Fetal blood volume responses to acute fetal hypoxia.

The purpose of this study was to explore the effects of acute hypoxia on the volume of blood circulating in the fetus. The oxygen content of air inspired by chronically catheterized, near-term pregnant sheep was reduced and supplemented with carbon dioxide in an attempt to maintain fetal carbon dioxide tension constant. Control fetal arterial oxygen tension averaged 22.3 +/- 0.7 (SE) mm Hg. During a 30-minute hypoxic period, reductions in fetal arterial oxygen tension by 2.4 to 12.5 mm Hg in individual animals were associated with decreases in fetoplacental blood volume of 1.1% to 14.3%. On the average (n = 14), fetal blood volume decreased by 6.7% +/- 0.8% when oxygen tension decreased by 7.7 +/- 0.9 mm Hg. Both arterial and venous pressures increased significantly during the hypoxic period. Upon the return to breathing of room air, there was a transient overshoot of fetal PO2 of 1.0 +/- 0.4 mm Hg at 10 minutes, but fetal blood volume did not return to normal until 30 minutes after the hypoxia. Thus, the data suggest that acute fetal hypoxia causes rapid and sustained reductions in fetal blood volume whereas volume returns to normal only slowly after elimination of the hypoxia.