BJA Advance Access originally published online on June 27, 2008
British Journal of Anaesthesia 2008 101(3):366-373; doi:10.1093/bja/aen186
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Nervus medianus evoked potentials and bispectral index during repeated transitions from consciousness to unconsciousness
1 Department of Anaesthesiology
2 Department of Legal Medicine, University Hospital Charité, Berlin, Germany
3 Department of Psychology, University of Michigan, Ann Arbor, MI, USA
* Corresponding author: Department of Anesthesiology, Charité—Universitätsmedizin Berlin, Campus Charité Mitte und Virchow, Charitéplatz 1, D-10117 Berlin, Germany. E-mail: ingrid.rundshagen{at}charite.de
Background: We investigated the relationship between median nerve somatosensory evoked potentials (SSEPs) and the bispectral index (BIS) during alternating periods of consciousness and propofol-induced unconsciousness.
Methods: Loss of consciousness (LOC) was repetitively induced by bolus injections of propofol in 24 patients undergoing elective surgery in spinal anaesthesia. SSEP and the BIS were recorded during LOC and recovery of consciousness (ROC). The level of consciousness was clinically assessed by the observer’s assessment of alertness/sedation scale. Propofol venous plasma concentrations were measured simultaneously.
Results: At LOC, all SSEPs latency components were prolonged (P<0.001), whereas amplitudes of the components 
45 ms were smaller (P=0.008) and the BIS values were lower (P<0.001). None of the EEG variables regained baseline levels during ROC. Regression analyses revealed that the SSEP components (five latencies and five amplitudes) explained 33% of the variance when predicting ROC; the BIS explained 12%. The combination of SSEP and BIS explained 37% of variance in this patient sample. Propofol venous plasma concentration was 1.2 (0.8) µg ml–1 during LOC and 0.4 (0.5) µg ml–1 during ROC.
Conclusions: The present results indicate the usefulness of combining variables of the evoked and spontaneous EEG to measure different levels of consciousness, because the SSEP provide additional information beyond the BIS. Inter-individual variability of all the EEG variables limits their predictive potency of ROC after propofol infusion.
Keywords: anaesthesia, depth; anaesthetics i.v., propofol; brain, electroencephalography
The results have been presented in part at the ‘Haupstadtkongress für Anästhesie und Intensivmedizin’ in Berlin, Germany, 2005 and at the Annual Meeting of the American Society of Anesthesiologists in Atlanta, GA, USA, 2005.