Behavioral activity during prolonged hypoxemia in fetal sheep.

Experiments were conducted in unanesthetized, chronically catheterized pregnant sheep to determine the fetal behavioral response to prolonged hypoxemia produced by restricting uterine blood flow. Uterine blood flow was reduced by adjusting a vascular occluder placed around the maternal common internal iliac artery to decrease fetal arterial O2 content from 6.1 +/- 0.3 to 4.1 +/- 0.3 ml/dl for 48 h. Associated with the decrease in fetal O2 content, there was a slight increase in fetal arterial PCO2 and decrease in pH, which were both transient. There was an initial inhibition of both fetal breathing movements and eye movements but no change in the pattern of electrocortical activity. After this initial inhibition there was a return to normal incidence of both fetal breathing movements and eye movements by 16 h of the prolonged hypoxemia. These studies indicate that the chronically catheterized sheep fetus is able to adapt behaviorally to a prolonged decrease in arterial O2 content secondary to the restriction of uterine blood flow.

Circulatory responses to prolonged hypoxemia in fetal sheep.

Experiments were conducted in 11 chronically catheterized pregnant sheep to determine the distribution of blood flow within the fetus during prolonged (48 hours) hypoxemia secondary to the restriction of uterine blood flow. Uterine blood flow was mechanically restricted with a polytetrafluoroethylene vascular clamp placed around the maternal common internal iliac artery such that mean (+/- SEM) fetal arterial oxygen tension decreased from 23.4 +/- 1.9 to 17.3 +/- 0.8 mm Hg at 1 hour of hypoxemia and remained low for 48 hours. There was an initial increase in fetal arterial carbon dioxide pressure from 48.5 +/- 0.9 mm Hg during the control period to 56.2 +/- 2.3 mm Hg at 1 hour; this parameter subsequently returned to control values, whereas base excess showed a transient decrease. Fetal cerebral, myocardial, and adrenal blood flows were significantly increased at 1, 24, and 48 hours of hypoxemia. In contrast, there was no change in nuchal muscle or renal blood flows with hypoxemia of this magnitude. Cotyledonary blood flow increased transiently by 38% at 1 hour of hypoxemia, but was not changed from control at 24 and 48 hours. These experiments demonstrate that the sheep fetus is able to maintain the normal protective circulatory adjustments seen with acute hypoxemia for up to 48 hours in the absence of progressive metabolic acidemia.