The effect of isoflurane-induced hypotension on cerebral blood flow and cerebral metabolic rate for oxygen in humans.

Deliberate hypotension was induced with isoflurane (mean inspired concentration 2.3 +/- 1.0%) in 12 patients undergoing craniotomy for clipping of cerebral aneurysms. Global cerebral blood flow (CBF) was measured before, during, and after hypotension. Arterio-venous O2 content difference was measured concomitantly, and the cerebral metabolic rate for oxygen (CMRO2) was calculated from these data. Mean arterial pressure (MAP) was reduced from 78 +/- 5 mmHg to 51 +/- 7 mmHg and then returned to 82 +/- 8 mmHg. Mean CBF before hypotension was 49 +/- 14 ml X 100 g-1 X min-1 and was unchanged during (45 +/- 12 ml X 100 g-1 X min-1) and after (49 +/- 15 ml X 100 g-1 X min-1) hypotension. The CMRO2 before hypotension was 2.0 +/- 0.6 ml X 100 g-1 X min-1. This was statistically significantly (P less than 0.025) reduced to 1.5 +/- 0.5 ml X 100 g-1 X min-1 during hypotension and then returned to 2.2 +/- 0.6 ml X 100 g-1 X min-1 on return to normotension. This indicates that the global cerebral O2 supply-demand balance was favorably influenced by isoflurane. No complications could be attributed to the hypotensive technique. We conclude that, with regard to global cerebral oxygenation, isoflurane is a safe agent with which to induce hypotension during neurosurgery.