Age-Dependent Pattern of Autonomic Heart Rate Control during Hypoxia in Fetal and Newborn Lambs

Abstract

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.

Maternally Administered Dexamethasone at 0.7 of Gestation Suppresses Maternal and Fetal Pituitary and Adrenal Responses to Hypoxemia in Sheep

“…Akagi et al (30) also demonstrated that maternal and fetal hypoxemia could be induced in sheep at 125 dGA by reducing maternal inspired O 2 content without inducing hypertension. Several studies have shown that late in gestation, fetal hypoxemia does elevate fetal blood pressure (23,24,26,27,36). In our study, hypertension supervened after hypoxemia in the DM-treated but not control fetuses at 105 dGA, producing the response characteristic of a more mature fetus.…”

“…However, relatively little is known about fetal responses to acute hypoxemia before 120 dGA. Cardiovascular responses of fetal sheep to hypoxemia before 100 dGA are different from those of fetal sheep at Ͼ100 dGA (26,27). In addition, gestational changes in organ blood flow (28) and neurohormonal regulation (29) suggest that the responses to acute hypoxemia in fetuses Ͻ120 dGA would be different from those Ͼ120 dGA.…”

Development of the ovine fetal cardiovascular defense to hypoxemia towards full term

“…Release of epinephrine (E) into the fetal circulation opposes vagal tone and returns FHR to basal levels (10). The fetal peripheral vasoconstriction is maintained by increased release of vasoactive agents into the fetal circulation, such as catecholamines, neuropeptide Y (NPY), and AVP (1,8,20,27,50,51,55,56,59).Although the cardiovascular responses to acute hypoxemia or asphyxia are known to change between mid and late gestation (7,30,33,34,64,65), no study has investigated developmental changes in cardiovascular responses to hy- …”

“…Although the cardiovascular responses to acute hypoxemia or asphyxia are known to change between mid and late gestation (7,30,33,34,64,65), no study has investigated developmental changes in cardiovascular responses to hy-…”

“…Although the cardiovascular responses to acute hypoxemia or asphyxia are known to change between mid and late gestation (7,30,33,34,64,65), no study has investigated developmental changes in cardiovascular responses to hy-poxemia after 130 days, when fetal basal cortisol concentrations rise. This study, therefore, tested the hypothesis that the fetal cardiovascular defense to hypoxemia matures close to full term, in association with the prepartum surge in fetal basal cortisol.…”