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Neonatal Biology – an Overview

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Neonatal Biology – an Overview NEONATAL BIOLOGY – AN OVERVIEW FACT FILE 2D For health professionals caring for neonates, it is important to understand the unique biological features of this special group in order to accurately assess normality and any potential deviation from the ‘norm’ which may require referral and further management. This Fact file provides an overview of key specific anatomical and physiological features of the term neonate as compared to the older child and / or adult. Any further differences in the preterm neonate will also be highlighted but more detail for this group of neonates can be seen in unit 3E. 1) Respiratory system Overall, the neonate’s respiratory system is smaller, shorter and less efficient in terms of the reserve and ability to cope with compromise. An increased basal metabolic rate means that oxygen consumption is significantly higher than that of an adult. A neonate will become tachypnoeic in response to compromise (e.g. hypoxia) rather than increasing vital capacity of the lungs as an adult would do. They have a relatively high respiratory rate and short inspiratory time and are obligatory nose breathers up to 3 months of age. Structurally, the epiglottis is floppy and situated at a higher position than a child of over 8 years of age. The narrowest part of the neonate airway is the subglottic region with a more funnel shape overall whereas, over 8 years, the airway shape is more cylindrical. The diaphragm is the main respiratory muscle. Neonates have short necks, small mouths, narrow nares and large tongues compared with adults. Therefore, there is less airway protection and more resistance. The thorax is softer with a greater proportion of cartilage which means there is lower elastic recoil during breathing. In the preterm neonate, there is an underdeveloped respiratory centre leading to a predisposition to apnoea of prematurity. There is also immature pulmonary function due to less alveoli growth meaning the surface area for gaseous exchange is reduced and the lung functional residual capacity is lower. This is further exacerbated by potential surfactant deficiency particularly at very early gestations meaning the alveoli are more fragile with a high surface tension. Therefore the lungs are less compliant and so more easily damaged by the shearing forces of mechanical ventilation. Page | 1 J Petty 2) Cardiovascular and haematological systems As with the respiratory system, the cardiovascular system is less efficient in terms of the reserves to deal with compromise and illness. The specific features are as follows; The neonatal heart is less contractile than the adult heart with lower myocardial reserve. Cardiac output is dependent on rate rather than stroke volume. There is also a relatively high pulmonary vascular resistance in the first few week s of life until this lowers to reach neonatal values after the first month of life. Heart rate variability is high between resting and active values. Being born preterm means the reduction in cardiac contractility is even greater and there is less sensitivity to the constricting effects of oxygen at birth to close the ductus arteriosus in the heart; therefore this may remain open. A neonate’s total blood volume is low (85 mls / kg). There is a higher percentage of haematocrit (45-65%) with high haemoglobin levels in the newborn period. These fall during the first 8-10 weeks of life to reach childhood values at approximately 6 months of age. In the newborn, the majority of haemoglobin is fetal (HbF)– this has an increased affinity to oxygen according to the oxygen dissociation curve with less being given up to the tissues (left shift). This HbF is rapidly broken down by haemolysis leading to a high risk of developing physiological jaundice. Red blood cells have a shorter life span in neonatal life (60-70 days approximately whereas it is up to 120 days in adults). This further increases the chance of physiological jaundice. There is no vitamin K production until the neonate is fully fed – this, along with liver immaturity can lead to prolonged clotting times and is the rationale for administering vitamin K at birth. In the preterm neonate, the total blood volume is further reduced, the red blood cells have an even shorter life span (approximately 30-40 days) and there is a more rapid decline in HB. Clotting times are also more prolonged. 