT were similar response . Although this could be inter­ to those seen during blood perfusion. An preted as mediation of the response by example of the hypoxic response is shown alpha adrenergic stimulation, we thought in the middle right panel of Figure 1. it more likely that phenoxybenzamine had Later, a graduate student, Ben Gorsky, merely left unopposed the dilatory beta­ showed that a small amount to plasma adrenergic effect of catecholamines pres­ potentiates the pressor responses to both ent in the tissue and perfusate at all oxy­ hypoxia and serotonin in lobes perfused gen tensions. with artificial perfusates but that small Several investigators had found that hypoxic responses occurred even in the the hypoxic pressor response could be absence of plasma. 1 Gorsky's results intro­ potentiated by acidosis, but this was not duced uncertainty into our earlier conclu­ universally agreed on. I thought our ap­ sion that some component of blood was proach provided better control of perfu­ needed for any hypoxic pressor response sion and ventilation parameters, includ­ to occur. ing gas and blood composition, and we The bottom panel of Figure 1 shows studied this problem . We found that the that potassium cyanide also evokes vaso­ hypoxic response was progressively de­ constriction and that this can be partially pressed by alkalosis , whether caused by reduced by raising alveolar %Oz from 14 hypocapnia or by administration of to 94%. Note that in the presence of cya­ NaHC0 3 or tris buffer. Sensitivity to pH nide, changing from 94 to 14% Oz caused was marked-a rise from a baseline pH of constriction, whereas before cyanide this 7.3 to 7.5 prevented an hypoxic pressor change made no difference. In a later response . The hypoxic response was pH study we found that dinitrophenol caused dependent but not specifically PCOz de­ effects similar to those of cyanide. 3 Thus, pendent. The uppermost trace of Figure 2 a pressor response could be produced by shows the arterial pressure response of a both cytotoxic and hypoxic hypoxia. perfused lobe when its ventilating gas Arterial wedge pressures were mea­ composition was changed from 6.5% COz sured with a 1-mm outside diameter cath­ in Oz to 6.5% COz in Nz· The second eter in several lobes of the initial study. tracing shows that when the lobe was Wedge pressure was approximately half­ made hypoxic with 100% Nz, there was no way between arterial and venous pres­ pressor response. When, however, the sures and showed characteristics consis­ lobe was ventilated with room air and the tent with pressure in small veins. Wedge pH of the perfusate was adjusted down­ pressure did not rise during hypoxia, and ward with lactic acid (HCl worked equally on a few occasions it fell to zero and lost well), there was a typical pressor response all responsiveness to alveolar and venous to 100% Nz· outflow pressures, as though it had been The question of mediation of the hy­ occluded during the pressor response . The poxic pressor response by nerve , even in wedge pressure measurements were taken excised lobes, had never been answered. as evidence that the pressor response to We added the following observations to hypoxia occurred in the arterial bed. the controversy. 5 Electrical stimulation at We were unsuccessful in blocking the the hilum of the excised perfused lobe hypoxic response with hexamethonium , caused a pressor response that was some­ atropine, and brom-lysergic acid diethyl­ what reduced by cooling perfusate from amide, thereby provisionally excluding 37 to 25 °c. However, raising perfusate pH mediation by autonomic ganglia, ace­ 0.16 or ventilating without Oz for 15 min tylcholine, or serotonin . Phenoxybenzam- had no significant effect on the response to 172 • THE PULMONARY CIRCULATION AND GAS EXCHANGE

CONTROL partial reversal that persisted as long as the stimulation was applied. We thought that these findings allowed us to conclude

20 that the responses to hypoxia and to nerve Pa 1 stimulation were independent events. 10 pH 7.34 An interesting and, I believe, provoca ­ pco 42 2 tive result occurred when we attempted to CONTROL stabilize vascular smooth muscle mem ­ brane potential using high concentrations O -CO N Al R 6 20 2 2 2 of procaine and tetracaine . I had expected Pa I 11111111111111111:.:.mRilllilliilhllliililhillilllllllmlihilil ... """"''""""' 11:.111.::.1.::.:.111 that this would impair the response of 10 t perfused lobes to hypoxia. Instead pro ­

