Effects of halothane, enflurane, and isoflurane in nitrous oxide on multilevel somatosensory evoked potentials

The effects of halothane, enflurane, and isoflurane were studied at 0.5, 0.75, and 1 MAC in 60% N2O on subcortical sensory evoked potentials recorded at the popliteal fossa (PF), the spine (L-3, C-6) and on cortical potentials recorded at the scalp (SC) following bilateral posterior tibial nerve stimulations at the ankle in 28 patients undergoing scoliosis surgery. Latencies and amplitudes of the resulting potentials at each level were compared with postinduction control values. With increasing MAC, latency and amplitude changes seen at C6 (subcortical) were also compared with those at SC (cortical). Increasing the concentrations of each agent resulted in a graded increase in latency and a graded decrease in amplitude, at all levels. At SC each increase in MAC with each agent resulted in an increase in latency (P less than 0.05) and a decrease in amplitude (P less than 0.005), respectively. The increases in SC latency at 0.75, 1 MAC were larger than the increase in latency at C-6 (P less than 0.005) and the decreases in SC amplitudes at 0.5, 0.75 and 1 MAC were greater than the decrease in amplitude at C-6 (P less than 0.01). Halothane, enflurane, and isoflurane in 60% N2O altered subcortical potentials less than cortical potentials. Enflurane and isoflurane at 0.5, 0.75, and 1 MAC, and halothane at 0.5, 0.75 MAC maintained subcortical and cortical potentials that were adequate for evaluation. However, 1 MAC of halothane suppressed cortical potentials but maintained subcortical potentials. Subcortical C-6 potential may serve as an additional monitor.