SUMMARY

Previous studies on depth of anaesthesia have shown graded changes in the auditory evoked response (AER) in patients anaesthetized with increasing concentrations of volatile anaesthetic agents [1,2]. In these studies the amplitudes of the early cortical waves, Pa and Nb, decreased progressively as the concentrations of halothane, enflurane and isoflurane were increased. However, when observed several minutes after induction of anaesthesia and before the administration of the volatile agent, the amplitudes of these early cortical waves appeared frequently to exceed their pre-anaesthetic values. In this period, anaesthesia was provided solely by nitrous oxide and the declining concentration of the induction agent, thiopentone. From these data it is not possible to differentiate between the effects of nitrous oxide alone, which could possibly have had an excitatory effect, and those effects produced by the potentially stressful sequence (induction of anaesthesia and tracheal intubation) that preceded it. Recent studies have shown that the depression of the early cortical AER by anaesthetic agents may be reversed partially by the stimulus of surgery [3]. We have conducted a cross-over study to examine the effects on these AER waves of the period immediately after induction and tracheal intubation in patients anaesthetized with equal MAC (minimal alveolar concentration) fractions of either nitrous oxide or isoflurane