pH 7.77 caine at concentrations from 2.5 to 10 mM, or tetracaine at one-tenth those concentra ­ p LACTIC ACID PC0 10 2 tions, had pressor effects of their own that resembled the effects of cyanide and dini ­ ...... ,- ...... AIR 20 trophenol. That is, when given while ven ­ Pa 1 tilating with 14% Oz they caused a modest 10 t t vasoconstriction that was reversed by rais ­ pH 7.12 pH 7.03 ing inspired Oz concentration to 94%. In pco 6 PC0 10 2 an equivalent fashion, the responses to 6 2 and 0% Oz were enhanced. But the most Figure 2. Effect of hypocapnic alkalosis on the remarkable effect was on the tolerance to pressor response to hypoxia in a perfused lung anoxia and on the useful life of the prepa ­ lobe. From reference 4, by permission. rations. Anoxia, which previously caused unsustained vasoconstriction and subse ­ electrical nerve stimulation. Cooling , mild quent unresponsiveness, now caused sus ­ alkalinization, and prolonged anoxia, tained vasoconstriction. In addition, the however , all prevented the pressor re­ life of the preparation during which re­ sponse to ventilation with 6% Oz, In con­ sponses to hypoxia could be demonstrated trast , procaine at a perfusate concentration was extended by several hours . Further­ of 1 mM or tetracaine at a concentration of more, when a lobe previously unexposed 0.1 mM greatly depressed responses to to the anesthetics became incapable of electrical stimulation but had no effect on responding to hypoxia after the usual 1 response to hypoxia . Pressor responses to hour of perfusion, addition of procaine epinephrine were unaffected by cooling, would restore and enhance hypoxic re­ but the effect of cooling on the serotonin sponsiveness, which then persisted sev­ response was interesting . Cooling slightly eral more hours. I was unable to find a reduced the serotonin peak response but sufficient explanation for those effects, but greatly prolonged the recovery, consistent it seems to me that there may be an impor­ with temperature sensitivity of the meta­ tant clue here . On which cells did the bolic removal of the agent. Several ani­ effect occur? Perhaps an explanation ex­ mals in this series were pretreated with ists that has not been brought to my atten­ either phenoxybenzamine or reserpine. tion. Until then, I will contend that this These treatments did not prevent re­ phenomenon merits further study . sponses to hypoxia. It was also interesting In 1966 I turned from studies in per­ to note that in lobes from animals given fused lobes to studies of excised vessels. phenoxybenzamine, electrical stimulation Methods were crude by today's standards: during a hypoxic pressor response caused although vessels were handled with care, Encounters with the Pulmonary Circulation • 173 no attempt was made to avoid endothelial that the hypoxic response depended on or to adjust baseline wall stresses to something released from lung paren­ optimum values for development of great­ chyma. I expected that such a mediator est active changes. In our first study,7 heli­ might be diluted to ineffectiveness if stud­ cal strips were prepared from segmental ies were made in an aqueous bath . To branches of pulmonary arteries of dogs reduce that possibility , subsequent studies and rabbits . Strips were immersed in a were made in a water-immiscible fluoro ­ bath of the same salt solution found earlier carbon that exhibited high salvation for 0 2 to be suitable for perfusion of lobes . Strips and CO2 but poor salvation of salts and were challenged with a number of vaso­ organic chemicals. In the first study, 8 we constrictor drugs as well as with electrical compared the effects of varying bath Po 2 field stimulation and hyperkalemia. To on responses of rabbit lobar arterial strips summarize, no strip, whether precon ­ with responses of similar strips to which a tracted or not, contracted when bath Po 2 thin layer of lung parenchyma remained was lowered progressively from 600 attached and to strips of lung parenchyma mmHg to near zero. Tensions of untreated of a similar size but as devoid of larger strips were unaffected by even severe hyp­ vessels and airways as could be achieved. oxia, but tensions of precontracted vessels The bath consisted of two phases: a lower fell as 0 2 was reduced . Reduction of strip (denser) phase of fluorocarbon on which tension required 0 2 depletions to near floated a layer of plasma diluted 50% with zero, as best shown by responses to electri ­ physiological salt solution . For the first cal stimulation : repeated electrical stimu­ 60 - 80 min the tissue strips remained in lation caused similar responses when Po 2 the fluorocarbon phase and were inte rmit ­ was reduced from 600 to 100 mmHg and tently challenged by lowering bath Po 2 then 40 mmHg , but further reduction to from 700 to 45 mmHg. We next attempted near zero reduced responses by about 30% to enhance any constrictor effect of hyp­ within 20 min. Strips prepared from per­ oxia with procaine as we had been able to fused dog lobes that moments before were do with perfused lobes. Procaine was fully responsive to hypoxia and stabilized added to the aqueous phase, and the strips with procaine behaved like all other were brought into that phase for 10 min strips. Addition of procaine to the baths of before being returned to the fluorocarbon previously untreated vessels did not for further challenges. We found that at no change their responses to hypoxia. In time did strips of isolated artery contract short , there was nothing to suggest that the in response to oxygen deprivation, al­ hypoxic pressor response of perfused though several relaxed with hypoxia after lobes resembled the effects of hypoxia on the procaine treatment , and all contracted isolated vessels . Instead , everything vigorously with electrical stimulation. In pointed to a depressor effect of severe contrast, arterial strips with attached pa ­ hypoxia, similar to the behavior of sys­ renchyma at first relaxed during hypoxia, temic vessel strips . In retrospect, there but , toward the end of the first hour and probably was endothelial injury in these prior to procaine treatment , about half preparations because acetylcholine usu­ underwent hypoxic contraction . After ally caused contraction that could be procaine treatment 9 of 10 strips con­ blocked with atropine . This may be im­ tracted as Po 2 was lowered. Contraction portant because some investigators now seem ed to begin at a Po 2 near 200 mmHg believe that endothelial factors may play a and reached its maximum near 100 role in the hypoxic pressor response. mmHg . After about 4 hours of study a Before undertaking the next experi­ subset of strips of artery with parenchyma ment with excised vessels , I hypothesized were removed from the fluorocarbon and 174 • THE PULMONARY CIRCULATION AND GAS EXCHANGE immersed in a bath of 10% plasma in salt not end in such a tidy way . There was one solution (with procaine) where further more set of experiments. 9 challenges with Po 2 variations were made . I wondered how vessels would re­

All strips removed from fluorocarbon and spond to variations in environmental Po 2 placed in the aqueous bath continued to if they were suspended in an even more display contractile responses to 0 2 reduc­ water-free environment. For this I chose tion. This suggested that the washout of first a bath of a more hydrophobic fluoro­ materials by an aqueous bath may not carbon and then a "bath" of humidified have been as important as I first thought. warm gas. This time we used strips of To our surprise, all the parenchymal strips rabbit aorta as well as parenchyma-free in the fluorocarbon bath contracted with rabbit pulmonary artery. The fluorocarbon

0 2 depletion both before and after the bath was used to study only pulmonary procaine treatment. The tension change of artery using the same protocol. However , the parenchymal strips was almost half only one strip was treated with procaine that of the artery-parenchyma combina­ or immersed in the aqueous phase above tion. Figure 3 illustrates the effects de­ the fluorocarbon . All strips of pulmonary scribed above. In each case tension (T) and artery contracted reversibly when bath bath P0 2 were simultaneously reco rded. Po 2 was reduced from 700 to 100 mmHg The upper left pair show parenchymal and the procaine -treated strip responded contractions obtained after procaine treat­ similarly to those not so treated. An exam­ ment. The upper right panel shows how ple is shown at the top of Figure 4. Con­ the parenchyma-covered arterial strips be­ tractions were modestly well sustained , haved before they began to show contrac­ but after several hours of study, contrac­ tions, and the lower curves display a re­ tions became less well sustained and there sponse after procaine treatment. These ex­ was often a rebound contraction when Po 2 periments were interpreted as evidence was raised. When aorta (AO) and pulmo­ that something from lung parenchyma nary artery (PA) were studied in the hu­ could evoke contraction in response to mid gas environment, each contracted as hypoxia and indeed was necessary for Po 2 was lowered. This is illustrated in the vessels to contract. bottom traces of Figure 4. In short here But my experiences with hypoxia did was a dilemma: isolated lengths of both

700

300 340 T 270 320

700 POz

100 ART. I! PARENCHYMA ~ PROCA INE

T HO -I min

Figure 3. Effect of bath Po 2 on tensions of strips of pulmonary artery , parenchyma, and artery with parenchyma attached . From reference 8, by permission. Encounters with the Pulmonary Circulation • 175

Pulm. Art. Strip .in FC 43 I was after the first. Furthermore, I am not convinced that contemporary studies of more carefully prepared smaller pulmo­ nary vessels, which are reported to show hypoxic contraction in aqueous baths, are suitable models of the excised lobe or whole-animal response to alveolar hyp ­

0 oxia. It seems to me that before venturing much further we need as much proof of the equivalence as can be obtained. One Arterial Strips in Wet Gas Chamber way to start might be to list the typical characteristics of the vascular response that most investigators agree are unique to the effects of hypoxia in situ and then to demonstrate that isolated vessels display each of them. These will probably have to be tested for species dependency. Several characteristics immediately come to mind, including sensitivity to cooling and to alkalosis, potentiation by procaine, evo­ cability by both hypoxic and histotoxic hypoxia, suppression following a period of severe hypoxia without loss of pharma­ cologic responses, and dependence on cal­ min cium movement. For now, I will have to rephrase the question first put to me: "You Figure 4. Effect of varying bath Pa z on tension of a pulmonary arterial strip immersed in fluor­ don't really believe that hypoxia causes ocarbon FC43 and of varying Oz composition of pulmonary vasoconstriction as a direct humidified gas in which were suspended strips smooth muscle effect, do you?" And it of aorta and pulmonary artery. From reference may be my turn to be wrong. 9, by permission. I thought that my studies with hyp­ oxia had come to an impass and that I was systemic and pulmonary artery contracted unable to answer the question that both­

when Po 2 was lowered from 700 to 100 ered me most: is the response a direct mmHg while suspended in humid gas, but muscle effect or is it not? The question of

both relaxed when Po 2 was lowered below neural control of the pulmonary circula ­ 40 mmHg while suspended in an aqueous tion seemed to offer more opportunities bath . Note in addition that the in vivo for forward movement. response takes place in the gap between

