In search of a reliable awareness monitor.

T he term anesthesia originally meant " the state in which a patient is insensible to the trauma of surgery. " Although, the science of anesthesiol-ogy has advanced rapidly, defining, measuring, and understanding depth of anesthesia has moved ahead slowly. Indeed, we are yet to define properly the phenomenon that we use in our everyday practice to render patients insensitive to the trauma of surgery. Prys-Roberts (1) defined anesthesia as the state in which, as a result of drug-induced unconsciousness, the patient neither perceives nor recalls noxious stimuli. He further stated that analgesia, muscle relaxation, and suppression of autonomic activity are not the components of anesthesia, but should be considered as desirable supplements to the state of anesthesia as a means to enable surgery to be performed. Although awareness during surgery was not unknown before the use of muscle relaxants, the use of small concentrations of anesthetic with muscle relaxants resulted in some patients being aware during surgery. The incidence of awareness during anesthesia and surgery is variable and depends on the type of surgery, the anesthetics used, and the timing of and technique for, evaluating awareness and recall. In two large series of patients, the incidence of awareness has been reported to be 0.2% (2) and 0.16%(3) and a more frequent incidence ranging from 1.1% to 1.5% during cardiac surgery (4,5). Awareness during general anesthesia can be a horrifying experience and may cause acute psychological trauma (6). It may also have medico-legal implications. Therefore, Eich et al. (7) believe that recall indicates a failure to anesthetize. A reliable indicator that would confirm that the level of anesthesia is adequate to ensure lack of awareness is obviously desirable. Initially, the hemodynamic response to laryngoscopy, endotracheal intubation and/or skin incision was used to assess the depth of anesthesia. Subsequently, electroencephalography (EEG) and processed EEG were used to relate drug concentration and clinical depth of anesthesia. However, application of these measures to assess clinical depth of anesthesia has not been very successful. The Bispectral index (BIS) is a variable derived from mathematical analysis of the EEG signal that estimates phase difference. It measures the hypnotic component of the anesthetic and is a potentially useful adjunct for monitoring the depth of anesthesia. The BIS is a di-mensionless number that varies from 0 to 100. The monitor assigns the BIS number based on a database of prior recordings and the expert opinion of the …