[Nitrous oxide action on the central nervous system: electrophysiological study as a sole agent or a coadjuvant].

BACKGROUND AND OBJECTIVES Nitrous oxide is the most widely used inhalational anesthetic worldwide. Its action mechanism is broadly discussed based on results of experimental studies and clinical evidences. The purpose of this study was to evaluate, through specific monitoring, nitrous oxide electrophysiological action on the central nervous system. METHODS Twenty-five patients of both genders, aged 6 to 25 years, undergoing orthopedic or corrective plastic surgery, were monitored by electroencephalogram bispectral index (EEG-BIS) and somatosensory evoked potential (SEP) during anesthesia. BIS and SEP baseline values were recorded, as well as after fractional alveolar (FA) 30%, 50% and 66% nitrous oxide administration. Then, nitrous oxide was withdrawn and isoflurane or desflurane were randomly administered in 0.5 and 1 MAC. While maintaining 1 MAC of one of those agents, nitrous oxide was again administered in the same previous concentrations. RESULTS Nitrous oxide as sole agent caused a BIS decrease which, although statistically significant, did not represent a hypnotic state. This decrease was also observed when nitrous oxide was used as a coadjuvant agent, however without clinical significance. As sole agent, nitrous oxide significantly depressed brain waves amplitude, with no increase in onset time. Isoflurane and desflurane decreased the amplitude and increased onset time of brain waves. The association of nitrous oxide to those agents further increased these effects on cortical waves. There were no significant changes in peripheral and spinal cord SEP waves. CONCLUSIONS Nitrous oxide has a minor hypnotic action, which is not completely captured by EEG-BIS. It has a pronounced action on cortical structures, both as sole agent or associated to isoflurane or desflurane, which may explain its satisfactory analgesic effect.