Awakening concentrations of isoflurane are not affected by analgesic doses of morphine.

A randomized, double-blind study was performed to determine how morphine 0.1 mg/kg IV, or placebo administered 80 +/- 11 (means +/- SE) minutes before the end of surgery affect recovery from isoflurane/oxygen anesthesia. End-tidal isoflurane remained constant at 1.10 +/- 0.02% (means +/- SE) in both groups intraoperatively, and no other anesthetics were given after the administration of the morphine or placebo. Duration of anesthesia did not differ significantly between the morphine (172 +/- 7 minutes) and placebo (163 +/- 18 minutes) groups. Times from discontinuation of isoflurane until eye-opening in response to verbal command were similar in the morphine (19 +/- 2 minutes) and placebo (22 +/- 3 minutes) groups. At the time of eye-opening, end-tidal isoflurane concentrations did not differ between subjects receiving morphine (0.20 +/- 0.02%) and placebo (0.18 +/- 0.01%). It is concluded that the awakening concentration (MAC-awake) during recovery from isoflurane anesthesia is approximately 0.19% and is not affected by analgesic doses or morphine.