Detection of awareness in surgical patients with EEG-based indicesÐbispectral index and patient state index

Background. Patient state index (PSI) and bispectral index (BIS) are values derived from the EEG, which can measure the hypnotic component of anaesthesia. We measured the ability of PSI and BIS to distinguish consciousness from unconsciousness during induction and emergence from anaesthesia and a period of awareness in surgical patients. Methods. Forty unpremedicated patients were randomized to receive: (1) sevo¯urane/remi-fentanil (<0.1 mg kg±1 min±1), (2) sevo¯urane/remifentanil (>0.2 mg kg±1 min±1), (3) propofol/ remifentanil (<0.1 mg kg±1 min±1), (4) propofol/remifentanil (>0.2 mg kg±1 min±1). Every 30 s after the start of the remifentanil, patients were asked to squeeze the investigator's hand. Sevo¯urane or propofol were given until loss of consciousness (LOC1). Tunstall's isolated fore-arm technique was used during neuromuscular block with succinylcholine. After tracheal intu-bation, propofol or sevo¯urane were stopped until return of consciousness (ROC1). Propofol or sevo¯urane were re-started to induce LOC2. After surgery, drugs were discontinued and recovery (ROC2) was observed. PSI and BIS at LOC (LOC1 and LOC2) were compared with those at ROC (ROC1 and ROC2) (t-test). Prediction probability (Pk) was calculated from val-ues at the last command before and at LOC and ROC. Values are mean (SD)