Effect of prolonged hypoxemia on fetal heart rate accelerations and decelerations in sheep

Experiments were conducted in 10 chronically catheterized fetal sheep to determine the effect of 24 hours of hypoxemia in the absence of progressive acidemia on fetal heart rate accelerations and decelerations. Fetal hypoxemia was produced by mechanically restricting uterine blood flow with a vascular clamp placed around the maternal common internal iliac artery. Fetal arterial oxygen tension decreased from 22.3 +/- 1.8 to 17.8 +/- 1.5 mm Hg at 2 hours and remained low for the 24-hour experimental period. Fetal pH decreased from 7.34 +/- 0.01 to 7.20 +/- 0.05 at 2 hours and returned to normal values by 12 hours. No significant change was observed in the number or characteristics of fetal heart rate accelerations or decelerations during the 24-hour control period. There was a significant increase in the number of accelerations from 48 +/- 4 to 63 +/- 4 per hour at 8 hours of hypoxemia followed by a return to control values by 12 hours. There was no significant change in the mean amplitude or duration of accelerations. There was a significant increase in the number of decelerations per hour with an associated increase in the mean amplitude but a decrease in the mean duration of decelerations during the first 16 hours of hypoxemia. We conclude that prolonged hypoxemia in fetal sheep leads to an initial increase in the number of both accelerations and decelerations in fetal heart rate followed by a return to normal patterns indistinguishable from those of the normoexemic fetus.