Median nerve somatosensory evoked potentials during isoflurane anaesthesia

Purpose

The effect of isoflurane on the subcortical P₁₄ component of the median nerve somatosensory evoked potential (SEP) is poorly known. We studied whether the P₁₄ wave from the upper brainstem, recorded with a nasopharyngeal electrode, was attenuated at the isoflurane-induced EEG burst-suppression level. We also compared the effect of isoflurane on the P₁₄, cervical N₁₃ and cortical N₂₀, N₃₅ and N₆₀ components.

Methods

Seventeen elective patients were anaesthetized with isoflurane. Somatosensory evoked potentials were recorded pnor to anaesthesia, at 0.5 MAC and I MAC end-tidal isoflurane as well as at the level when EEG was in burst-suppression (mean 1.9 vol% end-tidal isoflurane).

Results

Isoflurane had varying effects on the subcortical components of median SEP. The amplitude of nasopharyngeal P₁₄ was stable, but the mean latency increased from 14.4 ± 1.2 msec at 0.5 MAC to 15.2 ± 1.1 msec at burstsuppression level (P < 0.05). In contrast, the N₁₃ neck response amplitude was attenuated from 3.3 ± 0,6 μV to 2.6 ±0.5μV (P < 0.005) without latency changes. The latency of the cortical N₂₀ wave was increased from 19.7 ± 1.1 msec at awake to 24.4 ± 1.6 msec at burst-suppression level (P < 0.0001) and amplitude was reduced from 3.3 ± 1.1 μV to 1.3 ± 0.6 μV (P < 0.0001). The later cortical components were attenuated even during 0.5 MAC isoflurane and were not recordable during EEG burst-suppression.

Conclusion

We conclude that P₁₄ can reliably be recorded with nasopharyngeal electrodes during isoflurane anaesthesia, even during EEG burst-suppression, when the N₂₀ wave is attenuated. In contrast, the middle-latency SEP components are sensitive to isoflurane anaesthesia.