100 and 20 mmHg Po 2, specifically the range found to be without effect on the Responses of the isolated vessels. Pulmonary Vessels Do my observations with excised ves­ sels provide useful information? I am re­ One of the more challenging tasks has luctant to say that I have modeled the in been to test the hypothesis that increased vivo response to hypoxia with any of the pressure in the pulmonary arteries leads to isolated vessel techniques. Certainly I am reflex pulmonary arterial constriction. Al­ less confident after the second study than thought it would seem that such a positive 176 • THE PULMONARY CIRCULATION AND GAS EXCHANGE feedback mechanism would lead to a passive changes, and changes in pressure never-demonstrated sustained maximum might have been caused by changes in vasoconstriction, its existence has been flow. Unlike most others, who had chosen postulated, and perhaps demonstrated, to place balloons under indirec t observa ­ several times . The problem lies in the tion with chests closed, we opened chests technical difficulty associated with having of anesthetized dogs and attempted to stimulus and response in the same vessels place balloons under direct visual guid ­ and in securing certainty that one is in ance. We found that unless they were control of the experimental situation. I constrained, balloons in the left pulmo­ undertook a study of this problem with nary artery invariably "popped " retro­ Arthur Schneider, an anesthesiology post ­ gradely into the main pulmonary artery doctoral fellow. We believed we had when they were inflated sufficiently to found a way to discriminate between ac­ exert a force on the arterial wall. Ulti ­ tive and passive changes in pulmonary mately, we tied all lobar branches of the arterial pressure as left atrial pressure was left pulmonary artery and retrogradely progressively elevated. Experiments were placed a balloon in the parent vessel done in anesthetized dogs using bi ventric­ through an opening in the lower lobe ular bypass perfusion. We acquired what branch. This balloon remained in place. we thought was convincing evidence that Systemic flow was held constant by a left increasing pressure in the pulmonary vas­ ventricu lar bypass pump, and gas ex­ cular bed caused pulmonary vasoconstric­ change was provided externally so that tion and that this was mediated by the lung volume could be held invariant. We 27 28 sympathetic nerves . • We were wrong, were careful to preserve nerves, and we as I later showed in a subsequent study. 10 avoided dissection where they were What we had observed was related to a known to be located. Pressure in the main purely mechanical effect in the heart, pulmonary artery was recorded as a reflec­ whose activity was indeed affected by the tion of perfusion resistance of the right sympathetic . (I was later lung . No amount of distension of the left informed by Sol Permutt, who served as pulmonary artery influenced perfusion journal editor for all three manuscripts, pressure of the right lung, although right that one reviewer was unhappy when two lung vasoconstriction could be induced by papers he felt to be less than meritorious hypoxic ventilation or by stimulating the were published over his objections . When right stellate gangion. he had to accept a third that disclosed the Frustrated, and convinced that pres ­ error, he was incensed . Apparently the sure changes reported by others repre ­ publication and subsequent refutation of sented a passive mechanical effect, I errors was not the best way to expand my wanted to reproduce the findings of Craig curriculum vita. Sol periodically reminds Juratsch and his coworkers, who believed me of this.) they had evoked vasoconstriction by in ­ Several years later I attempted to flating a balloon placed in the main pul ­ reevaluate the problem by using balloons monary artery. The balloon was said not to to distend portions of the pulmonary arte­ have altered or occluded rial bed while monitoring resistance in major branches but instead to have caused other portions. Vasoconstrictor responses the main artery to be distended in accept ­ had been seen by other investigators with ing cardiac outpu t. Craig and I had met this approach, but there were inconsisten­ briefly and discussed his findings some cies among the results and questions about years before, and I wrote to him seeking to the methods. Most important, balloons borrow his balloon. To my surprise , he not might have caused unseen or unintended only had balloon catheters but said he Encounters with the Pulmonary Circulation • 177

INFLATE BALLOON

M I M I

Figure 5. Effect of inflating a balloon within the main pulmonary artery on systemic arterial pressure and flow and on pressures measured in right and left pulmonary arter ies. From reference 26, by permission. would like to come do the experiments nar y arteries measured through apical with me. Craig was, and remains, a de­ branches. Bursts at higher chart speed lightful individual, enjoyable to work with show waveforms, whereas intervals of even though he came fully convinced of mean pressure recording can be seen as the exis tence of reflex vasoconstriction, -free plateaus indicated by "M". The while I was convinced otherwise. We balloon was inflated at the up arrow and worked very hard for one week. On the deflated at the down arrow. Note that aor­ first day, using fluoroscop y, he passed his tic flow and pressure were not signifi­ balloon from a femoral vein of the anes­ cantly changed but that pressure in the left thetized dog into the pulmonary artery. pulmonary artery measured by both de­ Sure enough, inflation of the balloon was vices increased, while pressure in the followed by increased pressure down­ right fell. Calculated resistance of the left stream to the balloon in the left pulmonary lung actually fell during inflation of the artery where an extension of the catheter balloon. The changes caused by inflating lay. But then we opened the chest and the balloon were identical to those that placed catheters in apical lobe arterial followed tightening of a snare around the branches on each side. Pressures recorded right pulmonary artery. Throughout this from these during inflation of the balloon sequence of observations we were careful in the main pulmonary artery revealed to test after each step to assure that the that the right pulmonary artery was com­ original change observed while the chest pletely occluded by the inflated balloon. was closed continued to be present. This Every inflation and each subsequent ex­ experience cast doubt on the interpreta­ periment provided the same results. An tion of previous experiments using this example is shown in Figure 5. From top particular balloon technique but, of down are shown aortic pressure, aortic course, did not prove that all prior results flow, pressure in the left pulmonary artery were determined enti rely passively. We measured with the balloon (Laks) catheter, reported the results of this and the preced­ and pressures in the left and right pulmo- ing study. 20 •26 178 • THE PULMONARY CIRCULATION AND GAS EXCHANGE

