Development of autonomic nervous control of basal heart rate was studied in unanesthetized fetal lambs (93 days to term) and newborn lambs (2–29 days), using atropine and/or propranolol blockade. Fetal lambs showed a progressive increase in parasympathetic restraint of heart rate; vagal influence in the newborn lamb was similar to the term fetus. Sympathetic stimulation of fetal heart rate declined toward term, possibly due to the strongly increasing parasympathetic influence. Sympathetic influence in the newborn was similar to the early-gestation fetus. Intrinsic heart rate was about 185 beats/min throughout the fetal and newborn life span studied. Thus changes in basal heart rate resulted from a different balance of the sympathetic and parasympathetic components of the autonomic nervous outflow.

The immediate transient baroreceptor sensitivity was measured in 9 conscious fetal and 7 conscious newborn lambs for periods of at least 35 days following bolus injections of phenylephrine (20–50 μg/kg). Mean sensitivities were unchanged throughout gestation from 105 days at 6.7 ± 0.4 msec/cm H₂O (n = 45) and were insignificantly different from those in the newborn period, 5.9 ± 0.4 msec/cm H₂O (n = 78). In contrast, baroreflex sensitivities were less in 2 fetuses and 2 newborn lambs when pressures were increased by chronically implanted thoracic aortic balloon cuffs; they were 3.03 ± 0.11 (n = 127) and 0.91 ± 0.11 msec/cm H₂O (n = 61), respectively. ‘Steady-state’ heart period-arterial pressure curves indicate that the baroreflex operates down to levels of 40 cm H₂O in the fetus which is lower than that achieved in the adult of other species, rabbit and man.

Autonomic nervous control of heart rate (HR) during hypoxia was studied longitudinally using 9 chronically catheterized fetal lambs (109 day to term) and 10 newborn lambs (2–28 days old). Changes in heart rate (ΔHR) during hypoxia were age-dependent. Before 120 days of gestation ΔHR was insignificant, but between 120 days to term bradycardia occurred. The newborn response was marked tachycardia. Autonomic influences on HR were quantified using atropine and propranolol blockade. In fetal lambs, antagonistic increases in parasympathetic and sympathetic outflows were evident during hypoxia. In hypoxic lambs 120 days to term, net bradycaria reflected predominant parasympathetic cardio-deceleration; before 120 days of gestation both the parasympathetic and sympathetic outflows increased, but no net ΔHR occurred. In hypoxic newborn lambs, sympathetic and parasympathetic changes contributed synergistically to the net tachycardia. Thus the pattern of autonomic control of HR during hypoxia differs in fetal and newborn lambs. Changes in sympathetic and parasympathetic influences are antagonistic in the fetus, but synergistic in the newborn.

Respiratory activity (diaphragmatic electromyogram) was recorded in six unanesthetized in utero fetal lambs, between 0.7 of gestation and term. Respiratory patterns generated by the fetus showed developmental changes that included 1) an emergence of a periodic modulation of respiratory rate producing alternating active and quite phases (mean cycle length of 37 min between 130 and 140 days' gestation; 2) an increase in percentage apnea (expiratory time greater than 10 s) from 20% at 110 days to 60% at 140 days; and 3) a linear decrease in the 2-h average respiratory rate, while mean rate during active phases showed no consistent gestational decline. Electrocortical and electroocular activity was monitored in three of six fetuses; however, discrete sleep state patterns could not be consistently identified. The results demonstrate a gestational change in the respiratory patterns of the developing fetus and suggest an orderly maturation of the mechanisms controlling respiratory neuronal output.