Background and Goal of Study: Despite several progresses in the understanding of GA mechanisms, some questions remained unanswered like the precise mechanisms of awakening after a GA. Recovery from GA was perceived as passive pharmacokinetic process where the drugs would be eliminated and the circuits affected by the GA would reactivate in mirror of their deactivation1. But it may not be so simple, due to the intricated mechanisms leading to the loss of consciousness. We tested the hypothesis that 3 hours after an uneventful GA for inguinal hernia repair, frontal EEG is not back to normal in all the patients.

Materials and Methods: Anaesthesia was induced by sufentanil, ketamine, propofol and cisatracurium and maintained by sevoflurane. During the surgery, patients were continuously monitored with a multiparametric device Carescape monitor B850 (General Electrics HealthcareTM Finland Oy, Helsinki, Finland). On top of the usual monitoring, we included a 2-channel EEG recording at 100 Hz using the optional module E-EEG sold by GETM. To ensure proper detection of the anteriorization of oscillations described2,3, we used the frontal F4 electrode and the central C4 electrode with the common reference A1 (contralateral mastoid) using the international 10/20 classification. Three hours after the general anaesthesia, a new EEG was performed at the bedside. The three-hour delay was chosen based on the pharmocokinetic of the sevoflurane (half-life of 50 minutes). The Wilcoxon-Mann-Whitney test was used and P values less than 0.05 were deemed significant. All data processing was performed off-line using a commercial software package MATLAB 9.1 (The MathWorks Inc., Natick, MA, 2016). A clustering was made using the K-Means algorithm in order to extract relevant group from the data. After, a logistic regression was performed to construct a classification task.

Results and Discussion: From March to May 2017, 20 patients have been included. Several EEG parameters were compared before and after GA. Individual analysis of the Power Spectral Density (using the Welch's method), are presented in the Figure 1. Individual analysis of the results revealed a higher power in the 8-12 Hz for 6 patients (see figure 1, panel B) representing one third of the sample. Using an unsupervised algorithm (K-means clustering analysis), we identified the following risk factors for sustained alpha power: man, high blood pressure (HBP), significant post-operative pain, non-smoker, who underwent an anaesthesia longer than 90 minutes.

Conclusion(s): one third of the out-patients operated for inguinal hernia repair presented a sustained increase in power in the   band 3 hours after a GA. Attention on patient who presents the typical profile of HPAW (man with high blood pressure (HBP), post-operative pain, non-smoker, who went through a longer anaesthesia) should be reinforced as the clinical impact is still unknown.


10 / Electroencephalogram 3 hours after general anaesthesia (GA) is still not back to normal for all the patients