Late decelerations and brain tolerance of the fetal monkey to intrapartum asphyxia

Eight monkey fetuses near term were subjected to regulated asphyxia during labor by mechanically constricting the maternal abdominal aorta and diminishing blood flow to the uterus. A magnitude of asphyxia was produced and maintained for an initial three hours that was close to but not sufficient to elicit late decelerations. The asphyxia was then augmented during a fourth hour to cause late decelerations of magnitudes of 5 to 15 per cent of the initial heart rate. After termination of the fourth hour of asphyxia, the fetuses were delivered by hysterotomy and provided intensive care. During the three to nine months of survival after birth, all animals were neurologically intact; on necropsy the brains were free of pathologic changes both grossly and microscopically. These results support the thesis that fetal heart rate monitoring during labor exhibits a sensitivity sufficient to diagnose asphyxia of the fetus of clinical significance before it reaches a magnitude that may cause permanent neurological injury. The results are particularly pertinent to those clinical circumstances where the decreases in intervillous space blood flow brought about by uterine contractions are accentuated due to low maternal blood pressure.