Objectives: The present study was undertaken to provide basic information about bilateral frontal cerebral electrical activity after induction, before and after skin incision, and at a steady state during sevoflurane anesthesia at the end tidal concentration 1, 1.2, 1.4 and 1.6 MAC and determine the association between the electrical cerebral activity with other clinical end points, i.e. motor responses and post-operative recall.
Material and Method: the Dual Channel Brain Activity, ABM2 (DATEXR) was used to continuously monitor frontal EMG and electrical cerebral activities (i.e. frequency and amplitude) of both hemispheres in 20 adult female patients undergoing balanced anesthesia in Maharaj Chiang Mai Hospital. The eligible patients were randomly assigned to receive 1.0 MAC, 1.2MAC, 1.4MAC or 1.6MAC of a mixture of sevoflurane and 66.67% nitrous oxide in oxygen during anesthesia. The anesthesia was induced with 5 mg/kg thiopental and supplemented with 1-1.5 microgram per kilogram of fentanyl. The standard dose of pancuronium or atracurium was given during maintenance.
Results: After induction with 5 mg/kg thiopental, the mean (95% confidence interval) of frontal EMG significantly decreased from 2.66 (1.63,4.29) to 1.41 (0.2,1.61). When eyelash reflex was absent, the mean (95%CI) frequency and amplitude of the right frontal EEG was 3.89 (3.29, 4.497) Hz, 39.58 (32.11, 47.05) microvolt and left frontal EEG was 3.84 (3.43, 4.25) Hz, 33.55 (28.59, 38.61) microvolt. The findings were consistent with the raw EEG shown on the monitor, i.e. a progressive decrease in the frequency and an increase in the amplitude. During maintenance with the inhaled anesthetics, there was a statistically significant decrease in frequency of right frontal in those who had received the inhaled anesthetic concentration to reach 1.4 and 1.65 MAC (p < 0.05)(repeated measure ANOVA). At the steady state of end tidal concentration of the sevoflurane there was consistency in decreasing frequencies and increasing amplitudes of both hemispheres of the groups with higher MAC values (p < 0.05) (Table 3). However, the authors failed to demonstrate the relationships between EEG changes and other clinical responses.
Conclusion: The present study has provided basic information about cerebral electrical activity during the balanced anesthesia with sevoflurane. As anesthesia deepened by increased MAC, the frequency decreased and the amplitude increased.

Keywords: EEG, Balanced anesthesia, Hypnosis, Sevoflurane