Comparison of bispectral EEG analysis and auditory evoked potentials for monitoring depth of anaesthesia during propofol anaesthesia

We have compared the auditory evoked potential index (AEP(Index)) and bispectral index (BIS) for monitoring depth of anaesthesia in spontaneously breathing surgical patients. Twenty patients (aged 17-49 yr) undergoing day surgery were anaesthetized with computer-controlled infusions of propofol. The mean (so and range) of each measurement was determined during consciousness and unconsciousness and at specific times during the perioperative period. Mean values for AEP(Index) during consciousness and unconsciousness were 74.5 (so 14.7) and 36.7 (7.1), respectively. BIS had mean values of 89.5 (SD 4.6) during consciousness and 48.8 (16.4) during unconsciousness. AEP(Index) and BIS were greater during consciousness compared with during unconsciousness. The average awake values of AEP(Index) were significantly higher than all average values during unconsciousness but this was not the case for BIS. BIS increased gradually during emergence from anaesthesia and may therefore be able to predict recovery of consciousness at the end of anaesthesia. AEP(Index) was more able to detect the transition from unconsciousness to consciousness.