The effect of brief episodes of diminished uterine blood flow on breathing movements, sleep states and heart rate in fetal sheep.

Responses to brief episodes of occlusion of the middle uterine artery were examined in six fetal sheep in utero between 122 and 140 days of gestation. Continuous recordings were made of fetal breathing movements, electro-cortical (ECoG) and electro-ocular (EOG) activity, arterial pressure and heart rate. Five minutes of unilateral or bilateral arterial occlusion caused fetal hypoxaemia while changes in mean arterial PCO2 and pH were not statistically significant. Occlusions usually led to a cessation of rapid eye movements (REM) and respiratory activity (86% of trials), to the appearance of high-voltage slow waves in the ECoG (84% of trials) and to a fall in heat rate of more than 10% (81% of trials). With the exception of the changes in heart rate, these responses resemble those which frequently occur during non-labour contractions of the uterus. We conclude that the cessation of fetal breathing movements and rapid eye movements and the onset of slow waves in the ECoG seen in association with uterine contractions may be due to the mild hypoxaemia which accompanies them.