Control of Baseline Heart Rate in the Fetal Lamb

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Abstract
To investigate the mechanisms controlling baseline fetal heart rate (BFHR) at different gestational ages (GA) and behavioral sleep states, 10 chronically instrumented (nonvagotomized, control) and 9 vagotomized fetal lambs were used. BFHR was assessed in active and quiet behavioral states at 124–147 days gestation. Fetal electrocorticogram, electrooculogram, and breathing movements were used to distinguish behavioral states. Inverse relationships were found between BFHR and GA for both states in the control group, with a steeper regression slope during active sleep (P < 0.001). Vagotomized BFHR decreased with GA and the regression lines for both states nearly coincided. In active states before 135 days, BFHR in the controls was significantly higher than vagotomized BFHR. After 135 days, control BFHR was significantly lower than vagotomized BFHR [P < 0.001 (< 135 days), P < 0.005 (> 135 days)]. During quiet sleep, control BFHR was greater than vagotomized BFHR throughout the entire gestational period [P < 0.001 (< 135 days), P < 0.05 (> 135 days)]. These results suggest that in the active state, vagal influence on fetal heart rate is enhanced after 135 days. During quiet sleep, vagal activity remains low before and after 135 days. In addition, sympathetic activity appears to be diminished in the vagotomized sheep.

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