Effects of birth on baroreceptor-mediated changes in heart rate variability in lambs and fetal sheep.

1. In adult unanaesthetized sheep, there is a V-shaped relationship between mean arterial pressure (MAP) and heart rate variability (HRV), measured in both time and frequency domains. In contrast, in fetal sheep, there is only a positive direct relationship between MAP and HRV, which is determined by the cardiac vagus. We postulated that by the time lambs were 8-10 days old, the 'adult like' V-shaped relationship between MAP and HRV would be present and it may appear at or after birth. To test these hypotheses, experiments were performed in six chronically catheterized fetal sheep (aged 132-138 days gestation), 10 newborn sheep (within 10 h of birth) and 10 lambs (aged 8-10 days). The relationships between MAP and HRV (in both time and frequency domains) were studied before and during beta-adrenoreceptor blockade with propranolol and before and during cardiac vagal blockade with atropine. 2. In 8-10-day-old lambs, V-shaped relationships between MAP and HRV (measured in both time and frequency domain) were obtained. The negative limb of this V-shaped relationship between MAP and HRV was present after cardiac vagal blockade. The positive slope of the V was present after beta-adrenoreceptor blockade. 3. In 4-h-old newborn sheep, there was no relationship between MAP and HRV (measured in the time domain), but between 7 and 10 h of age a negative relationship was found during treatment with atropine and a positive relationship was found during beta-adrenoreceptor blockade, when HRV was measured in both time and frequency domains. 4. As described previously, there was only a positive relationship between MAP and HRV in fetal sheep, which was abolished by atropine but not affected by beta-adrenoreceptor blockade. 5. Thus, until relatively late in fetal life, baroreceptor- modulated changes in efferent cardiac sympathetic tone, determined by measuring the effects of autonomic blockade on HRV, could not be elicited, although reflex vagal pathways were active. By 7-10 h after birth, baroreceptor-modulated changes in efferent cardiac sympathetic tone were present. It was possible using measurements of HRV made in the time domain to show that these baroreceptor-modulated cardiac sympathetic effects became stronger over the first 10 days of life (P < 0.01). These studies are the first to show that the influence of baroreceptor-mediated changes in cardiac sympathetic tone on HRV increases in early life. This is probably because maturation of sympathetic innervation of the fetal sheep heart is occurring at this age.