Bispectral Index Does Not Correlate with Observer Assessment of Alertness and Sedation Scores During 0.5% Bupivacaine Epidural Anesthesia with Nitrous Oxide Sedation

The bispectral index (BIS) has been used as a measure of the degree of sedation and level of hypnosis for IV hypnotics and sedatives, potent volatile anesthetics. We evaluated the effect of increasing concentrations of nitrous oxide (N2O) on BIS and compared it with the Observer's Assessment of Alertness and Sedation (OAA/S) scale in patients undergoing regional anesthesia. We studied 48 unpremedicated, ASA physical status I–II adult patients scheduled for lower extremity surgery under lumbar epidural anesthesia. N2O was given in oxygen to achieve measured end-tidal concentrations of 33%, 50%, and 67% N2O by a tight-fitting facemask, and each N2O concentration was maintained for 20 min. Paired measurements of BIS and OAA/S scores were obtained just before each increase in N2O concentration. Forty of the 48 subjects completed the study. Increasing N2O concentrations produced no changes in BIS despite a significant decrease in OAA/S scores at 50% and 67% N2O concentrations. The prediction probability for BIS and OAA/S calculated by Somers' dx · y were 0.60 and 0.84, respectively. Anesthesiologists should be aware that the BIS monitor may not be sensitive enough to provide an adequate measure of the depth of sedation and hypnosis when using N2O alone for sedation. It may be better to monitor sedation clinically (e.g., with the OAA/S scale) to determine the dose requirement and the adequacy of depth of sedation and hypnosis.