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Differential Effects of Isoflurane/Nitrous Oxide on Posterior Tibial Somatosensory Evoked Responses of Cortical and Subcortical Origin

Received from the Department of Anesthesiology of the Veterans' Administration Medical Center, La Jolla, California; the Department of Anesthesiology, University of California, San Diego; and the Department of Anesthesiology, U.S. Naval Hospital, San Diego, California.

Abstract

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 isofluvane/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 < 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 µV during 0.5 MAC isoflurane/N₂O to 0.28 ± 0.29 µV during 1.5 MAC/N2O. Elimination of N₂O resulted in an increase in amplitude of approximately 100% (P < 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.

Key Words: ANESTHETICS, VOLATILE—isoflurane • ANESTHETICS, GASES—nitrous oxide • MONITORING, EVOKED RESPONSES—somatosensory • BRAIN, EVOKED RESPONSES—somatosensory

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Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999