Nitrous Oxide Decreases Electroencephalogram Complexity, But Not Frequency Content at Loss of Consciousness  Sarah Eagleman , Teo Jeon Shin, M.D.,PhD (Seoul National University); M. Bruce MacIver, M.Sc., Ph.D. (Anesthesiology, Perioperative, Stanford University, Palo Alto, CA )   C20

Background: Nitrous Oxide (N2O) is an analgesic, sedative, and hypnotic gas, often used to supplement surgical anesthesia and reduce pain in dentistry. N2O has a unique mechanism of action and is associated with electroencephalogram (EEG) effects that differ from other sedatives and hypnotics.1,2 It has been reported that little or no change is observed in the frequency content EEG signals during N2O sedation. Additionally, commercially available brain monitors fail to detect changes even at concentrations that produce marked sedation.3,4 We hypothesized that a geometric, phase-space complexity measure derived from nonlinear dynamics could significantly discriminate between brain states produced by N2O, as this measure works well using other sedatives, regardless of their effects on EEG spectral content. Methods: We compared synchronicity, spectral, and complexity measures produced by 50% N2O in oxygen, across 15 participants receiving only this agent to produce loss of response, LOR. Results: In agreement with earlier reports, N2O produced only subtle effects on EEG signals, with no significant differences seen for delta, theta, alpha, beta, or gamma frequency bands.3,5 During sedation, a predominant theta/alpha rhythm was present that significantly synchronized wide regions of cortex during LOR compared to recovery (assessed using Pearson's correlation between Fp1 and P4, p < 0.04). Complexity measures clearly and significantly decreased in sedated participants evidenced by a 'flattening' in 3D attractor shapes created from EEG signals. This flattening was statistically significant (p < 0.02; n=15) comparing EEG signals 30 seconds before versus after LOR. Conclusions: These results indicate that information complexity in the EEG is significantly depressed by N2O, similar to effects produced by other volatile agents at LOR. This indicates a common breakdown in the brain's ability to process information is observed in EEG signals, even when there is no significant effect on amplitude or spectral measures of the EEG.