MAC-AWAKE OF ISOFLURANE, ENFLURANE AND HALOTHANE EVALUATED BY SLOW AND FAST ALVEOLAR WASHOUT

End-tidal anaesthetic concentrations at first eye opening inresponse to a verbal command during recovery from anaesthesia(MAC-awake), were measured for isoflurane (n = 16), enflurane(n = 16) and halothane (n = 14). MAC-awake was measured duringeither slow or fast alveolar washout. Slow washout was obtainedby decreasing anaesthetic concentrations in predetermined stepsof 15min, assuming equilibration between brain and alveolarpartial pressures. Fast alveolar washout was obtained by discontinuationof the inhalation anaesthetic, which had been maintained at1 MAC for at least 15 min. Mean MAC-awake obtained with slowalveolar washout was similar for isoflurane (0.25 (SD 0.03)MAC), and enflurane (0.27 (0.04) MAC) and significantly greaterthan values obtained by fast alveolar washout (isoflurane: 0.19(0.03) MAC; enflurane: 0.20 (0.03) MAC). The MAC-awake of isofluraneand enflurane was significantly less than that of halothane,which was 0.59 (0.10) MAC as evaluated by the slow and 0.50(0.05) MAC as evaluated by the fast alveolar washout method.Recovery time from anaesthesia with fast alveolar washout was8.8 (4.0) min for halothane, which was not different from isoflurane(15 (2.5) min), but significantly shorter than for enflurane(22 (10) min), reflecting differences in the anaesthetic concentrationgradient between MAC and MAC-awake values. These data do notsupport the hypothesis of a uniform ratio between MAC and MAC-awakevalues.