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.

Keywords:
Baroreceptor-heart rate reflex, Autonomic nervous system, Development of circulatory control, Fetal maturation, Gestation, Heart rate, Blood pressure
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© 1977 S. Karger AG, Basel
1977
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