Fetal heart rate and blood pressure during prolonged partial asphyxia in the rhesus monkey

Fetal heart rate (FHR) and fetal blood pressure (FBP) were recorded continuously in 20 anesthetized fetal monkeys near term while their oxygenation was progressively decreased by a stepwise reduction in uterine blood flow. Fetal blood was sampled periodically from the carotid artery and analyzed for pH and PO2. Hemoglobin saturation with oxygen (S) was derived from nomogram. During the study period, uterine contractions were either minimal or absent. FHR remained constant as long as S was greater than 55%. Below that it began to decline by 0.34%/% saturation. When S was less than 31%, FHR fell by 0.93%/% saturation. The correlation between FHR and PO2, and FHR and pH, was less consistent. Arterial FBP was maintained over the entire range of pH, PO2, and S. It is concluded that, under the conditions described, an unequivocal decrease in mean FHR signals a substantial reduction in the availability of O2 to the fetus.