Cardiovascular responses to hypoxemia and acidemia in fetal lambs

Circulatory responses to hypoxemia and acidemia were studied in 10 fetal lambs in utero with gestational ages of 122 to 142 days. Vinyl catheters were placed in fetal and maternal vessels, and the fetuses were studied 2 to 5 days postoperatively. Fetal heart rate, arterial pressure,Po2,Pco2, and pH were measured during a control period and while the standing ewe breathed 6 per cent oxygen and 3 per cent carbon dioxide through a plastic bag over its head. Fetal cardiac output and distribution and absolute organ blood flows were calculated from injections of 15μ nuclide-labeled microspheres, during the control and hypoxemic states. One group of 5 fetuses only became hypoxemic (meanPo2 12 and mean pH 7.36), but the other 5 fetuses also developed acidemia (meanPo2 12 and mean pH 7.28). Fetal arterialPco2 values were normal throughout. During hypoxemia, fetal arterial pressure increased, and fetal heart rate decreased. Although cardiac output fell in all but one fetus, the decrease was significant only in the acidemic group. Blood flow to the fetal body decreased in all, but the change was significantly greater in the acidemic group. Umbilical blood flow was maintained in all fetuses during hypoxemia. The per cent distribution of cardiac output to the placenta rose from 41 to 48 per cent and from 41 to 57 per cent in the hypoxemic and acidemic groups, respectively. Blood flow to the brain, heart, and adrenals increased two- to three-fold in all fetuses during hypoxemia while pulmonary, renal, splenic, gut, and carcass flows decreased. The changes were of greater magnitude in fetuses with combined hypoxemia and acidemia. These studies quantitate the fetal circulatory changes that occur in unanesthetized fetal lambs in utero during maternal hypoxemia.