The physiology of the neonate is best considered in relation to the various body systems. Some of these are considered elsewhere in this book, but a brief summary of the main points follows. Physiological factors that result in specific differences in drug handling by the neonate are also considered here. It should be remembered that functioning of the neonate’s organ systems is closely related to the gestation at which it is born. Finally, factors that are important in the fetus may be equally important after birth; thus many of the following points refer to both the fetus and the neonate.
Circulatory system
In the fetus, oxygenated blood returning from the placenta is directed through the foramen ovale via the left atrium into the left ventricle, and thence preferentially to the brain. Deoxygenated blood from the brain passes via the right atrium and ventricle into the pulmonary artery; since the pulmonary vascular resistance is high, the blood passes through the ductus arteriosus into the aorta and thence via the two umbilical arteries (arising from the internal iliac arteries) to the placenta (Figure 159.1).
At birth, the systemic vascular resistance increases as the umbilical arteries close, whereas the pulmonary vascular resistance decreases as air is drawn into the lungs. Thus, the circulation takes up the adult pattern, although the circulation remains transitional for
Figure 159.1 Diagram of fetalcirculation. IVC, inferior vena cava; SVC, superior vena cava. Arrows denote flow of blood. Figures refer to the approximate oxygen saturation.
about 2 weeks in term neonates, and fetal circulation may persist if pulmonary vascular resistance remains high (e.g. caused by hypoxaemia, acidosis, hypovolaemia or hypothermia). The neonate relies mainly on heart rate for maintenance of cardiac output, the stroke volume being relatively fixed.
Respiratory system
Two-thirds of the pulmonary fluid is expelled from the chest by compression during delivery (reduced in caesarean section and if the neonate is small) and the remaining fluid is rapidly absorbed. Lung inflation is important to assist transition from the fetal to the adult circulation and to promote pulmonary surfactant production. Surfactant is required to enable alveolar expansion and is present in only small amounts up until about 34 weeks’ gestation, although its production can be stimulated by maternal steroid therapy.
Fetal haemoglobin (containing a and у chains) comprises about 80% of the circulating haemoglobin in the fetus. Its oxyhaemoglobin dissociation curve is shifted to the left compared with that of adult haemoglobin; thus transfer of oxygen from maternal to fetal blood is encouraged. Fetal haemoglobin normally persists for about 2-3 months after delivery, but this may be extended if there is a haemoglobinopathy affecting adult haemoglobin.
The response of the neonate to hypoxia is discussed in Chapter 160, Neonatal resuscitation.
Neurological system
The blood-brain barrier is generally accepted to be less complete in the neonate than in the adult, making the neonate more susceptible to depressant drugs such as opioids, and its function may be compromised further in the presence of hypoxia or acidosis. In premature babies, the fragile periventricular vessels are susceptible to fluctuations in arterial blood pressure and hypoxia, resulting in intraventricular haemorrhage.
Other systems
Heat production in the neonate is achieved by oxidation of brown fat, which results in increased oxygen requirements. It may be inadequate if there has been fetal growth restriction. Thus, a warm environment for delivery is especially important.
Pharmacology
Uptake of drugs by the fetus is considered in Chapter 16, Placental transfer of drugs. The low plasma protein-binding capacity of the fetus and neonate results in a greater amount of free drug in the plasma compared with that in the adult. Lipid-soluble drugs (e.g. anaesthetics) are extensively bound to fetal and neonatal tissues, offsetting this effect. Finally, fetal acidosis may result in the ‘trapping’ of drugs, which may persist postpartum if the neonate remains acidotic. The relative immaturity of both target organs (e.g. brain) and organs involved in metabolism (e.g. liver) makes the neonate more susceptible to many drugs administered to the mother before delivery or directly to the neonate after delivery.
Key points
• Major circulatory and respiratory changes occur at birth.
• The neonate may exhibit the effects of intrapartum insults, and remains susceptible to insults occurring postpartum
Further reading
Morton S, Brodsky D. Fetal physiology and the transition to extrauterine life. Clin Perinatal 2016; 43: 395-407.