Brain-surface Oxygen Tension and Cerebral Cortical Blood Flow during Hemorrhagic and Drug-induced Hypotension in the Cat

Abstract

Cerebral blood flow (CBF) is higher during drug-induced hypotension than during hypotension resulting from hemorrhage and, among the hypotensive drugs, CBF may be higher during nitroprusside (SNP) than during trimethaphan (TMP) administration. Increased perfusion of the brain does not guarantee better tissue O2 supply, since perfusion of the capillary microcirculation may be inhomogeneous. Minute areas of ischemia may exist while CBF values are normal. Regional cerebral blood flow (rCBF) was measured in circles of parietal cortex .apprx. 1.5 cm in diameter in the cat. O2 tension values (PBO2) were measured in minute areas of cortical surface by use of 15 .mu.m P O2 electrodes. Control measurements were made during light anesthesia; blood pressure (BP) was then decreased to 30-35 torr by hemorrhage, TMP, or SNP. In the animals rendered hypotensive with TMP or SNP, a .beta. blocker, practolol, was administered. rCBF was higher with SNP (68 ml/100 g per min) than with TMP (45 ml/100 g per min). PBO2 values showed a marked hypoxic shift during hemorrhagic hypotension, but no shift from control during SNP-induced hypotension. The PBO2 pattern with TMP was intermediate between the SNP and hemorrhage patterns. PBO2 were less than 10 torr in 25% of cortical areas examined during hemorrhage, 12 and 7% of areas examined during TMP-induced hypotension (2 studies) and only 1% of areas examined during SNP-induced hypotension. These results agree with those of others in showing higher rCBF during SNP-induced than during TMP-induced hypotension. The PBO2 values showed that the well-maintained cerebral perfusion during SNP administration was associated with homogeneous perfusion of the microcirculation. Brain tissue O2 availability is greater during SNP-induced than during either TMP-induced or hemorrhagic hypotension.