Middle latency auditory evoked responses and electroencephalographic derived variables do not predict movement to noxious stimulation during 1 minimum alveolar anesthetic concentration isoflurane nitrous oxide anesthesia

The electroencephalogram (EEG) and middle latency auditory evoked responses (MLAER) have been proposed for assessment of the depth of anesthesia. Howe ver, a reliable monitor of the adequacy of anesthesia has not yet been defi ned. In a multicenter study, we tested whether changes in the EEG and MLAER after a tetanic stimulus applied to the wrist could be used to predict sub sequent movement in response to skin incision in patients anesthetized with 1 minimum alveolar anesthetic concentration (MAC) isoflurane in N2O. We al so investigated whether the absolute values of any of these variables befor e skin incision was able to predict subsequent movement. After the inductio n of anesthesia with propofol. and facilitation of tracheal intubation with succinylcholine, 82 patients received 1 MAC isoflurane (0.6%) in N2O 50% w ithout an opioid or muscle relaxant. Spontaneous EEG and MLAER to auditory click-stimulation were recorded from a single frontoparietal electrode pair . MLAER were severely depressed at 1 MAC isoflurane. At least 20 min before skin incision, a 5-s tetanic stimulus was applied at the wrist, and the ch anges in EEG and MLAER were recorded. EEG and MLAER values were,evaluated b efore and after skin incision for patients who did not move in response to tetanic stimulation. Twenty patients (24%) moved after tetanic stimulation. The changes in the EEG or MLAER variables were unable to predict which pat ients would move in response to skin incision. Preincision values were not different between patients who did and did not move in response to skin inc ision for any of the variables. MLAER amplitude increased after skin incisi on. We conclude that it is unlikely that linear EEG measures or MLAER varia bles can be of practical use in titrating isoflurane anesthesia to prevent movement in response to noxious stimulation. Implications: Reliable estimat ion of anesthetic adequacy remains a challenge. Changes in spontaneous or a uditory evoked brain activity after a brief electrical stimulus at the wris t could not be used to predict whether anesthetized patients would subseque ntly move at the time of surgical incision.