The midlatency auditory evoked potentials predict responsiveness to verbal commands in patients emerging from anesthesia with xenon, isoflurane, and sevoflurane but not with nitrous oxide

Background: It has recently been demonstrated that the approximately 40-Hz spectral power of the midlatency auditory evoked potential (MLAEP) correlates well with wakefulness during desflurane or propofol anesthesia. The aim of this study was to characterize how other inhalational anesthetics affects the MLAEP as the patients regain responsiveness to simple verbal command during emergence from anesthesia. Methods: Sixty patients were randomly assigned to receive xenon, isoflurane, sevoflurane, or nitrous oxide (N2O) supplemented with epidural anesthesia. During emergence, the concentration of an anesthetic was decreased in 0.1-minimum alveolar concentration (MAC) decrements from 0.8 MAC or from 70% in the case of N2O, and each new concentration was maintained for 15 min. Every 5 min during each equilibration period, the MLAEP was recorded and the patients were asked to open their eyes and squeeze and release the investigator's hand. This process was repeated until the first response to either of these commands was observed. Results: Thirteen patients were excluded because of technical reasons. The preanesthetic MLAEP showed a periodic wave-form, where the Na-Pa-Nb complex was the most prominent component contributing to the high energy around 29-39 Hz in the power spectrum. Emergence from xenon, isoflurane, and sevoflurane anesthesia produced similar changes in the MLAEP. The spectral power for the frequency 29 Hz or greater was severely suppressed at 0.8 MAC but significantly recovered between the concentration only 0.1 MAC higher that permitting the first response to command and that associated with the first response. In contrast, N2O hardly affected the MLAEPs, even at the concentrations producing unresponsiveness. Two patients did not lose responsiveness even at the highest concentration tested (70%). Conclusions: The MLAEP is closely associated with responsiveness to verbal command during emergence from anesthesia with xenon, isoflurane, and sevoflurane but not with N2O.