Fast Oscillatory EEG Activity Induced by Analgesic Concentrations of Nitrous Oxide in Man

EEGs during inhalation of analgesic concentrations of 30%, 50%, and 70% nitrous oxide in oxygen, in the absence of other medication were studied in man. Although nitrous oxide has been considered to produce no significant change in EEG activity, it was found to produce characteristic fast oscillatory activity (FOA) in a predictable fashion. FOA was associated with unconsciousness or unresponsiveness to stimuli and appeared predominantly in the frontal areas. Power spectrum analyses showed that the frequency of FOA was remarkably constant across subjects and across trials with a peak frequency of 34 Hz. Amplitude and quantity of FOA increased as the concentration of nitrous oxide increased approaching 100 microV and more than 90%, respectively, under 70% nitrous oxide. It took approximately 1 hour after cessation of nitrous oxide before EEGs had completely returned control pattern with little FOA in spite of expected rapid decrease of alveolar nitrous oxide concentration. FOA can serve as a clinical monitor of the effectiveness of nitrous oxide analgesia.