Changes in the bispectral index and the spectral edge frequency 90 during different phases of anesthesia with sevoflurane

INTRODUCTION: Electroencephalographic monitoring is one of the techniques used to measure hypnosis during anesthesia. Efforts to find a means to apply this monitoring function are justified by cases of intraoperative awakening and are in the interest of guaranteeing patient welfare and controlling anesthesia. OBJECTIVES: To determine the changes in two electroencephalographic parameters, the bispectral index (BIS) and the spectral edge frequency 90 (SEF90), during the different phases of anesthesia.Patients and method. Forty-eight patients undergoing abdominal surgery were studied prospectively. Anesthesia was induced by propofol, fentanyl or sevoflurane. Anesthesia was maintained with sevoflurane and O2/N2O at a concentration of 70% to maintain BIS between 40 and 60. Variables monitored were heart rate, blood pressure, arterial oxygen saturation (with a pulse oximeter), expired CO2 partial pressure, inspired and expired sevoflurane partial pressure, esophageal temperature, SEF90 (defined as the frequency below which 90% of the power in the electroencephalogram resides) and BIS. All variables were recorded during three phases: induction, maintenance and recovery from anesthesia. We then performed an analysis of variance, with p < 0.05 considered significant. RESULTS: During induction mean BIS decreased from 95.6 to 34.7 after intubation, while SEF90 fell from 20.5 Hz to 11.9 Hz. During maintenance BIS, SEF90 and expired sevoflurane partial pressure remained constant. During recovery mean BIS increased from 59.2 to 92.3 upon extubation, and SEF90 went from 15.1 to 22.2 Hz; although heart rate increased, blood pressure did not. The mean electroencephalographic values recorded when movement occurred were 77 for BIS (range 58-96) and 18 for SEF90 (range 13-18). CONCLUSION: Electroencephalographic monitoring is useful for distinguishing between states of consciousness and unconsciousness during anesthesia, and is valid for the phases of induction and recovery. BIS values over 58 and SEF90 values over 13 may indicate inadequate hypnosis.