Being awake intermittently during propofol-induced hypnosis: A study of BIS, explicit and implicit memory

Background: Being awake during anaesthesia is a serious complication. An an aesthetic depth monitor must discriminate in real time between wakefulness and unconsciousness. The present study created a period of wakefulness duri ng propofol-induced hypnosis. Bispectral index (BIS), explicit and implicit memories of the awake period were investigated. Methods: Ten volunteers were studied. The calculated brain concentration of a target controlled infusion of propofol was increased until loss of respo nse (LOR) to verbal command and then propofol was stopped. When fully awake , volunteers were presented with a picture, sound and smell. Propofol infus ion was restarted until LOR and then ceased. BIS and the calculated brain c oncentration of propofol were recorded every minute. A structured interview was conducted for explicit memories after awakening and for explicit as we ll as implicit memories the day after. Results: Median BIS-index for the transition between awake and asleep and v ice versa differed significantly. It was not possible, however, to establis h any threshold value or zone for discriminating between wakefulness and LO R due to the large interindividual variations in BIS-index. No volunteer co uld explicitly recall any of the stimuli presented during the period of wak efulness. Conclusion: The BIS-index decreases with increasing sedation but because of the large individual variations, the real-time BIS-index for the individua l subject cannot reliably discriminate wakefulness from unconsciousness dur ing propofol infusion. Propofol causes such profound amnesia that lack of p ostoperative recall does not assure that episodes of awareness have not occ urred during propofol-induced hypnosis.