After several attempts I had found no I 5 O 1------HR support for the hypothesis that pulmonary per min arterial distension would lead to arterial constriction, and I decided to examine 50 other potential initiators of neurogenic pulmonary vasomotion. and/or increased intra­ cranial pressure may be associated with pulmonary and pulmonary edema, and some have proposed that there may be a neurogenic pulmonary vasomo­ tor component . had dem­ onstrated that increased intracranial pres­ sure caused proportional systemic arterial vasoconstriction sufficient to raise arterial pressure above and thereby restore cerebral blood flow. This vasoconstriction was shown to depend on sympathetic efferent activation. Extension 0 2 of the Cushing experiment to include pul­ Figure 6. Systemic and pulmonary vascular monary vascu lar pressures could be ex­ responses in the . From refer­ ence 12 , by permission. pected to provide observations that would bear not only on pulmonar y responses to intracranial misadventures but also on the temic arterial pressure (Pao) increased. general problem of the role of the sympa­ (HR) rose transiently with thetic nerves in control of the pulmonary increased Pie but then fell. Atropine elimi­ circulation. Others had used it for that nated the but not the tachy­ purpose, but results were controversial cardia. There were very small but statisti­ because it was unclear whether pulmo­ cally significant changes in pulmonary ar­ nary vascular pressure changes were de­ terial pressure (Ppa), which as a group did termined actively or as a result of changes not differ significantly from the small in th e heart and systemic vessels. I be­ changes in left atrial pressure (Pla) that lieved we had techniques that could better also occurred. The atrial pressure rise exclude the secondary consequences. probably occurred as a result of a mechan­ We opened the chests of anesthetized ical effect of the bradycardia on discharge dogs, collected venous systemic and pul­ rate through the atrial cannula because it monary venous blood into external reser­ was reduced or eliminated by atropine. In voirs, and substituted two pulsatile-flow spite of the similarity of average arterial pumps for the ventricles. This allowed and atrial pressure changes, the Ppa-Pla control of flow and left atrial pressure gradient rose an average of 0.36 cmH 20 while we observed changes in systemic which, though small, was statistically sig­ and pulmonary arterial pressures in re­ nificant. We were unable to find a signifi­ sponse to brief intervals of increased intra­ cant change in pulmonary arterial pulse cranial pressure. Intracrani al pressure was pressur e during the time of increased Pie, varied by forcing physiological salt solu­ suggesting arterial wall stiffness did not tion into the subarachnoid space through a change. There was a graded relationship hollow plug screwed into the skull. Figure between the magnitude of the imposed 6 shows a typical response. When intra­ increase in Pie and the increase in Pao . cranial pressure (Pie) was increased, sys- During increased Pie we also noted a rise Encounters with the Pulmonary Circulation • 179 in levels of the external reservoirs. This and to control secondary events. Second ­ was not quantified but was believed to ary events were to include not only other reflect the considerable systemic vasocon­ , but also changes in systemic striction that was induced. We briefly flow, lung mechanics, and gas exchange. ventilated the lungs with N2 and found a Furthermore, our efforts were to be di­ reversible pulmonary pressor response . rected toward measuring outcome in This was taken as evidence that pulmo­ terms of physiological sequelae rather nary vessels were capable of constricting than as changes in afferent neural activity, under the experimental conditions. These for we contended that the former best experiments were interpreted as showing represents the reflex whe reas the latter that even massive activation of sympa­ only represents one form of the input to thetic efferent activity was incapable of the controller. evoking a physiologically significant pul­ Most work prior to ours had implied monary vasomotor response, although that distension of components of the left there was evidence of a small but statisti­ heart and pulmonary circulation would cally significant effect. The systemic vas­ cause systemic vasodilation and cardiac cular changes and the changes in heart slowing. Controversy existed. Some inves­ rate were in keeping with prior observa­ tigators were unable to find vasomotor tions. effects of left atrial and/ or pulmonary vein I was not encouraged by any of the distension while others found reflex foregoing experiments to believe that . In other hands distension of there is significant neural control of the the pulmonary arteries had been found to adult canine pulmonary vascular bed. My cause systemic vasoconstriction. The hy ­ attention was then directed to the possibil­ pothesis of our first study was that retro ­ ity that the lung vessels may be an impor­ grade pulmonary hypertension brought tant starting point for reflexes, rather than about by increasing left atrial pressure an important destination. would cause systemic vasodilation and a fall in heart rate and that these responses would be eliminated by vagotomy and Systemic Vasomotor partially suppressed by interaction of sys­ Changes Initiated by High temic arterial . We used Pulmonary Vascular biventricular bypass to isolate the stimu ­ Pressure lus to the left heart and pulmonary vessels and to isolate the response from interact ­ My curiosity was initially drawn to ing or secondary effects. This was reflex cardiovascular changes that might achieved in anesthetized dogs by opening occur in response to increased left ventric­ the chest, stopping the ven tricular beat by ular end -diastolic pressure. There was inducing ventricular fibrillation, draining abundant evidence that reflexes might systemic and pulmonary venous blood arise from distension of the left atrium and from the right ventricle and left atrium, ventricle, from intraparenchymal pulmo ­ and perfusing the systemic and pulmo ­ nary vessels , and from the major pulmo­ nary circulations through cannulas in the nary arteries . Most information, however, aorta and left lower lobe pulmonary ar­ was either in the form of afferent nervous tery. Pressure in the left heart was con ­ activity in response to pressure changes or trolled by a Starling resistor in the left hemodynamic variables measured under atrial drain line . The pulmonary perfusion conditions where primary changes were pump ran at constant rate, but provision insufficiently separated from secondary was made to servocontrol the systemic phenomena . I wanted to isolate stimuli pump so that could be held 180 • THE PULMONARY CIRCULATION AND GAS EXCHANGE constant. We measured left atrial pressure, O=c P•c pulmonary and systemic arterial pres­ sures, and systemic arterial flow. Heart ~---~ rate was acquired from the left atrial pres­ sure pulse. Systemic vascular resistance HR was continuously computed and re­ corded. All illustration of effects with ( P = Pao------~ C) and without (Q = C) servocontrol of ~------arterial pressure is shown in Figure 7. Qao r-_. When left atrial pressure (Pla) was raised ------../ "-- TIME (min) abruptly from Oto 20-25 cmH 20 the typi­ cal response was a transient fall of sys ­ temic vascular resistance (SVR) averaging 35% that gave way over a period of 40 - 60 JL_ ____Jl_ s to a plateau decline of 21 % . When sys­ temic arterial pressure (Pao) was not ser­ Figure 7. Heart rate and systemic vascular changes that occur in response to increased vocontrolled, the respective changes were pressure in the left heart and pulmonary circu ­ declines of 26 and 12%, significantly less lation. From reference 11, by permission. than with servocontrol. Fall of resistance was accompanied by a fall in heart rate (HR) from tachycardic baseline rates of any reflex dependent on that strain. And 150 - 200 beats/min. In a few instances, indeed we found that an increase of end ­ increased left atrial pressure brought expiratory pressure from 5 to 12 cmH 20 about a rise in systemic vascular resis­ significantly reduced the response to in­ tance and heart rate. That outcome creased left atrial pressure. seemed to occur most often when baseline In the next study 13 of the cardiopul ­ resistance and blood pressure were unu­ monary , we acquired stimulus­ sually and abnormally low. Bilateral cer­ response relationships using both step and vical vagotomy eliminated all effects of sinusoidal input pressures of variable increased left atrial pressure. Use ofpulsa­ magnitude. In addition we examined the tile, rather than continuous, pulmonary relationship between sinusoidal forcing perfusion did not alter response, although frequency and response, while using a it might have been expected to increase a constant stimulus amplitude. The prepa­ response if mediated by pulmonary arte­ ration was the same as that used earlier, rial with properties similar although systemic arterial pressure was to the systemic arterial . We not servocontrolled. Step pressure also tested the hypothesis that a change in changes were applied from a baseline left lung volume would alter the response to atrial pressure of O cmH 20. The magni­ increased left atrial pressure. There were tudes of both the transient and sustained three reasons for this hypothesis: (1) lung falls of systemic vascular resistance were volume changes can change the effective essentially linearly related to left atrial perivascular pressure and perhaps the step pressure magnitude over the range 10 vasc ular strain imposed by a change in to 35 cmH 20. For phasic pressure forcing, intravascular pressure, (2) slowly adapt­ we servocontrolled left atrial pressure us­ ing pulmonary receptors reflexly alter sys ­ ing a low frequency signal generator and a temic vasomotor tone and may have an third pump to impose a back pressure on influence on further neural modulation, atrial drainage such that left atrial pres­ and (3) greater may mechan­ sure varied sinusoidally. At frequencies ically limit en largement of the heart and below 0.03 Hz , the systemic arterial pres - Encounters with the Pulmonary Circulation • 181 sure output waveform was cyclic but not nary artery and atrial appendage. While sinusoidal, whereas from 0.03 Hz to 0.08 one compartment was gas pressurized , we Hz the output waveform appeared to be an opened a drain from the other so that any undistorted sinusoid whose amplitude be­ transcapillary gas passage would not came progressively less as forcing fre­ cause a pressure change in the second quency was raised . Above 0.08 Hz, the compartment. Step changes of pressure of response was too small to be detectably various magnitudes were used to deter ­ cyclic. Over the frequency range 0.03 to mine pressure-response curves . We found 0.08 Hz the input-output characteristics that pressurization of the veno-atrial com ­ resembled those of a linear second-order partment caused systemic vascular resis­ system, and on that basis we determined tance to fall in amounts that were essen­ stability characteristics. We found that the tially linearly proportional to forcing pres­ system behaved with a natural frequency sure over the range 10 to 30 cmH 20 but of 0 .05 Hz and that its gain and phase that, on average, a plateau was achieved at margins assured stability. When left atrial about 30 cmH 2 0. Those results were in­ pressure was forced to vary sinusoidally at distinguishable from the earlier experi­ 0.03 Hz using a range of amplitudes from 5 ments in which the entire cardiopulmo­ to 30 cmH 20 , the induced systemic arte­ nary compartment was pressurized as a rial pressure variations were linearly re­ unit. A similar linear response and plateau lated to the magnitudes of the forcing pres ­ were found when pressurizing the pulmo­ sure. nary arterial compartment, but responses In all preceeding experiments with were about one-third those obtained from the cardiopulmonary baroreflex the stimu ­ the vein-atrium, and the plateau occurred lus pressure was imposed equally at 60 cmH 20. The vein -atrial reflex was throughout the lung vessels and left heart further characterized to show that re­ chambers . Our next goal was to determine sponses varied in proportion to rate of rise the roles of individual subcompartments . of pressure if a ramp stimulus was used The first of those studies 14 demonstrated and that greatest responses would follow a responses to pressurization of the pulmo­ uniform 5 cmH 20 step change if that step nary arterial compartment and pressuriza­ was introduced from a baseline pressure tion of the left atrium-pulmonary vein of 15 cmH 20 . compartment. Open-chest anesthetized The gas insufflation experiments did dogs were provided with external gas ex­ not include pressurization of the pulmo­ change and systemic vascular perfusion nary capillary bed , nor did they separate by draining systemic venous blood from pulmonary veins from left atrium. Further the right atrium and , after gas exchange, discrimination was made in preparations returning it to the aorta by a continuous ­ using the same basic bypass perfusion flow pump. Systemic arterial pressure was technique but in which all pulmonary sevocontrolled. Valve orifices at the base veins were tied at their atrial junctions so of the heart were occluded by clamping that the pulmonary and left heart cham­ the fibrillating ventricles with a single bers could be individually pressurized large clamp placed just below the atrio ­ with blood. Responses to pulmonary vas­ venous groove . A small drain was placed cular pressurization was essentially the in the left ventricular cavity. Recalling same as that found with isolated arterial that gas does not traverse capillaries ex­ distension , whereas the response to left cept at high pressure , we were able to heart distension was similar to vein-atrial distend the arterial and veno-atrial com­ pressurization .16 Mild pulmonary edema partments selectively by admitting 5% was induced with sustained pressure in a

