Effects of cephalic hypotension, hypertension, and barbiturates on fetal cerebral flood flow and metabolism.

The effect of altered perfusion pressure (cephalic arterial pressure minus sagittal venous pressure) on cerebral blood flow was examined in chronically catheterized fetal sheep under ganglionic blockade. Perfusion pressure was decreased by partial inflation of a brachiocephalic artery occluder and increased by an intravenous angiotensin II infusion. Cerebral flood flow was measured with radiolabeled microspheres. On average, 45% +/- 3% (SEM) reductions in perfusion pressure were accompanied by 49% +/- 4% decreases in cerebral blood flow. Increases in perfusion pressure of 42% +/- 3% were associated with 45% +/- 7% increments in cerebral flood flow. The cerebral oxygen consumption was not significantly altered in either case. We also investigated the effect of pentobarbital on fetal cerebral oxygen consumption and cerebral blood flow at normal and at altered perfusion pressure. At normal perfusion pressure pentobarbital caused a 27% +/- 5% decrease in cerebral oxygen consumption that was associated with a 44% +/- 4% decrement in cerebral blood flow and a slight widening of the arteriovenous oxygen content gradient. In this hypometabolic cerebral state, alterations in perfusion pressure were again accompanied by proportional changes in cerebral blood flow. Both perfusion pressure and the cerebral oxygen consumption appear to be important determinants of fetal cerebral blood flow.