Comparison Of The 40-Hz Auditory Steady State Response Index And The Bispectral Index For Monitoring Depth Of General Anesthesia

ObjectiveThe purpose of this study was to compare the 40-Hz auditory steady state response index (40-Hz ASSR index) and bispectral index for monitoring the depth of the general anesthesia. The investigation was designed to evaluate the usefulness of the 40-Hz auditory steady state response index in assessing the sedation levels with target controlled propofol infusion induction and invest the changes of the 40-Hz auditory steady state response index and the bispectral index during anesthesia, surgery and the period of enmergency from anesthesia.MethodsThirty-three patients of ASA physical status I adult patients (aged 19-60 yr) with normal hearing scheduled for elective surgery under general anesthesia were enrolled in this study. The depth of anesthesia was monitored by 40-Hz auditory steady state response index, bispectral index (BIS for short) and alertness/sedation scale (OAA/S). Anesthesia was induced with target controlled propofol infusion of seven defferent concentration. It was maintained by the 1.0%-1.5%isoflurane-60%N2O combined with the continued infusion of remifentanyl after the intubation. The endotracheal tube would be extubated only after the patients could be response to verbal commands. The parameters of BIS, 40-Hz ASSR index, SBP, DBP, HR and OAA/S sedation scores were recorded in order at different 15 time points for analysis.ResultsThe 40-Hz ASSR index and the BIS were decreasing with the asending of the propofol concentration. The correlation coefficients of 40-Hz ASSR index and BIS to OAA/S sedation score were 0.766 and 0.856 respectively (P<0.01). 40-Hz ASSR index decreased to 22 (SD13) and the BIS decreased to62 (SD11) when the patients became unconscious (P<0.01). The 40-Hz ASSR index and BIS would keep on a low level during the whole surgery. During the period of emergency from anesthesia the 40-Hz ASSR index and BIS would increase. But the 40-Hz ASSR indexes were significantly higher than all average values during unconsciousness but this was not the case for BIS. BIS increased gradually during emergence from anesthesia and may therefore be able to predict recovery of consciousness at the end of anesthesia. 40-Hz ASSR index was more able to detect the transition from unconsciousness to the recovery of reponsivness of verbal commands.ConclusionsThe 40-Hz ASSR index was useful for assessing the level of sedation during the induction procedure with target controlled infusion of propofol. When compared with BIS, the 40-Hz ASSR index was more able to detect the transition from consciousness to unconsciousness. At the period of emergency from anesthesia, the 40-Hz ASSR index was more able to detect the transition from unconsciousness to the recovery of responsiveness to verbal commands.