Cerebrovascular effects of central depressants: a study of nitrous oxide, halothane, pentobarbital and ethanol during normocapnia and hypercapnia in the rat.

The effect of the central depressants nitrous oxide, halothane, pentobarbital and ethanol upon cerebral blood flow (CBF), cerebral oxygen consumption (CMRO2) and cerebral vascular reactivity to carbon dioxide were measured using the rapid and repetitive intraarterial 133Xenon injection technique modified for rat studies. The cerebrovascular resistance (CVR) during normocapnia in the pentobarbital group was significantly higher (P less than 0.01) than in the nitrous oxide group thus indicating a vasoconstrictor effect of pentobarbital that may be clinically important, because the ability of barbiturates to contract vessels in healthy brain regions may partly explain the protective properties of these drugs against cerebral ischemia. The results indicated that pentobarbital and ethanol may act synergistically with carbon dioxide in depressing CMRO2 and cerebral vascular reactivity. Finally, it is concluded that nitrous oxide anaesthesia (70% N2O: 30% O2) is suitable as a reference situation in rat studies of the effect of pharmocological agents on CBF, CMRO2 and cerebrovascular reactivity.