CO2 in 0 2 through catheters in the pulmo- subset of this study , and more severe 182 • THE PULMONARY CIRCULATION AND GAS EXCHANGE edema was caused by alloxan in another ily appreciated during servocontrol of sys­ subgroup. Edema per se had no detectable temic arterial pressure wherein it was of­ effect on systemic vascular resistance or ten necessary to quickly double systemic on subsequent responses to pulmonary flow to maintain pressure. One thing not vascular pressurization . disclosed is whether the left atrium ever The experiences with control of sys­ "sees" the necessary transmural pressure: temic vascular resistance by a cardiopul­ in the intact animal, restraints by the peri­ monary baroreflex indicated that, although cardium or by mechanical cardiopulmo­ vasodilatory responses could occur conse ­ nary interaction may significantly reduce quent to increased pressure in the pulmo­ the strain in response to any given change nary arteries, a much larger response fol­ in intraatrial pressure. lowed distension of the left atrium. (We had also shown that distension of the fi­ brillating ventricle was ineffective over Effects of Cardiac and the pressure range used to evoke the atrial Pulmonary Vascular response. 15 ) I had anticipated that pulmo­ Pressures on nary congestion would play a bigger role and, anticipating a role for C-fiber re­ Inspired by reports of cardiodynamic flexes , that capillary congestion would hyperpnea and of the effects of congestion show itself to be important. But this was on slowly and rapidly adapting pulmo ­ not so . Furthermore , atrial and pulmonary nary receptors and on C-fiber afferent ac­ arterial baroreflexes did not seem to sum­ tivity, I next addressed the question of the mate when induced together. Although it role of cardiac and pulmonary vascular was apparent that the greatest effect on pressures in the control of breathing. resistance was transient, this was not de­ Our first effort made use of the meth ­ tectably different from the time course of ods described above in which ligation of the systemic arterial baroreflex, which we pulmonary veins and cross-clamping of often displayed by abruptly increasing set the ventricles of dogs on cardiopulmonary point pressure of the servocontroller . In bypass perfusion enabled independent what seemed appropriate, the optimum distension of pulmonary vascular and left baseline pressure from which to acquire atrial compartments . In addition to re­ the left atrial baroreflex corresponded cording cardiovascular pressures , breath­ rather closely with normal transmural ing was monitored by recording the dia ­ pressure and the threshold, and plateau phragm electromyogram. Note that in pressures of the pulmonary artery re ­ these preparations in which external gas sponse curve exceeded those of the atrial exchange was provided there was no need response by an amount roughly in propor ­ for lung ventilation. Consequently, lungs tion to the normal mean pressures of those were held at a single baseline transpul­ compartments. Teleologically (and to ad­ monary pressure during the period of per ­ dress utility is not without meri t}, the fusion. This approach decoupled breath­ vasodilatory cardiopulmonary baroreflex ing activity from lung volume movements would seem destined to respond to and gas exchange and was an important acutely higher left ventricular end­ aspect of our experiments on the control of diastolic pressures in a way that, at least breathing because many secondary effects temporarily, reduces ventricular load and were prevented in this way. Based on promo tes stability. In my experience it has observations of others, I had anticipated been a robust and reproducible reflex ob­ that lung congestion would cause servable under a wide range of conditions. tachypnea, but I had no firm preconvic ­ Its effects are dramaticall y large, most eas- tions regarding the effect of atrial disten - Encounters with the Pulmonary Circulation • 183

