The effect of high-dose fentanyl on human median nerve somatosensory-evoked responses

Median nerve somatosensory evoked responses (MnSSERs) were recorded in nine neurologically normal adult cardiac patients before and during the administration of high-dose fentanyl. MnSSERs were recorded prior to induction and at t = 20 min and t = 45 min postinduction. Fentanyl was administered as a slow bolus (53.2 +/- 9.1 micrograms X kg-1), followed by a continuous infusion at 10-20 micrograms X kg-1 X hr-1 (total dose 63.6 +/- 10.1 micrograms X kg-1). All MnSSER waveform components remained recordable and easily identifiable during anaesthesia. The effect of fentanyl was more pronounced on cortical waveform components, leaving subcortical components largely unaffected. There was a significant increase in the latency of the cortical MnSSER at t = 20 min, e.g., for the initial negative cortical wave, N1, the latency was 21.18 +/- 1.55 ms preinduction versus 22.18 +/- 1.42 ms at t = 20 min. There was also a significant decrease in the amplitude of the cortical response at t = 20 min, i.e., 2.04 +/- 1.30 microV preinduction versus 1.31 +/- 0.74 microV at t = 20 min. However, the degree of change was quite variable (range = 0-65 per cent). No further changes occurred at t = 45 min. The authors conclude that MnSSERs can be consistently and reliably monitored during high-dose fentanyl anaesthesia. However, fentanyl produces modest but significant changes in the MnSSER which should be taken into account lest they be misinterpreted as neurologic injury in evolution.