Physiology of transition from intrauterine to extrauterine life

Abstract

The transition from fetus to newborn is the most complex adaptation that occurs in human experience. Lung adaptation requires coordinated clearance of fetal lung fluid, surfactant secretion, and onset of consistent breathing. The cardiovascular response requires striking changes in blood flow, pressures, and pulmonary vasodilation. Energy metabolism and thermoregulation must be quickly controlled. The primary mediators that prepare the fetus for birth and support the multiorgan transition are cortisol and catecholamine. Abnormalities in adaptation are frequently found following preterm birth or cesarean delivery at term, and many of these infants need delivery room resuscitation to assist in this transition.

Figure 1

Catecholamine response to delivery of term lambs, preterm lambs and term lambs following adrenalectomy. Fetal term (145±2 day gestation) and preterm (130±1 day gestation) lambs were delivered at 0 time following fetal catheter placement. The adrenalectomy lambs had adrenal glands removed at 138±1 days and received continuous cortisol supplemental until delivery at 142 days gestation. Epinephrine and norepinephrine values are expressed relative to the values measured 10 min prior to delivery. A. Epinephrine increased about 12 fold over the -10 min value for the term lambs, and this increase was ablated by adrenalectomy. The increase in epinephrine was much larger for the preterm lambs. B. There was a similar pattern for the norepinephrine responses. C. Blood pressure increased in term and preterm animals but not in term adrenalectomized animals. D, E. Glucose and free fatty acids in blood increased more for term than preterm lambs with minimal increases following adrenalectomy. Data from Padbury et al (6, 8).

Figure 2

Antenatal cortisol alters postnatal catecholamine secretion and blood pressure responses to delivery in preterm lambs. Fetal sheep had vascular catheters placed at 122–125 days gestation, and the fetuses were randomized to a 60 hr cortisol or vehicle infusion at 128 days gestation. The fetuses were delivered and supported on mechanical ventilation. During transition, epinephrine (A) and norepinephrine (B) increased more in control lambs than in cortisol-exposed lambs. Nevertheless, blood pressure (c) was higher in the cortisol-exposed newborns than the control animals. Data from Stein et al (11).