Long-Latency Auditory Evoked Potentials During General Anesthesia

The N1 and P3 auditory evoked potentials were recorded to evaluate their usefulness as measures of the level of consciousness in 14 ASA physical status I and II patients undergoing elective surgery. The anesthetic agents were thiopental, fentanyl, and isoflurane with or without nitrous oxide. Recordings were carried out before induction (preinduction) and during induction, surgical anesthesia, emergence, and recovery from anesthesia. The auditory response was evoked by 700-Hz tones that occurred occasionally and unpredictably in a train of 500-Hz tones delivered at 40 per second. The patients were asked to press a button whenever they detected a 700-Hz tone. Studies with normal subjects have repeatedly shown that detected tones (HITS) evoke N1 and P3 waves, whereas undetected tones (MISSES) evoke no recognizable waves. The responses evoked by HITS were compared with those evoked by MISSES. The amplitudes of N1 and P3 were significantly different from zero for HITS before induction and during induction and recovery but not during emergence. The amplitudes of N1 and P3 were not different from zero for MISSES during induction, surgery, and emergence. During recovery, the N1 and P3 for MISSES were small and the P3 for HITS was significantly larger than for MISSES. The results indicate that except during emergence, HITS were associated with clear N1 and P3 waves, whereas MISSES were not. The lack of either N1 or P3 for HITS during emergence perhaps occurred because the patients, although responsive, were not yet fully conscious. The N1 and P3 components of the auditory evoked potential may provide specific indicators for consciousness.