Effects of halothane, enflurane, and isoflurane on somatosensory evoked potentials during nitrous oxide anesthesia.

The effects of 0.5, 0.75, and 1 MAC of halothane, enflurane, and isoflurane in 60% nitrous oxide on somatosensory cortical evoked potentials were studied in 30 patients undergoing corrective surgery for scoliosis. The evoked potentials were averaged at the scalp from the electroencephalogram following repeated bilateral posterior tibial nerve stimulation at the ankle. Latencies and amplitudes of the resulting potentials were measured and compared with the post-induction control values. Graded increase in latencies and graded decrease in amplitudes were found with increasing concentrations of all the three agents (P less than 0.05), confirming that the effects were dose related. Reductions in amplitudes were more marked than increase in latencies. The authors conclude that, during nitrous oxide-based anesthesia, enflurane, and isoflurane resulted in less alteration of somatosensory cortical evoked potentials than halothane. In conjunction with 60% nitrous oxide, 0.5 and 0.75 MAC of halothane, 0.5, 0.75, and 1.0 MAC of isoflurane and enflurane, respectively, were found to be compatible with the generation of waves adequate for evaluation.