Baroreflex control of the circulation in chronically instrumented fetal lambs

The role of the arterial baroreceptors in regulating the fetal circulation was studied in 18 chronically instrumented fetal lambs, 120-136 days gestation. The results were compared to those obtained in three sinoaortic denervated adult sheep. Nine fetuses underwent sinoaortic denervation and nine fetuses served as sham-operated controls. Two to 3 days after surgery, heart rate and blood pressure were recorded continuously and analyzed over 1-hour periods. In the denervated fetuses, marked fluctuations of the arterial pressure and heart rate were observed. Frequency distribution curves for mean arterial pressure and heart rate showed broader and flatter curves in the denervated fetuses. The average mean arterial pressure and heart rate were not significantly different between the two groups. However, the standard deviation as an index of variability of the mean arterial pressure increased from 2.7 +/- 0.9 (mean +/- standard deviation) in the control group to 6.5 +/- 2.4 in the denervated fetuses, and the standard deviation of the heart rate increased from 10.6 +/- 2.1 to 21.5 +/- 5.7. In the adult sheep, mean arterial pressure and heart rate increased on the 1st day after denervation and returned to near control values on the 3rd day. The variability of the mean arterial pressure increased, but that of the heart rate did not change. Fetal cardiac output and its distribution were measured by the radionuclide-labeled microsphere technique. Combined ventricular output was not statistically different between the groups, but the blood flow to the fetal body was 27% higher in the denervated fetuses. The calculated fetal body vascular resistance was 31% lower in the denervated lambs. This study indicates that the arterial baroreceptors are important in regulating variability of the arterial pressure and heart rate in fetal life, and also may have a significant role in maintaining the normal peripheral vascular resistance.