Effects of Enflurane, Isoflurane, and Nitrous Oxide on Somatosensory Evoked Potentials during Fentanyl Anesthesia

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Abstract
The effects of N2O, enflurane, and isoflurane on cortical somatosensory evoked potentials (SEP) were studied in 29 patients undergoing intracranial or spinal operations. Anesthesia was induced with fentanyl (25 .mu.g/kg, i.v.) plus thiopental (0.5-1.0 mg/kg, i.v.). In one group of patients (n = 12), N2O (50%) was compared with enflurane (0.25-1.0%), and in another group (n = 12) N2O (50%) was compared with isoflurane (0.25-1.0%). In a 3rd group of patients (n = 5) with preexisting neurologic deficits, N2O (50%) was compared with enflurane (0.25-1.0%). In all 3 groups, one gas was administered for 30 min, and then the alternate gas was administered for 30 min; then the cycle was repeated for a total of 2 adminstrations of each of the 2 anesthetics. SEP were determined before and after induction of anesthesia and at the end of each 30-min study period. The latencies and amplitudes of the early cortical components of the upper- and lower-extremity SEP were examined. Induction of anesthesia resulted in increases of latency in both upper- and lower-extremity SEP without any alteration of amplitude. N2O, enflurane, and isoflurane each decreased the amplitude of the upper-extremity SEP compared with the postinduction value. The amplitude of the upper-extremity SEP was less during nitrous oxide than with either enflurane or isoflurane. N2O decreased the amplitude of lower-extremity SEP below postinduction value, while enflurane and isoflurane had no effect. Isoflurane and enflurane increased the latency of both upper- and lower-extremity SEP slightly, while N2O had no effect. In patients with preexisting neurologic deficits, nitrous oxide decreased amplitude more than enflurane. During fentanyl-based anesthesia either enflurane or isoflurane (0.25-1.0%) results in less alteration of cortical SEP than does N2O (50%), and these concentrations of enflurane or isoflurane are compatible with the generation of waves that are adequate for evaluation.