sion. We were surprised to find that lung E MG congestion decreased breathing frequency through prolongation of expiratory time. Pp a = 0 mm H g Changes appeared and disappeared imme­ diately on change in vascular pressure. We used vascular pressures ranging from 20 to 70 cmH 20 and found that there was a threshold near 30 cmH 20 . Prolongation of expiration varied directly with pressure Pp a = 3 2 mm Hg above that threshold. There were no significant effects on inspiratory time , peak inspiratory magnitude, or the rate of inspiratory activity . An example is shown in Figure 8. Although in 6 of 15 experi­ ments expiration time shortened for two or three breaths at the onset of vascular Pp a = O mm Hg pressurization, the respiroinhibitory effect of congestion that prevailed in all experi­ ments resembled that of lung inflation , considered to be a response to stimulation of slowly adapting airway receptors. Congestion did not influence breathing seconds after bilateral cervical vagotomy . Figure 8. Effect on the diaphragm electro­ The effect of left atrial distension on myogram of increasing and then lowering pres ­ breathing was inconstant and inconclu­ sures in the left atrium and lung vessels, re­ sive in that early study. However, when corded as pulmonary arterial pressure (Ppa). there was an effect on breathing it seemed From reference 17, by permission. to be weakly excitatory . Subsequent stud­ ies21·24·25 using similar but improved how this would interact with the respiro­ methods have shown that breathing fre­ inhibitory effect oflung congestion, which quency can be stimulated by distension of should be simultaneously active under the left atrium . Shortening of both expi­ most conditions in the intact animal. We ration and inspiration contribute to this undertook further investigations of the ef­ change, and, in addition, there may be a fects of lung vascular congestion and of small reduction in depth of inspiration. interaction between lung and cardiac re­ Breathing frequency was, by inspection , a flexes. linear function of left atrial pressure above As noted earlier, the anticipated re­

a threshold of 8 cmH 20 . An atrial pressure sponse to lung congestion was tachypnea

