Differential effects of isoflurane/nitrous oxide on posterior tibial somatosensory evoked responses of cortical and subcortical origin.

Posterior tibial somatosensory evoked responses (SSERs) were recorded during administration of isoflurane and nitrous oxide. Responses arising from cortical and subcortical neural generators were examined to compare their relative resistance to anesthetic-related degradation. Recordings were performed in ten adults during anesthesia with 0.5 MAC isoflurane/60% N2O, 1.0 MAC isoflurane/60% N2O, and 1.5 MAC isoflurane/60% N2O. Thereafter, N2O was omitted and recordings were repeated during anesthesia with 1.5 and 1.0 MAC isoflurane/O2. Isoflurane resulted in a significant (P less than 0.001) dose-related decrease in the amplitude of cortical waveforms. The amplitude loss was substantial; e.g., for the first cortical waveform, amplitude decreased from 1.21 +/- 0.67 microV during 0.5 MAC isoflurane/N2O to 0.28 +/- 0.29 microV during 1.5 MAC/N2O. Elimination of N2O resulted in an increase in amplitude of approximately 100% (P less than 0.04). By contrast, the amplitude of the subcortical response as recorded in vertex to linked mastoid and vertex to upper cervical spine derivations was not significantly altered by changing concentrations of isoflurane or N2O. The results suggest that subcortical SSERs may be preferable to those of cortical origin for spinal cord monitoring in situations where isoflurane and nitrous oxide, especially in varying concentrations, are the primary anesthetic agents.