Human auditory steady-state response, electroencephalogram, and late auditory evoked potentials during general anesthesia.

In order to identify neurophysiological correlates of the changes in the level of conciousness associated with general anesthesia, the 40 Hz auditory steady-state response (ASSR), the electroencephalogram (EEG) and the N1 and P3 components of the transient auditory evoked potential were recorded before anesthesia (pre-induction), at the onset of anesthesia (induction), during surgical anesthesia, at the time of emergence and during recovery from anesthesia. The amplitude of the ASSR, was reduced significantly during late induction and dropped below noise levels during surgical anesthesia. It increased during emergence and further increased during recovery although the amplitude during recovery was significantly less than pre-induction values. Total EEG power increased significantly after induction. The EEG median frequency and 95% quantile frequency decreased significantly during surgery and increased significantly during emergence. Muscle artifacts could account for many of the EEG changes. The results for the transient auditory evoked potential indicated that, except during emergence, detection was associated with clear N1 and P3 waves whereas undetection was not. The lack of either N1 and P3 for detection during emergence perhaps occurred because the patients were not yet fully conscious. (Abstract shortened by UMI.