Early cortical auditory evoked response in anaesthesia: comparison of the effects of nitrous oxide and isoflurane.

Previous studies showing graded changes in the early cortical waves Pa and Nb of the auditory evoked response (AER) with increasing concentration of volatile anaesthetic agents demonstrated high amplitudes of these waves in the period immediately following induction of anaesthesia and tracheal intubation, when the patient breathed nitrous oxide alone. These high amplitude waves were not consistent with extrapolation of the data or observations of patients under steady-state nitrous oxide anaesthesia. In order to discriminate between effects in the period immediately following induction of anaesthesia and tracheal intubation, and effects caused by nitrous oxide alone, a randomized cross-over study was performed. Eight patients breathed either nitrous oxide or isoflurane at 0.6 MAC for three consecutive 10-min periods following intubation and before surgery. The amplitudes of Pa and Nb were significantly less for isoflurane with respect to the same MAC fraction of nitrous oxide in all periods, but for both agents the amplitudes were significantly greater in the 10 min following intubation than in subsequent periods, presumably as a result of stimulation.