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The bispectral index (BIS), a value derived from the electroencephalograph (EEG), has been proposed as a measure of anesthetic effect. To establish its utility for this purpose, it is important to determine the relation among BIS, measured drug concentration, and increasing levels of sedation. This study was designed to evaluate this relation for four commonly used anesthetic drugs: propofol, midazolam, isoflurane, and alfentanil.Seventy-two consenting volunteers were studied at four institutions. Volunteers were given either isoflurane, propofol, midazolam, or alfentanil. Each volunteer was administered a dose-ranging sequence of one of the study drugs to achieve predetermined target concentrations. A frontal montage was used for continuous recording of the EEG. At each pseudo-steady-state drug concentration, a BIS score was recorded, the participant was shown either a picture or given a word to recall, an arterial blood sample was obtained for subsequent analysis of drug concentration, and the participant was evaluated for level of sedation as determined by the responsiveness portion of the observer's assessment of the alertness/sedation scale (OAAS). An OAAS score of 2 or less was considered unconscious. The BIS (version 2.5) score was recorded in real-time and the BIS (version 3.0) was subsequently derived off-line from the recorded raw EEG data. The relation among BIS, measured drug concentration, responsiveness score, and presence or absence of recall was determined by linear and logistic regression for both the individual drugs and, when appropriate, for the pooled results. The prediction probability was also calculated.The BIS score (r = 0.883) correlated significantly better than the measured propofol concentration (r = -0.778; P < 0.05) with the responsiveness score. The BIS provided as effective correlation with responsiveness score of the OAAS as did the measured concentration for midazolam and isoflurane. None of the volunteers given alfentanil lost consciousness and thus were excluded from the pooled analysis. The pooled BIS values at which 50% and 95% of participants were unconscious were 67 and 50, respectively. The prediction probability values for BIS ranged from 0.885–0.976, indicating a very high predictive performance for correctly indicating probability of loss of consciousness.The BIS both correlated well with the level of responsiveness and provided an excellent prediction of the loss of consciousness. These results imply that BIS may be a valuable monitor of the level of sedation and loss of consciousness for propofol, midazolam, and isoflurane.