Determination of loss of consciousness

Abstract

BACKGROUNDComputer-processed algorithms of encephalographic signals are widely used to assess the depth of anaesthesia. However, data indicate that the bispectral index (BIS), a processed electroencephalography monitoring system, may not be reliable for assessing the depth of anaesthesia.OBJECTIVEThe aim of this study was to evaluate the ability of the BIS monitoring system to assess changes in the level of unconsciousness, specifically during the transition from consciousness to unconsciousness, in patients undergoing total intravenous anaesthesia with propofol. We compared BIS with the electroencephalogram (EEG), and clinical loss of consciousness (LOC) defined as loss of verbal commands and eyelash reflex.DESIGNThis was an observational cohort study.SETTINGUniversity Hospital Linkoping, University Hospital orebro, Finspang Hospital and Kalmar Hospital, Sweden from October 2011 to April 2013.PATIENTSA total of 35 ASA I patients aged 18 to 49 years were recruited.INTERVENTIONSThe patients underwent total intravenous anaesthesia with propofol and remifentanil for elective day-case surgery. Changes in clinical levels of consciousness were assessed by BIS and compared with assessment of stage 3 neurophysiological activity using the EEG. The plasma concentrations of propofol were measured at clinical LOC and 20 and 30min after LOC.MAIN OUTCOME MEASURESThe primary outcome was measurement of BIS, EEG and clinical LOC.RESULTSThe median BIS value at clinical LOC was 38 (IQR 30 to 43), and the BIS values varied greatly between patients. There was no correlation between BIS values and EEG stages at clinical LOC (r=-0.1, P=0.064). Propofol concentration reached a steady state within 20min.CONCLUSIONThere was no statistically significant correlation between BIS and EEG at clinical LOC. BIS monitoring may not be a reliable method for determining LOC.CLINICAL TRIALS REGISTRYThis trial was not registered because registration was not mandatory at the time of the trial.