INTERACTION OF ENFLURANE AND NITROUS-OXID E ON MEDIAN EEG FREQUENCY

The aim of this study was to quantify the interaction of enflurane and nitrous oxide at a constant median EEG frequency. Methods. Thirty pat ients were studied during laparotomies. Nitrous oxide was randomly adm inistered in concentrations of 0, 20, 40, 60, and 75 vol.-% for 10 pat ients for each nitrous oxide concentration. Enflurane vaporizer settin gs were chosen so that the Median EEG frequency was kept constant at 2 -3 Hz. The relationship between nitrous oxide concentrations and the r equired enflurane concentrations was examined with the isobole method. Results. Nitrous oxide decreases the enflurane requirement linearly. Addition of every 10 vol.-% of nitrous oxide decreases the enflurane r equirement by 0.042 vol.-%. The total anaesthetic requirement of enflu rane and nitrous oxide, expressed in terms of previously reported MAC values, increases significantly with increasing nitrous oxide concentr ations. Conclusions. The interaction of enflurane and nitrous oxide in the dose range from 0 to 75 vol.-% on median EEG frequency is compati ble with additivity. The potency of nitrous oxide as a substitute for enflurane is less than might be expected when adding up the MAC values .