Bradycardia in the Lamb Fetus in Response to Circulatory Distress

Abstract

Fetal lambs in the last trimester of pregnancy were delivered by Cesarean section, the placental circulation maintained intact, and breathing prevented. Heart rate and carotid blood pressure were recorded. The reflex bradycardia resulting from temporary occlusion of the umbilical cord was studied. It occurs when the umbilical arteries alone or the umbilical veins alone are occluded. It is abolished (i.e., prompt cardiac slowing) by bilateral vagotomy. When arteries are occluded, there is a rise in blood pressure; when veins are occluded, there is a fall in blood pressure. Reflex bradycardia is not, therefore, dependent on an aortic depressor reflex. Interference with cord venous return (of some 2/3rds of the entire cardiac output) appears to set up reflex action from the large veins which affects the heart rate. The bradycardia response is not modified by exclusion of carotid sinus receptor reflexes or by excision of both stellate ganglia (including the 3rd and 4th sympathetic ganglia on each side).