Comparison of the effects of sevoflurane and propofol on cortical somatosensory evoked potentials

Background. Propofol (P) and sevoflurane (S) are potential anaesthetic agents if electrophysiological monitoring is required during spinal surgery. They allow rapid recovery and do not depress cortical somatosensory evoked potentials (SSEP) as much as other agents. The effects of these agents on SSEP have not been compared before. Methods. Twenty-four patients were allocated randomly to receive either S (n=12) or P (n=12). SSEP evoked by electrical stimulation of the posterior tibial nerve at the ankle were recorded before anaesthesia. The cortical potential P40 was recorded (latency P40 and amplitudes N29P40 and P40N50). The anaesthetic concentration was adjusted gradually to obtain three predetermined ranges of values of bispectral index (BIS): 45-55, 35-45 and 25-35. For each range, a stable state was maintained for 10 min and SSEP were recorded. Results. For the BIS 45-55 range, compared with preoperative values, P40 latency increased during S [mean change +2 (sd 0.6) ms] but not during P [+0.4 (0.2) ms (P= 0.12)] and both amplitudes (N29P40 and P40N50) decreased with S. Increasing S concentration caused a dose-dependent depression of SSEP. P did not have a statistically significant effect on the recordings and the signals remained stable in each BIS range. Conclusion. Sevoflurane affected SSEP recordings in a dose-dependent fashion. Propofol had a minimal effect on SSEP recordings.