40 - Monitoring the State of the Brain and Central Nervous System During General Anesthesia and Sedation

Abstract

General anesthesia is a drug-induced reversible state consisting of anti-nociception, unconsciousness, amnesia and akinesia with maintenance of physiological stability. Continuous physiological monitoring is critical of ensuring an adequate anesthetic state. Heart rate and blood pressure, and in the absence of muscle relaxants, muscle tone and movement, are the primary variables that anesthesia providers monitor to infer anesthetic state and guide drug dosing. Use of these variables to track anesthetic state is justified by the autonomic nociceptive pathway, which governs the body’s autonomic and arousal responses to nociceptive stimuli. For approximately twenty years now, some anesthesia providers have also used electroencephalogram (EEG)-based indices to track anesthetic state. These indices, such as the Bispectral Index and the Physiological State Index, use proprietary algorithms to analyze the EEG in near real-time and determine the adequacy of the anesthetic state. Recently, anesthesia providers have begun to use the unprocessed EEG and its spectrogram to monitor the level of unconsciousness in patients receiving general anesthesia or sedation. This approach is predicated on the observation that anesthetic-induced EEG oscillations are likely a primary mechanism through which anesthetics produce unconsciousness. Anesthetic-induced EEG oscillations change systematically with drug dose, anesthetic class and patient age. Measuring entropy and changes in functional connectivity are other approaches currently being refined to monitor anesthetic state. The ability to track physiological markers in real time makes possible the use of computer-driven closed loop anesthesia delivery systems to control anesthetic state. The development of real-time monitors of nociception is further enhancing anesthetic state monitoring.