The effect of combined hypoxemia and cephalic hypotension on fetal cerebral blood flow and metabolism

The effect of hypoxemia and cephalic hypotension, alone and in combination, on hemispherical CBF and metabolism was examined in seven chronically catheterized fetal sheep. Hypoxemia was induced by lowering the maternal inspired oxygen fraction and cephalic hypotension was generated by partial occlusion of the fetal brachiocephalic artery. CBF was measured with radionuclide-labeled microspheres. During control, the arterial blood oxygen content (CaO₂) was 3.2 ± 1.0 (SD) mM and CBF averaged 131 ± 21 (SD) ml min^-1 100 g^-1. The cephalic perfusion pressure (PP, mean cephalic arterial - sagittal venous) was 40 ± 4 mm Hg and cerebral vascular resistance (CVR, PP/CBF) was 0.31 ± 0.06 mm Hg ml^-1 min 100 g. During induced hypoxemia, CaO₂ was 1.4 ± 0.7 mM and CBF was elevated to 223 ± 60 ml min^-1 100 g^-1. PP was not different from control and CVR was lower at 0.19 ± 0.04 mm Hg ml^-1 min 100 g, reflecting cerebral vasodilation. With cephalic hypotension alone (PP = 21 ± 4 mm Hg; CaO₂ = 3.4 ± 0.9 mM), CBF fell to 83 ± 23 ml min^-1 100 g^-1 and there was no significant change in CVR (0.26 ± 0.05 mm Hg ml^-1 min 100 g). During combined hypoxemia and hypotension (CaO₂ = 1.5 ± 0.8 mM and PP = 18 ± 4 mm Hg), CBF was significantly greater than during hypotension alone (100 ± 6 ml min^-1 100 g). CVR was 0.19 ± 0.05 mm Hg ml^-1 min 100 g, identical to that measured in normotensive hypoxemia and significantly less than found during hypotension alone. Cerebral oxygen consumption was lower during combined hypoxemia and cephalic hypotension than during hypoxemia alone. Cerebral glucose uptake was significantly higher than control in both the hypoxemic and combined hypoxemic-hypotensive conditions. The glucose:oxygen quotient (6 x molar glucose uptake/molar oxygen consumption) was not different from unity during control or hypotension but was 2.31 ± 1.16 and 3.63 ± 1.99 during the hypoxemic and hypoxemic-hypotensive conditions, respectively, suggesting an anaerobic glucose utilization. No significant lactate efflux could be measured in any of these conditions.