IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 53, NO. 4, APRIL 2006 617 Quantifying Cortical Activity During General Anesthesia Using Wavelet Analysis Tatjana Zikov, Member, IEEE, Stéphane Bibian*, Student Member, IEEE, Guy A. Dumont, Fellow, IEEE, Mihai Huzmezan, Senior Member, IEEE, and Craig R. Ries Abstract—This paper reports on a novel method for quantifying the cortical activity of a patient during general anesthesia as a sur- rogate measure of the patient’s level of consciousness. The proposed technique is based on the analysis of a single-channel (frontal) electroencephalogram (EEG) signal using stationary wavelet transform (SWT). The wavelet coef- ficients calculated from the EEG are pooled into a statistical representation, which is then compared to two well-defined states: the awake state with normal EEG activity, and the isoelectric state with maximal cortical depression. The resulting index, referred to as the wavelet-based anesthetic value for central nervous system monitoring ( ), quantifies the depth of consciousness between these two extremes. To validate the proposed technique, we present a clinical study which explores the advantages of the in comparison with the BIS monitor (Aspect Medical Systems, MA), currently a reference in consciousness monitoring. Results show that the and BIS are well correlated during periods of steady-state despite fundamental algorithmic differences. However, in terms of dynamic behavior, the offers faster tracking of transitory changes at in- duction and emergence, with an average lead of 15–30 s. Further- more, and conversely to the BIS, the regains its prein- duction baseline value when patients are responding to verbal com- mand after emergence from anesthesia. We conclude that the pro- posed analysis technique is an attractive alternative to BIS moni- toring. In addition, we show that the dynamics can be modeled as a linear time invariant transfer function. This index is, therefore, well suited for use as a feedback sensor in advisory systems, closed-loop control schemes, and for the identification of the pharmacodynamic models of anesthetic drugs. Index Terms—Consciousness monitoring, depth of anesthesia, depth of hypnosis, electroencephalogram (EEG), wavelet trans- form (WT). Manuscript received July 10, 2004; revised July 17, 2005. Asterisk indicates corresponding author. T. Zikov was with the Department of Electrical and Computer Engineering, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada. She is currently with the NeuroWave Division, Cleveland Medical Devices Inc., Cleve- land, OH 44103 USA. *S. Bibian is with the Department of Electrical and Computer Engineering, the University of British Columbia, Vancouver, BC V6T 1Z4 Canada (e-mail: bibians@ece.ubc.ca). G. A. Dumont is with the Department of Electrical and Computer Engi- neering, The University of British Columbia, Vancouver, BC V6T 1Z4 Canada. M. Huzmezan is with the Department of Anesthesiology, Pharmacology & Therapeutics, The University of British Columbia, Vancouver, BC V6T 1Z3 Canada, and also with the United Technologies Research Center, United Tech- nologies Corporation, East Hartford CT 06415 USA. C. R. Ries is with the Department of Anesthesiology, Pharmacology & Ther- apeutics, The University of British Columbia, Vancouver, BC V6T 1Z3 Canada. Digital Object Identifier 10.1109/TBME.2006.870255 I. INTRODUCTION I N the operating room, the anesthesiologist is essential in providing optimal working conditions to surgeons, and in ensuring patient safety and comfort. To achieve these end- points, anesthesiologists employ a variety of drugs to alter cognitive processing, regulate cardiorespiratory functions, and block muscle movement. One major aspect of the practice is to use these drugs in quantities that warrant unconsciousness and the absence of response to surgical stress, while avoiding pharmacological toxicity (e.g., cardiac morbidity). Although the understanding of the underlying mechanisms of anesthesia is still limited, it is commonly accepted that anes- thesia is comprised of at least 3 functional constituents: immo- bility, analgesia and hypnosis. The third component describes drug-induced unconsciousness, and the fact that all anesthetic drugs depress cognitive processing and provoke loss of con- sciousness. As of today, there is still no accepted scientific def- inition of hypnosis and consciousness, let alone a gold standard as to the quantification and measurement of these endpoints. With the central nervous system (CNS) being the target of anesthetic drugs, the electroencephalogram (EEG) signal has received considerable attention from the anesthesia research community. The effects of anesthetic drugs on the EEG have been well described and documented since the early 1940s. Although EEG patterns appear to be drug-dependent and not specific to anesthesia only, most anesthetic agents alter cortical activity in a dose-dependant manner. Moreover, it has been recently shown that there exist reversible invariant electroen- cephalographic changes, independent of the drug used, and independent of the anesthesia protocol [1]. Hence, it is advo- cated that the EEG can provide a reliable basis for deriving a surrogate measurement of hypnosis. In recent years, the anesthesia community has witnessed the development of a number of EEG-based monitors of conscious- ness. These monitors quantify the patient’s cortical activity in order to estimate the hypnotic depth, also referred to as depth of anesthesia. One such monitor, the BIS monitor (Aspect Med- ical Systems, MA), first made available in 1996, has received considerable attention from the research community. Based on bispectral analysis of the EEG, the BIS monitor calculates and displays a dimensionless descriptor of the EEG, referred to as the bispectral index scale (BIS). The original motivation for the BIS monitor was the detection and prevention of intra-operative awareness. Two large scale studies (B-AWARE and SAFE 2) have recently substantiated 0018-9294/$20.00 © 2006 IEEE