Hemodynamic Changes in Fetal Lamb in Utero in Response to Asphyxia, Hypoxia, and Hypercapnia

The effects of hypoxia (administration of 10 per cent O 2 in N 2 and constriction of the uterine arteries), of hypercapnia (administration of 8 per cent CO 2 in air), and of asphyxia (obstruction of the umbilical circulation) were studied on the fetus in utero in near-term pregnant ewes under spinal anesthesia. Hypoxia and hypercapnia produced increases in the maternal arterial pressure, maternal heart rate, and uterine blood flow. Fetal arterial pressure and carotid and umbilical blood flows increased, while the fetal heart rate decreased. Fetal femoral flow decreased during hypoxia and increased during hypercapnia. There was a delay of five to six minutes between the initiation of hypoxia or hypercapnia and the appearance of fetal circulatory changes. Transient reduction of uterine blood flow evoked insignificant hemodynamic alterations in the mother and fetus. A reduction of flow greater than 50 per cent of control values produced a fetal bradycardia along with increases in fetal arterial pressure, carotid and umbilical flows. Constriction of the umbilical veins with the umbilical arteries intact produced a prompt bradycardia together with a decrease in fetal arterial pressure and regional blood flows. Constriction of the umbilical arteries alone also produced a bradycardia with an initial rise in arterial pressure and carotid flow. Similar changes were observed after cutting the umbilical cord. Elimination of the low resistance of the placenta played a major role in these changes. Placing the lamb in the head-down position caused a marked increase in carotid blood flow without greatly modifying arterial pressure.