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Conventional electroencephalographic (EEG) analysis techniques do not use the phase information from the Fourier analysis. This study used a new technique of EEG analysis, bispectral analysis, which measures interfrequency phase relationships in the EEG. Using a reference database, and a process of multivariate discriminant analysis, we developed a univariate bispectral variable, the bispectral index (BIS). This study was designed to test the efficacy of BIS in predicting movement to incision during either an isoflurane/alfentanil anesthetic or a propofol/alfentanil anesthetic technique. Fifty consenting patients were randomized to two groups; one received isoflurane/alfentanil and the other, propofol/alfentanil for anesthesia. EEG was recorded using a microcomputer system and the data were analyzed off-line. Hemodynamic variables were also recorded. After skin incision, each patient was observed carefully for 2 min to detect purposeful movement. A significant difference was found between the BIS values for movers versus nonmovers within each of the two treatment groups (P < or = 0.002). However, isoflurane/alfentanil nonmovers could not be distinguished from propofol/alfentanil movers (P < or = 0.180), suggesting a treatment group effect independent of response classification. Preincision hemodynamic variables did not predict patient movement in response to skin incision. These findings suggest the possibility that different anesthetics have different effects on BIS, and thus BIS may not be independent of the anesthetic. Interfrequency phase coupling, a nonlinear feature of the EEG which is measured with bispectral analysis, may contain clinically useful information for the assessment of anesthetic adequacy. In this study, BIS was a better predictor of patient response than other currently available variables including hemodynamic status.
Vernon et al. (Sat,) studied this question.