3) Immune system Neonates have reduced or immature specific and non-specific immune systems. For example... Specific immunity is via the action of B and T lymphocytes which form immunoglobulins against certain diseases or antigens. Page | 2 J Petty Immunoglobulins such as IgM and IgG start to be produced in utero in mid trimester but the full quantity required for c omplete immuno-protection is not reached until at least 1 year of age. IgG can cross the placenta but then the levels will fall in the first year of life rendering the neonate prone to infection. Protection against certain diseases and general defences tak e at least the first year to build up as immunoglobulins are produced and specific antibodies are formed against certain antigens. Immunisations are essential to further assist with the build of defences against specific conditions. Prematurity means tha t neonates will fail to receive the transfer of IgG across the placenta during the last trimester and will be further immune -compromised. Non- specific immunity refers to the action of granulocytes which engulf bacteria and the release of Opsonins, chemi cal agents necessary for phagocytosis. Neonates have a less effective phagocytic action as well as impaired opsonic activity with relatively reduced complement levels. 4) Digestive System The digestive tract is structurally complete at term but functional ly immature due to reduced levels of certain digestive enzymes in the first 9 -12 months of life The first feed triggers postnatal changes in the function of the gastro -intestinal tract so that hormones are then stimulated and the gut matures. Production o f natural skin flora also commences at this stage, essential to protect the gut from infection and a source of natural vitamin K. Gastric acid production is lowered with relatively prolonged rate of gastric emptying The cardiac sphincter of the stomach is often weak predisposing the neonate to gastro-oesophageal reflux. Until feeding is established, the healthy newborn is able to cope with the sudden cessation of glucose via the placenta by relying on alterative substrates such as ketones for brain energy and metabolism. Following this metabolic adaptation and once feeding starts, glucose is obtained from an exogenous source and levels normalise. 5) Hepatic System The term neonate has immature liver and hepatic enzyme systems. This results in a slower rate of metabolism of many endogenous and exogenous substances such as clearance of drugs, breakdown of haem in bilirubin metabolism and reduced formation of clotting factors. Liver metabolism is further slowed, the lower the gestation. Page | 3 J Petty Physiological jaundice is common in the term neonate due to liver enzyme immaturity (e.g. gluconeryl transferase) and even more so in prematurity. A delay in establishing feeding can lead to slow digestive transit time leading to increased reabsorption of bilirubin back into the hepatic pathway and then circulation. 6) Renal and urinary-genital Systems Kidney development is complete by week thirty-six of pregnancy but the nephrons continue to mature for longer. At birth, there is relatively low glomerular filtration rate (GFR) compared to the older child / adult which does not mature until approximately one year of age. There is therefore, a poorer response to excessive fluid load with the kidneys being unable to tolerate large volumes with a lower ability to concentrate urine, excrete excess acids or solutes or drugs and toxic waste. Kidney immaturity is even greater in the preterm neonate. Increased renal vascular resistance is also present initially. At birth, there is a contraction of the extracellular fluid volume in the immediate postnatal period, another aspect of adaptation to extra-uterine life. 75% of the birth weight is total body water with a higher extracellular to intracellular fluid ratio compared to adults. Neonates lose 10% of their body water is lost in the first week of life demonstrated by postnatal weight loss. External genitals should be fully developed and sex differentiation should be clearly visible at birth. In the term male, the testes should have descended into the scrotum and are palpable with the scrotal skin showing deep rugae. In the female, the labia majora should cover the labia minora and clitoris fully. In the preterm neonate, differences may be seen in the genitalia due to immaturity – for example, the testes may not have descended and the labia majora may not cover the minora, depending on the gestational age. Page | 4 J Petty 7) Thermoregulatory System The neonate has an immature thermoregulatory system and a greater predisposition to heat loss for the following reasons... The hypothalamus, the central control centre for thermoregulation, is immature. This has a central role in non-shivering thermogenesis so this process may be hindered leading to an inability to generate heat physiologically. Neonates have a high surface area to volume ratio with relatively large heads, a site of potentially high heat loss via exposure to cooler environmental air temperature or convective air currents, from radiation towards cooler surfaces and through direct contact with cold surfaces from conduction. In addition, newborns due to their wet skin can easily lose significant and rapid heat through evaporation. The most effective way to prevent heat loss by these means in the healthy, term newborn is to dry, cover the head and place on to the mother’s bare chest for skin- to-skin contact which has proven physiological benefits.
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