Electroencephalogram Bispectral Analysis Predicts the Depth of Midazolam‐induced Sedation

Background The electroencephalogram (EEG) has been used to study the effects of anesthetic and analgesic drugs on central nervous system function. A prospective study was designed to evaluate the accuracy of various EEG parameters for assessing midazolam‐induced sedation during regional anesthesia. Methods Twenty‐six consenting adult patients were administered 4.5–20 mg intravenous midazolam (in increments of 0.5–1‐mg bolus doses every 6–10 min) until they became unresponsive to tactile stimulation (i.e., mild prodding or shaking). The EEG was continuously recorded from a bifrontal montage (FP1 ‐Cz and FP2 ‐Cz) to obtain the bispectral index (BI), 95% spectral edge frequency (SEF), median frequency (MF), and delta, theta, alpha, and beta power bands. Sedation was assessed clinically at 6–10‐min intervals using the Observers' Assessment of Alertness/Sedation (OAA/S) scale, with 1 = no response (unconsciousness) to tactile stimulation to 5 = wide awake. The EEG parameters were correlated with the OAA/S scores using nonparametric Spearman's rank‐correlation analysis. Kruskal‐Wallis analysis of variance was used to determine significant changes in EEG parameters during the onset of and recovery from midazolam‐induced sedation. Results Of the EEG parameters studied, the BI exhibited the best correlation with OAA/S scores during both the onset (Spearman's Rho = 0.815) and recovery (Spearman's Rho = 0.596) phases. With increasing sedation, there was a progressive decrease in the BI (OAA/S score of 5: BI = 95.4+/‐2.3; 4:90.3+/‐4.5; 3:86.6+/‐4.6; 2:75.6+/‐9.7; 1:69.2+/‐13.9). A similar pattern was found for the 95% SEF as the OAA/S score decreased from 4 to 1. Similarly, EEG‐BI increased with recovery from the sedative effects of midazolam (OAA/S score = 2: BI = 75.2+/‐10.2; 3:82.3+/‐7.3; 4:90.8 +/‐6). However, no consistent changes were found with the other EEG parameters. The mean EEG values between OAA/S scores 3 and 2 and between OAA/S scores 2 and 1 during the onset and recovery phases from midazolam‐induced sedation, defined as EEG50 values for response to verbal command (EEG50‐VC) and to shaking of the head (EEG50‐SH), were 79.3+/‐8 and 70.8+/‐14.3, respectively, for EEG‐BI. The EEG‐BI displayed the smallest coefficients of variation for the EEG sub 50‐VC and EEG50‐SH values. Conclusions The EEG‐BI appears to be a useful parameter for assessing midazolam‐induced sedation and can predict the likelihood of a patient responding to verbal commands or to shaking of the head during midazolam‐induced sedation.