Awareness during general anesthesia: new technology for an old problem

The possibility of awareness during general anesthesia causes apprehension for the patient and the Certified Registered Nurse Anesthetist (CRNA). The goals of general anesthesia are to prevent the sensation of pain and produce a state of sedation, hypnosis, and unconsciousness so the patient will not remember the surgical procedure. An inadequate level of anesthesia can result in patient awareness during surgery. The current practice of anesthesia relies on indirect hemodynamic measurements such as blood pressure and heart rate to monitor the sedative hypnotic state of the patient's brain during general anesthesia. Hemodynamic responses are not reliable for predicting awareness just as blood pressure and heart rate are not indicative of consciousness. Electroencephalogram (EEG) waveforms are known to be affected by anesthetics. Characteristic EEG waveforms are a direct indication of the patient's level of consciousness. Unprocessed and computer-processed EEG recordings have been used in an attempt to monitor the patient's level of consciousness during general anesthesia. A raw or unprocessed EEG recording to monitor the level of consciousness during general anesthesia is problematic. The EEG signal is complex, affected by artifact, and it requires a dedicated interpreter. Conventional processed EEG monitoring systems are problematic because of the complexity of the equipment and technical difficulty of reading the EEG recording. The purpose of this article is to describe the history of awareness during anesthesia and introduce a new processed EEG monitor, the Bispectral Index (BIS) (Aspect Medical Systems, Inc., Natick, MA) with implications for future clinical use and research.