Baroreflex activity in conscious fetal and newborn lambs.

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 mug/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 cuff; 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.