Changes in the auditory evoked potentials and the bispectral index following propofol or propofol and alfentanil

Background: Midlatency auditory evoked potentials (MLAEP) show graded chang es with increasing doses of hypnotics but little change with opioids, The e ffect of their combination on the MLAEP was evaluated. Also, the bispectral index (BIS) was compared with the ability of MLAEP to correlate with sedat ion and predict loss of consciousness. Methods: Twenty healthy volunteers were randomly assigned to receive steppe d increases in propofol concentration (10 subjects) or propofol plus alfent anil 100 ng/ml (10 subjects). At baseline and at each targeted effect site concentration the mean MLAEP, BIS, measures of sedation, and drug concentra tion were obtained. The relation among MLAEP, BIS, and sedation score was d etermined. The prediction probability (P-k) was calculated and compared for BIS and MLAEP. Results: The BIS and MLAEP patterns showed significant changes (Pa and Nb d ecreased in amplitude and increased in latency) with increasing level of se dation (P < 0.0001). The BIS correlated better with sedation scores (0.884) than did the MLAEP (P < 0.05). Pa and Nb latencies showed the best correla tion with sedation levels (0.685 and 0.658, respectively). The addition of alfentanil did not affect the relation between MLAEP and loss of consciousn ess (P > 0.15), The BIS (P-k = 0.952) was a better predictor of loss of con sciousness than were Pa and Nb amplitude (P < 0.05) but were comparable to Pa and Nb latency (P-k = 0.869 and 0.873, respectively). Conclusion: MLAEP changes, like the BIS, correlate well. with increasing se dation produced by propofol, and these changes in the MLAEP are independent of the presence of an opioid. Among all the MLAEP parameters, Pa and Nb la tencies are the best predictors of increasing sedation and loss of consciou sness.