NITROUS-OXIDE HAS DIFFERENT EFFECTS ON THE EEG AND SOMATOSENSORY-EVOKED POTENTIALS DURING ISOFLURANE ANESTHESIA IN PATIENTS

Background: Electroencephalogram (EEG) and somatosensory evoked potent ials (SEPs) are altered by inhalation anaesthesia. Nitrous oxide is co mmonly used in combination with volatile anaesthetics. We have studied the effects of nitrous oxide on both EEG and SEPs simultaneously duri ng isoflurane burst-suppression anaesthesia. Methods: Twelve ASA I-II patients undergoing abdominal or orthopaedic surgery were anaesthetize d with isoflurane by mask. After intubation and relaxation the isoflur ane concentration was increased to a level at which an EEG burst-suppr ession pattern occurred (mean isoflurane end-tidal concentration 1.9 ( SD 0.2) %. With a stable isoflurane concentration, the patients receiv ed isoflurane-air-oxygen and isoflurane-nitrous oxide-oxygen (FiO(2) 0 .4) in a randomized cross-over manner. EEG and SEPs were simultaneousl y recorded before, and after wash-out or wash-in periods for nitrous o xide. The proportion of EEG suppressions as well as SEP amplitudes for cortical N-20 were calculated. \ Results: The proportion of EEG suppr essions decreased from 53.5% to 34% (P<0.05) when air was replaced by nitrous oxide. At the same time, the cortical N-20 amplitude was reduc ed by 69% (P<0.01). Conclusion: The results suggest that during isoflu rane anaesthesia, nitrous oxide has a different effect on EEG and cort ical SEP at the same time. The effects of nitrous oxide may be mediate d by cortical and subcortical generators. (C) Acta Anaesthesiologica S candinavica 41 (1997).