of 30 cmH 20 caused an average change in secondary to stimulation of juxtacapillary frequency of 20%. Although the changes C-fiber endings (sometimes referred to as were not large, the reflex was robust: it type J receptors). In contrast, our results could be demonstrated under a wide range suggested that slowly adapting receptors of conditions, it was reproducible, and it had been stimulated. Recall that in those could be evoked by changes in left heart experiments lung volume did not change loading or by the increased pressure con­ throughout the breathing cycle . If conges­ sequent to induction of atrial fibrillation. I tion stimulated or "sensitized" the slowly concluded that this left atrial reflex could adapting receptors under normal condi­ play a role in tachypnea of exercise or left tions , where lung volume varies with ventricular failure, but it was not clear breathing, congestion may result in 184 • THE PULMONARY CIRCULATION AND GAS EXCHANGE tachypnea. The reasoning behind this is 32 1, 2 - control 1 3 - cong estion that because the slowly adapting receptors 4 28 4, 5 - recovery may activate the inspiratory off switch , enhancement of their activity will lead to 24 shallow tachypnea if is 20 to be preserved (for example, by the CO - C 2 ~ related chemoreflex) . This suggested an '- 16 .Cl experiment to test the hypothesis that con­ .!: 12 gestion enhances the sensitivity of the Hering-Breuer reflex, a reflex attributed to 8 slowly adapting airway receptors. 4 Anesthetized dogs were prepared by draining systemic venous blood from the 0 2 4 6 8 10 12 14 16 18 right atrium to an external gas exchanger 0 Paw, cm H 0 and perfusion pump that returned it to the 2 aorta at constant flow. Ventricular fibrilla­ Figure 9. Relationship between transpulmon­ tion was induced, and drains were placed ary pr essure and breathing frequency before, in the right and left ventricular cavities . A during , and after congesting the lung vascular bed. From reference 22, by permission. second pump perfused the lungs and left heart chambers with blood from the exter­ nal reservoir admitted through a cannula breathing frequency fell in a nearly linear in the main pulmonary artery. Perfusion way as airway pressure (Paw) was raised. flow rate of this second system was inten­ Note also that congestion had no signifi­ tionally low-about 500 ml/min. Pressure cant effect on frequency of (fr) within the lung-heart compartment was at low airway pressure but that as Paw adjusted by regulating the resistance of rose, frequency did not decrease as much outflow from the left ventricle. During as it had while lung vessels and left heart baseline conditions, pressure in the pul­ were not distended. Those characteristics monary artery was approximately 0 pertained to the group as a whole. The cmH 20. Breathing movements of the dia­ failure of frequency to fall as much during phragm were monitored . The Hering­ congestion implied that the Hering-Breuer Breuer reflex was characterized by the reflex was less rather than more effective relationship between breathing frequency during congestion. In each of these experi ­ and transpulmonary pressure. Transpul ­ ments we also infused a small amount of monary pressure was increased from 2 oleic acid into the lung vessels to cause an cmH 20 to 20 cmH 20 , or to the point of acute chemical injury. This was associ ­ if that occurred first. Each step was ated with an increased frequency of held for 20 s while breathing movements breathing at low airway pressures , which stabilized . After making observations un­ fell readily with inflation such that at der baseline vascular conditions, the ef­ higher airway pressures there was no ap­ fect of congestion was tested by raising parent effect of the acid. The result could pressure in the pulmonary artery to 60 be interpreted as enhancement of the Her ­ cmH 20. About 1 min was allowed for ing -Breuer reflex by lung vascular injury. stabilization before a Hering -Breuer reflex In the experiments with the Hering ­ response relationship was obtained as be­ Breuer reflex we noted that congestion of fore. Vascular pressure was then returned lungs and left heart did not cause a sus­ to baseline, and another response to infla­ tained fall in breathing frequency as it had tion was obtained. An example of the re­ in the earlier experience with isolated sults is provided in Figure 9. Note that lung vascular congestion, although there Encounters with the Pulmonary Circulation • 185 was usually a transient dip at the onset of 50 ]Group 1: Lungs _,,.,.,-,...__.·~---. the vascular pressure rise. I wondered if - - this reflected the combined effects of the \ __,_...... r"'--~---:, depressor pulmonary reflex and the exci ­ t ~ I tr 10 tor cardiac reflex. Two groups of dogs br/min t-- 1 min~ were prepared using methods for perfu­ 50 1 ...r""""'"'-----·,,__._,-.,-v"L- vagal block f""-~ '"'\,I"" ._,,...-.,..,."----~-- sion and stimulus isolation already de­ 30 t I scribed. In one group we looked for the effects of isolated pulmonary vascular congestion and in the other the effects of Group 2 : Lungs+ le ft heart combined lung vessel and left heart dis­ tension. In both groups we also attempted to block conduction of myelinated vagal afferent fibers by nerve cooling in an at­ tempt to bring out C-fiber afferent effects. As predicted, congestion of isolated ves­ sels caused marked initial transient de­ Figure 10. Changes in breathing frequency pression of breathing followed by a sus­ evoked by pressurizing (up arrow) and depres­ surizing (down arrow ) the lung vessels alon e or tained plateau at an intermediate level. in combination with left heart chambers before Pressurization of combined vessels and and during vagal cold block. From reference left heart, however , caused an initial tran­ 23, by permission. sient depression followed by an increased frequency significantly above the prestim­ sures that were confined to th e closed ulus baseline. During vagal block, pressur­ extraparenchynmal arterial compartment. ization of isolated lung vascular or com­ My experiences with reflex effects on bined lung-heart compartments caused breathing that arise from the heart and stimulation of breathing. An example of pulmonary vessels have shown that small each of these effects is shown in Figure 10. (5-25%) upward changes in breathing fre­ To complete our studies of the effects quency can be generated by increased on breathing of pressures in the pulmo­ pressure in the extraparenchymal arterial nary circulation, we confined pressure portion but more particularly by increased changes to the beating right ventricle and pressure in the left atrium. Similar in ­ the extraparenchymal pulmonary arteries. creases can be caused by lung congestion Again, bypass perfusion and gas exchange if a dominant and much larger depressor provided control of secondary variables. effect is prevented by blocking myelinated We found that if outflow resistance from afferent fibers. The pressures shown to be the pulmonary arterial component was effective are within the range of expected raised sufficient to increase arterial pres­ pressures in the intact animal, at least at sure to about 65 cmH 20, there would be a times of stress, and this is particularl y true small increase in breathing frequency. 19 for the left atrial reflex. The lack of uni­ This result was not seen in all experiments form direction of change makes it impossi­ and was not as repeatable as were changes ble to anticipate the combined effects of caused by congestion of intrapar ­ these reflexes in the intact situation. Our enchynmal vessels or the left heart. Our demonstrations only provide evidence results conflicted with those of others who that certain things may happen. They have suggested that breathing may be im­ could not test the hypothesis that card io­ portantly modulated by right heart load­ pulmonary pressure variations are effec­ ing but confirmed studies that showed tive in the control of breathing in the intact only a small effect of high arterial pres- animal. If nothing else, these experiments 186 • THE PULMONARY CIRCULATION AND GAS EXCHANGE serve as useful reminders of the complex ­ pressure was raised in the lung vessels ity of reflex control systems and the near ­ and left heart but that changes of the same futility of creating a valid large-system direction and size were present after bilat ­ model without more sophisticated and ex­ eral cervical vagotomy. Induced changes tensive information. In that regard, perfor­ were approximately 20% of baseline val ­ mance of these experiments was a reward­ ues . The effect of vagotomy itself was to ing but at the same time a cautionary increase compliance and reduce resis­ experience . tance . In each experiment we were able to find the expected vagally-mediated change in systemic arterial pressure, con­ Effect of Lung Vessel and firming the presence of reflex activity sec­ Left Heart Pressures on ondary to congestion. I inferred from these Airways observations that the changes in lung me­ chanics brought about by congestion were , Although well established in the ex­ in this case , passive and not by reflex. periences of clinicians, cardiac asthma Because others had found the trachea had not been well documented in the to undergo reflex contraction typica l of physiology laboratory, and I couldn't re­ smaller airways, I speculated that this may sist a look to see if left heart and/or lung be a more sensitive site from which to vascular congestion caused reflex changes detect changes. In a second series of exper­ in lung mechanics. This was done in two iments, dogs were once again prepared series of experiments, both of which used with cardiopulmonar y bypass, but in this the now-familiar technique of cardiopul­ group all pulmonar y veins were tied at the monary bypass perfusion and external gas atrium so that the pulmonary vessels and exchange. Both studies were combined in left heart could be individually pressur ­ a single report. 18 In one study we looked at ized with blood drawn from the external changes in and resis ­ reservoir. Lung ventilation was stopped tance brought about by increased pressure and an endotracheal tube was passed that in the combined chambers of the left heart had a long compliant cuff. The cuff was and pulmonary circulation. In this study, inflated to exert a small force on the tra ­ after establishing cardiopulmonary by­ chea . Changes in cuff pressure were used pass perfusion, the lungs and left heart to detect changes in tracheal muscle tone. were perfused with a second pump at low We recorded cuff pressure while pressur­ flow admitted through a cannula in one izing the left heart and the pulmonary pulmonary arterial branch and drained vessels and while imposing different from cannulas in atrium and ventricle. transpulmonary pressures through the en­

The lungs were ventilated with 5% CO2 in dotracheal tube . We found that distension

0 2 by a piston respirator connected of the left heart consistently caused tra ­ through a that exceeded tidal cheal contraction. Pulmonary vascular volume , incorporated to minimize gas ex­ congestion caused contraction in 6 ani ­ change and drying. Airway pressure and mals but relaxation in 4 others. Lung infla ­ flow were continuously monitored, while tion caused tracheal relaxation in 8 ani ­ pressure in the left heart and lung vessels mals but relaxation in 2. Bilateral cervical

was raised from zero to 45 cmH 20 by vagotomy led to tracheal relaxation and partially occluding the outflow of blood. loss of changes during lung inflation, vas­ Dynamic lung compliance and resistance cular congestion, or left heart distension. were calculated from the airway pressure An example of tracheal contract ions with and flow records. We found that compli­ left heart and pulmonary vascular conges ­ ance fell and resistance rose whenever tion is shown in Figure 11. In this record - Encounters with the Pulmonary Circulation • 187

