Journal of Clinical Monitoring and Computing 17: 377^381, 2002. ß 2003 KluwerAcademic Publishers. Printed in the Netherlands. BIASING EFFECT OF THE ELECTROMYOGRAM ON BIS: A CONTROLLED STUDY DURING HIGH-DOSE FENTANYL INDUCTION Maurizio Renna, MD FRCA,TimWigmore, FRCA, Ali Mofeez, FRCA, and Charles Gillbe, FRCA From the Department of Anaesthesia, Royal Brompton Hospital, Sydney Street, London SW3 6NP, U.K. Received Jan 23, 2003. Accepted for publication Mar 18, 2003. Address correspondence to Dr Maurizio Renna, Department of Anesthesiology, Section of Cardiothoracic Anesthesia, Washington University School of Medicine, 660 South Euclid Avenue, Box 8054 St Louis, MO, 63110 U.S.A. E-mail: rennam@notes.wustl.edu Renna M, Wigmore T, Mofeez A, Gillbe C. Biasing e¡ect of the electromyogram on BIS: A controlled study during high-dose fen- tanyl induction. J Clin Monit 2002; 17: 377^381 ABSTRACT. Objective. A biasing e¡ect of the electromyogram (EMG) on the Bispectral Index (BIS) may explain discrep- ancies in previous studies assessing BIS in the presence of neuromuscular activity. Our aims were: to evaluate variations of BIS in the presence of high EMG activity associated with muscular rigidity after administration of high-dose fentanyl; to compare muscular rigidity, as measured by the EMG variable of the BIS monitor, in patients who were adminis- tered two di¡erent dosages of fentanyl at induction of cardiac anaesthesia. Methods. 26 patients undergoing CABG sur- gery, after premedication with morphine 0.15 mg/kg, were randomized to receive either fentanyl 50 mcg/kg (group F) or fentanyl 10 mcg/kg plus etomidate 0.2 mg/kg (group EF). The induction dose was administered over 2 minutes. Patients were manually ventilated with O 2 via face mask. Five minutes after induction was complete, patients were clinically assessed using the Responsiveness portion of the Observer’s Assess- ment of Alertness/Sedation scale (OAAS). Haemodynamic data were recorded and arterial blood samples obtained at the time of OAAS observation. Patients were administered a neuromuscular blocking agent only after the OAAS assess- ment. BIS (3.4) was recorded from an A-2000 Õ EEG monitor (Aspect Medical Systems) using disposable sensors (BIS Sen- sor Õ , Aspect Medical Systems) applied per manufacturer’s instructions. Data were recorded on a PC for o¡-line analysis. Results. At the time of OAAS observation, mean (95% CI) BIS in group F was 85 (77^92) compared to 67 (56^79) in group EF (p = 0.01). Similarly, mean (95% CI) EMG was 50 dB (45^56) in F and 41 dB (35^47) in EF (p = 0.01). Correlation between BIS and EMG was very high (r 2 = 0.88). OAAS scores were signi¢cantly higher in group F (p = 0.03). Non signi¢cant correlation was observed between BIS and OAAS scores (r 2 = 0.32, p = 0.1). Backward stepwise multiple regression analysis including EMG, pH, CO 2 ,O 2 and OASS scores showed EMG as strong predictor of BIS (p < 0.0001, r 2 = 0.7). Regression of EMG against BIS yielded the equa- tion: BIS = 3.7 + (1.6  EMG). Conclusion. During fen- tanyl-induced muscular rigidity BIS recordings re£ect EMG variations.When assessing BIS in the absence of neuromuscular blockade, it is necessary to evaluate the e¡ect of EMG on BIS before making conclusions about depth of sedation. Fentanyl- induced rigidity appears to be a dose-related phenomenon which the EMG variable of BIS 3.4 is able to quantify. KEY WORDS. Bispectral index, EEG, EMG, opioids, rigidity, high-dose fentanyl, sedation scores. INTRODUCTION Numerous studies have evaluated the Bispectral Index (BIS Õ , Aspect Medical Systems) as a reliable tool for monitoring hypnosis during clinical anaesthesia [1^4]. However, the application of this technology in clinical