Baroreflex Activity in Conscious Fetal and Newborn Lambs

Abstract

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 H<sub>2</sub>O (n = 45) and were insignificantly different from those in the newborn period, 5.9 ± 0.4 msec/cm H<sub>2</sub>O (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 H<sub>2</sub>O (n = 61), respectively. ‘Steady-state’ heart period-arterial pressure curves indicate that the baroreflex operates down to levels of 40 cm H<sub>2</sub>O in the fetus which is lower than that achieved in the adult of other species, rabbit and man.