Changes in the rapidly extracted auditory evoked potentials index and the bispectral index during sedation induced by propofol or midazolam under epidural block

Background. The bispectral index (BIS) and the rapidly extractedauditory evoked potentials index (A-line ARX Index or AAI) havebeen proposed as methods to measure the depth of sedation. Aprospective study was designed to assess the performance ofboth these methods for measuring the depth of sedation inducedby propofol or midazolam under epidural block. Methods. Thirty-two ASA I and II adult patients undergoing electivegynaecological surgery under low-thoracolumbar epidural blockwere studied. Eighteen patients received propofol (Group P:20 mg bolus every 3 min) and 14 received midazolam (Group M:0.5 mg bolus every 5 min) until an observer’s assessmentof alertness/sedation (OAA/S) scale score of 1 was achieved.AAI and BIS were monitored for different OAA/S scores. Results. AAI and BIS decreased and increased following the changeson the patients’ OAA/S scores and correlated with sedationsignificantly. During the onset phase, the coefficients of Spearman’srank correlation for AAI and BIS were respectively 0.958 and0.898 (P<0.001) for Group P, and 0.973 and 0.945 (P<0.001)for Group M. During the recovery phase in Group P, the coefficientswere respectively 0.946 and 0.702 (P<0.001). Linear regressionanalysis showed that both AAI and BIS were linearly relatedto the OAA/S scores. The coefficients of Spearman’s rankcorrelation and linear regression for AAI were all greater thanthose for BIS (P<0.05). Conclusions. Both AAI and BIS correlated well with the depthof sedation induced by propofol or midazolam under epiduralblock. AAI may be more valuable when monitoring depth of sedation. Br J Anaesth 2002; 89: 260–4