Determination of loss of consciousness

Eva-Lena Zetterlund, Henrik Gréen, Anna Oscarsson, Svante Vikingsson, Magnus Vrethem, Maj-Lis Lindholm, Christina Eintrei

Computer-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. The 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. This was an observational cohort study. University Hospital Linköping, University Hospital Örebro, Finspång Hospital and Kalmar Hospital, Sweden from October 2011 to April 2013. A total of 35 ASA I patients aged 18 to 49 years were recruited. The 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 30 min after LOC. The primary outcome was measurement of BIS, EEG and clinical LOC. The 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 20 min. There was no statistically significant correlation between BIS and EEG at clinical LOC. BIS monitoring may not be a reliable method for determining LOC. This trial was not registered because registration was not mandatory at the time of the trial.