Background and objective: Changes in the middle latency auditory evoked response following the administration of opioids have been shown. However, it remains unclear as to whether these changes are due to a direct depressant effect of opioids on the middle latency auditory evoked response itself, or an indirect effect on account of their action in attenuating central nervous system arousal associated with noxious stimuli. By comparing changes in the middle latency auditory evoked response in intubated and non-intubated patients, receiving saline or remifentanil in different doses, this study attempts to answer this question.
Methods: Fifty-four patients were anaesthetized with isoflurane and nitrous oxide (0.9 MAC) and randomized to 1–6 groups. Groups 1–3 received a bolus injection of either saline 0.9%, low-dose remifentanil (1 μg kg−1) or high-dose remifentanil (3 μg kg−1) prior to intubation of the trachea. Groups 4–6 were not intubated following the bolus injection.
Results: Pa and Nb amplitudes of the middle latency auditory evoked response increased by 82% and 79% with intubation in the saline group (P < 0.005) and these changes were not seen in the patients given remifentanil. There was a significant linear trend for the reduction in Pa and Nb amplitude with increasing remifentanil dose (P < 0.05). In the absence of endotracheal intubation remifentanil had no effect on either the amplitudes or latencies of the waves Pa and Nb and there was no effect of dose. For the haemodynamic measurements remifentanil attenuated the pressor response to intubation (P < 0.001) and had a significant dose-related effect (P < 0.001) in the absence of intubation.
Conclusions: We demonstrated an effect of remifentanil on both the middle latency auditory evoked response and haemodynamic changes to endotracheal intubation. For the non-intubated patients there was only an effect of remifentanil on the haemodynamic measurements. This suggests that remifentanil has an effect on the middle latency auditory evoked response in attenuating the arousal associated with intubation of the trachea but has no effect in the absence of a stimulus.