NITROUS OXIDE-ISOFLURANE ANESTHESIA CAUSES MORE CEREBRAL VASODILATIONTHAN AN EQUIPOTENT DOSE OF ISOFLURANE IN HUMANS

To compare the cerebral vascular and metabolic effect of an isoflurane -nitrous oxide mixture to an equipotent dose of isoflurane at 1.1 mini mum alveolar anesthetic concentration (MAC), and to study the interact ion between nitrous oxide and isoflurane anesthesia, we measured right middle cerebral artery blood flow velocity (V-max) and cerebral arter iovenous oxygen content difference (AVDO(2)) in six healthy patients d uring normocapnia and normothermia under the following sequence of ste ady-state anesthetic conditions: Condition A, 0.5 MAC of isoflurane, C ondition B, 0.5 MAC of isoflurane + 0.6 MAC of N2O, Condition C, 1.1 M AC of isoflurane + 0.6 MAC of N2O, and Condition D, 1.1 MAC of isoflur ane. The study entry sequence was randomized. V-mca and AVDO(2) during 1.1 MAC of isoflurane (Condition D) was 48 +/- 7 cm/s and 3.9 +/- 0.6 vol%, respectively. Substituting 0.6 MAC of isoflurane with an equipo tent concentration of N2O (Condition B) resulted in an increase in bot h V-mca and AVDO(2) of approximately 20% (P < 0.05). These findings su ggest that the increase in flow was accompanied by an even greater inc rease in metabolic rate. Adding 0.6 MAC of N2O to 1.1 MAC of isofluran e (Condition C) also increased V-mca (P < 0.05). We conclude that N2O is a more potent cerebral vasodilator than an equipotent dose of isofl urane alone in humans.