CAROTID, NOT AORTIC, CHEMORECEPTORS MEDIATE THE FETAL CARDIOVASCULAR-RESPONSE TO ACUTE HYPOXEMIA IN LAMBS

The fetal cardiovascular response to acute hypoxemia consists of a decrease in heart rate, a variable change in mean arterial pressure, and an increase in peripheral vascular resistance. This response is mediated by the arterial chemoreceptors. To determine whether chemoreceptors in the carotid artery or in the aorta mediate the fetal cardiovascular response to acute hypoxemia, we studied the response to acute hypoxemia in fetal lambs at 125 to 130 d of gestation after selective carotid (six fetuses) or aortic (five fetuses) denervation. One to 3 d after insertion of catheters, hypoxemia was induced by inflating a balloon occluder around the ewe's hypogastric artery or by giving the ewe 95% N2 and 5% O2 to breathe. The chemoreflex response was measured as decrease in heart rate per decrease in Hb O2 saturation. To validate our results, we also studied the response to chemical stimulation of the chemoreceptors by injection of sodium cyanide into the inferior vena cava. We found that carotid denervation abolished the heart rate and peripheral vascular resistance responses to hypoxemia but that aortic denervation did not. Responses after injection of sodium cyanide were similar to those seen during acute hypoxemia. We conclude that the carotid chemoreceptors, and not the aortic chemoreceptors, mediate the fetal cardiovascular response to acute hypoxemia.