Effect of isoflurane on the auditory evoked response in man

We have examined the effects of isoflurane (0.6-2.9% end-tidal) on the auditory evoked response (AER) in six patients before elective surgery. Isoflurane produced significant dose-related changes in the AER: reductions in amplitude and increases in latency of the cortical waves Pa and Nb, and increases in the latency of the brainstem waves III and V. When isoflurane was compared with halothane and enflurane using an MAC-based comparison, we found no differences in the effect of the three agents on the amplitude of the early cortical waves, although the latencies showed significant differences. The consistent dose-related effect on the amplitudes of the cortical waves implies that the AER could be a promising index of the depth of anaesthesia.