NEONATAL PHYSIOLOGY

Neonates are not miniature adults. Their physiology is unpredictably different from adults. The neonatal period is the physiologic transition period from the aquatic, self-contained fetal life with its full dependence on maternal nutrition, waste handling and environmental homeostasis to independent extrauterine life where birth and breaking the umbilical cord literally means the neonate must adapt its physiology to provide all the necessities of independent life. The term “neonate” is derived from the Latin natus (to be born) and refers to a newborn during the first weeks of life during the physiologic transition. Although strictly speaking it should encompass the entire period until the transition is complete for all organ systems, by convenience it is usually defined as the first 3 to 4 weeks of life in most domestic species. The physiology of the fetus is much different in very many aspects than the physiology of the adult. At times the fetal physiology seems counterintuitive to those familiar with adult physiology. For instance we understand the logic of the adult kidney producing concentrated urine in response to hypovolemia as it will help maintain vascular volume. But when the fetus produces concentrated urine which may increase the osmolarity of the fetal fluids it may not only tend to prevent reabsorption of the fluids, it may actually draw more fluid from the fetus into this fluid reserve leading to a negative effect on volemia. On the other hand when the fetus produces dilute urine the resulting decrease in fetal fluid osmolarity tends to enhance reabsorption of fetal fluids by the fetus having a positive effect on volemia.1 Another example is the fetal heart rate response to hypoxemia. In the adult, hypoxemia stimulates tachypnea and tachycardia as the physiology adjusts in an attempt to deliver more oxygen to tissues. But in the fetus hypoxemia results in bradycardia. This bradycardia is a logical adaptation to hypoxemia. Unlike the adult who can bring more oxygen in contact with blood by increasing its minute ventilation, the fetus has no way to communicate to the mother that it needs an increase in maternal placental perfusion. The fetus meets this challenge by maximizing perfusion of fetal placenta and by increasing vascular tone directing blood flow to vital organs. But this increase in afterload will increase cardiac work and thus oxygen demand. By slowing its rate the heart adapts to the new circulatory pattern but simultaneously requires no more oxygen than it required before the hypoxemia. Although these examples are of fetal physiology, the neonate has a strong memory of fetal physiology. Disease states may slow the transition to pediatric physiology and even when the neonate has made a complete transition, with stress, the neonate may revert to the more familiar fetal physiology. These examples emphasize the importance of understanding fetal and neonatal physiology to appreciate the neonate’s response to disease and more importantly to predict its response to our therapeutic interventions. My clinical experience is with foals, calves, kids, lambs and crias which has allowed me to temper information from experimental studies in these species and comparative information from studies in other species, emphasizing what is clinically relevant. But as I have no clinical experience with puppies or kittens I submit the following ideas for your consideration. Some of the ideas presented here may have little clinical relevance. It should also be fully understood that although superficially it appears that the organ systems have similar maturational patterns between species with the major difference being when in that maturational process the fetus is born, the unpredictable impact of interspecies differences on organ development and maturational processes makes extrapolation of data obtained in one animal species describing fetal and neonatal physiology responses to other animal species error prone.