EFFECTS OF NITROUS OXIDE ON CEREBRAL METABOLIC RATE IN RATS ANAESTHETIZED WITH ISOFLURANE

It has been demonstrated in several species that a significantincrease in cerebral blood flow (CBF) occurs when nitrous oxideis added to a volatile anaesthetic. This blood flow responsecould result from an increase in cerebral metabolic rate orfrom a direct effect of nitrous oxide on cerebral vessels. Toinvestigate this, the cerebral metabolic rate for glucose (CMRglu)was determined autoradiographically in rats receiving isofluraneanaesthesia with or without nitrous oxide. An increase in anaestheticdepth from 0.5 to 1.0 MAC achieved with isoflurane alone causeda significant reduction in CMRglu (52 (SD11) µmol/100gmin^–1 vs 39 (8) µmol/100 g min^–1). In contrast,the addition of 70% nitrous oxide (0.5 MAC) to 0.5 MAC isofluraneanaesthesia (1 MAC total) left CMRglu unchanged (54 (4) µmol/100g min^–1). We conclude that 70% nitrous oxide does notalter cerebral metabolic rate when administered with 0.5 MACisoflurane. Because CBF increases substantially under very similarconditions, our data indicate that the CBF effects of nitrousoxide, when administered with a volatile agent, were directand mediated by factors other than changes in cerebral metabolicrate. Presented at the American Society of Anesthesiologists AnnualMeeting 1989, New Orleans, U.S.A.