Developmental physiology of the fetus and newborn and physiologic changes related to birthcomma; cerebral adaptationcomma; and apnea

摘要

Dramatic physiologic changes are the hallmark of fetal and neonatal life. Birth itself is accompanied by instantaneous alteration in respiratory and cardiovascular physiology. Recent studies have begun to separate the effects of individual stimuli at birth. Lung distension affects circulatory distribution but does not initiate continuous breathing. Distension and oxygenation further alter blood flow and can stimulate continuous breathing. Umbilical cord occlusion also can initiate continuous breathing. None of these stimuli alone sufficiently explains the physiologic changes that occur at birth. After birth, neurologic injury and apnea are common problems. Doppler ultrasound studies indicate that cerebral blood flow increases over gestation and significantly decreases at birth. Preterm infants have more rapid increases than do fetuses of a comparable age. In addition, autoregulation in preterm infants may occur over a narrow range of blood pressures. Consequently, preterm neonates are predisposed to neurologic injury. Sleep state, oxygenation, and thermal environment all contribute to the regulation of breathing. The effects of these factors in newborns are now being studied, and findings will contribute to our understanding of the pathophysiology of apnea.