Cyclic variation in fetal heart rate and sympathetic activity.

The effects of alpha- and beta-adrenergic blockade on electrocortical activity-related cyclic variability in fetal heart rate and mean arterial pressure were investigated in eight fetal lambs between 119 and 138 days of gestation. In the absence of adrenergic blockade, fetal heart rate during high-voltage slow activity was significantly higher than that during low-voltage fast activity. Propranolol (2.0 mg/hr) produced a decrease in fetal heart rate, but the decrease was only statistically significant during high-voltage slow activity. Phentolamine (5.0 mg/hr) induced a significant increase in fetal heart rate with a small but statistically significant decrease in mean arterial pressure. The cyclic variability in fetal heart rate between low-voltage fast activity and high-voltage slow activity was abolished by propranolol and enhanced by phentolamine. These results indicate that the cyclic variability in the fetal heart rate associated with electrocortical activity can be accounted for by cyclic fluctuation in sympathetic activity.