ently dominated by slowly adapting re­ 20 ceptors was reminiscent of the effect of d I congestion on breathing frequency. 0 [ Epilogue It has been my pleasure for over more than 30 years to explore the lung circula­ [ 120 tion, lung mechanics , lung-heart interac­ I min 40 tions, and the control of breathing. Whereas some have preferred to focus Figure 11. Variations in pressure in an en­ more narrowly and to greater depth, I have dotracheal tube cuff caused by lung inflation found satisfaction in covering a range of and deflation and by distension of the left heart topics in lung physiology. I am somewhat chambers or lung vessels. From reference 18, by permission . concerned that a narrow path is tread at some peril , at least to one's students, who may not appreciate the whole from a few ing the same transducer was used to ac­ of its parts. The whole is too fascinating to quire both the left heart (Plh) and the let pass by . pulmonary vascular (Pves) pressure by Looking back over my experiences, I switching from one catheter to the other in see the lung circulation as a largely pas ­ the interval between the two episodes of sive system , albeit with complicated phys ­ pressurization. Tracheal activity, reflected ical properties, whose most prominent ac­ in the tracheal cuff pressure (Pcuff), can tive regulation comes in response to re ­ also be seen to vary with lung inflation (i) gional oxygen composition. The lung and deflation (d). Small changes in aortic vessels seem able to play an interesting pressure (Pao) are also apparent. but as yet uncertain role as the afferent site Unlike the first study, the second pro ­ in several reflexes. None of the above is vided evidence in support of reflex in­ without controversy , and I am left with crease in airway tone consequent to left more questions than when I began . Look­ heart or lung vascular distension . How­ ing forward to other topics of personal ever, we also saw evidence of reflex air­ interest, I see the prospect for demonstra­ way relaxation with lung congestion. This tions of control of vascular smooth muscle should come as no surprise because con ­ by a wide range of local and circulating gestion has been documented to cause compounds, the regu lations of which re­ stimulation of each of the several afferent main to be explored . I also envision receptor types in the lung. Typically , stim ­ greater interest in an immense vascular ulation of slowly adapting receptors leads surface area that exchanges more with to airway re laxation, whereas rapidly blood than just the respiratory gases and adapting receptor or C-fiber ending stimu­ that must be understood in those terms. lation results in . Note I look forward to better understanding that lung inflation , which also stimulates lung microvascular mechanics and the each receptor group, had a similar diver ­ rheology of pulmonary blood flow, topics gent effect among the experiments , al­ about which our knowledge is sketchy though the dominant effect seemed to be and which are in urgent need of further that attributable to slowly adapting recep­ study. tors. The appearance of reflex effects of The universe , at least this little part of congestion on airways that were appar- it, is still expanding. 188 • THE PULMONARY CIRCULATION AND GAS EXCHANGE

References 16. Lloyd, T. C., Jr. Cardiopulmonary barore­ flexes: effects of pulmonary congestion 1. Gorsky, B. H., and T. C. Lloyd, Jr. Effects of and edema. J. Appl. Physiol. 43: 107- 113, perfusate composition on hypoxic vaso­ 1977. constriction in isolated lung lobes. J.Appl. 17. Lloyd, T. C., Jr. Effects of pulmonary con­ Physiol. 23: 683- 686, 1967. gestion and of left atrial distention on 2. Lloyd , T. C., Jr. Effects of alveolar hypoxia breathing in dogs. f. Appl. Physiol . 45: on pulmonary vascular resistance . J.Appl. 385- 391, 1978. Physiol . 19: 1086- 1094, 1964. 18. Lloyd, T. C., Jr. Reflex effects of left heart 3. Lloyd, T. C., Jr. Pulmonary vasoconstric ­ and pulmonary vascular distension on air­ tion during histotoxic hypoxia. f. Appl. ways of dogs . J. Appl. Physiol . 49: 620- Physiol. 20: 488- 490, 1965. 626, 1980. 4. Lloyd, T. C., Jr. Influence of blood pH on 19. Lloyd , T. C., Jr. Effect on breathing of acute hypoxic pulmonary vasoconstriction. J. pressure rise in pulmonary artery and right Appl. Physiol. 21: 358- 364, 1966. ventricle. J. Appl . Physiol. 57: 110-116 , 5. Lloyd, T. C., Jr. The role of nerve pathways 1984. in hypoxic pulmonary vasoconstriction. J. 20. Lloyd, T. C., Jr. Pulmonary artery disten­ Appl. Physiol. 21: 1351- 1355, 1966. sion does not cause pulmonary vasocon­ 6. Lloyd, T. C., Jr. P0 2-dependent pulmonary striction. f. Appl. Physiol. 61: 741-745, vasoconstriction caused by procaine. J. 1986. Appl. Physiol. 21: 1439- 1443, 1966. 21. Lloyd, T. C., Jr. Control of breathing in

7. Lloyd , T. C., Jr. Influences of P0 2 and pH anesthetized dogs by a left heart barore­ on resting and active tensions of pulmo ­ flex. f. Appl. Physiol . 61: 2095- 2101, 1986 . nary arterial strips. J. Appl . Physiol. 22: 22. Lloyd, T. C., Jr. Effects of lung congestion 1101- 1109, 1967. and oleic acid injury on the Hering-Breuer 8. Lloyd , T. C., Jr. Hypoxic pulmonary vaso­ reflex. J.Appl. Physiol. 64: 832-836, 1988. constriction: role of perivascular tissue . f. 23. Lloyd, T. C., Jr. Breathing response to lung Appl. Physiol. 25: 560-565, 1968. congestion with and without left heart dis­ 9. Lloyd, T. C., Jr. Responses to hypoxia of tension. J. Appl. Physiol. 65: 131- 136, pulmonary arterial strips in non-aqueous 1988 . baths. f. Appl. Physiol . 28: 566- 569, 1970. 24. Lloyd, T. C., Jr. Effect on breathing of 10. Lloyd , T. C., Jr. Relation of left ventricular abruptly loading and unloading the canine diastolic and lung vascular pressures: left heart. J. Appl. Physiol. 66: 2216- 2222, atrial effects. J.Appl. Physiol. 30: 703- 707, 1989. 1971. 25. Lloyd , T. C., Jr. Effect of increased left 11. Lloyd, T. C., Jr. Control of systemic vascu­ atrial pressure on breathing frequency in lar resistance by pulmonary and left heart anesthetized dog. f. Appl. Physiol. 69: baroreflexes. Am. f. Physiol. 222: 1511- 1973-1980, 1990. 1517, 1972. 26. Lloyd, T. C., Jr., and C. E. Juratsch . Pulmo­ 12. Lloyd, T. C., Jr. Effects of increased intra­ nary hypertension induced with an in­ cranial pressure on pulmonary vascular traarterial balloon: an alternative mecha­ resistance. f. Appl. Physiol . 35: 332- 335, nism. J.Appl. Physiol. 61: 746-751, 1986. 1973. 27. Lloyd, T. C., Jr., and A. J. L. Schneider. 13. Lloyd , T. C., Jr. Cardiopulmonary barore­ Relation of pulmonary arterial pr essure to flexes: integrated responses to sine- and pressure in the pulmonary venous system. square-wave forcing . J. Appl. Physiol . 35: J.Appl. Physiol. 27: 489- 407, 1969. 870-874, 1973. 28. Lloyd, T. C., Jr., and A. J. L. Schneider . 14. Lloyd , T. C., Jr. Cardiopulmonary barore­ Reflex pulmonary vascular response to flexes: effects of staircase, ramp and distention of lung vessels and left heart. f. square-wave stimulation. Am . J. Physiol. Appl. Physiol . 29: 318-322, 1970. 228 :47 0-476, 1975 . 29. Lloyd, T. C., Jr., and G. W. Wright. Pulmo­ 15. Lloyd, T. C., Jr. Cardiopulmonary barore­ nary vascular resistance and vascular flexes: left ventricular effects. Am . f. Phys­ transmural gradient. J. Appl. Physiol. 15: iol. 232: H634 - H638, 1977. 241- 245, 1960.