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 (AEPIndex) andbispectral index (BIS) for monitoring depth of anaesthesia in spontaneouslybreathing surgical patients. Twenty patients (aged 17-49 yr) undergoing daysurgery were anaesthetized with computer-controlled infusions of propofol.The mean (SD and range) of each measurement was determined duringconsciousness and unconsciousness and at specific times during theperioperative period. Mean values for AEPIndex during consciousness andunconsciousness were 74.5 (SD 14.7) 36.7 (7.1), respectively. BIS had meanvalues of 89.5 (SD 4.6) during consciousness and 48.8 (16.4) duringunconsciousness. AEPIndex and BIS were greater during consciousnesscompared with during unconsciousness. The average awake values of AEPIndexwere significantly higher than all average values during unconsciousnessbut this was not the case for BIS. BIS increased gradually during emergencefrom anaesthesia and may therefore be able to predict recovery ofconsciousness at the end of anaesthesia. AEPIndex was more able to detectthe transition from unconsciousness to consciousness.