[Is measuring the depth of anesthesia sensible? An overview on the currently available monitoring systems]

Abstract

Without any doubt there is an increasing need for accurately measuring depth of anesthesia - from the viewpoint of the anesthesiologist and also of the patient. The ideal monitoring should fulfill the following criteria: It should be applicable for any type of anesthesia (intravenous as well as inhalative anesthesia); the monitor must have an extremely high sensitivity (each patient being awake must be recognized by the device).If the monitor does not have a high sensitivity, the monitor itself might lead to an increasing number of patients being awake during anaesthesia, because the anesthesiologist might rely on the monitor and does not deepen anesthesia while the patient is awake. Specificity is not as important as sensitivity. As incidence of interoperative awareness is low, one must monitor more than 750 patients to recognize only one patient who is awake. Finally, the monitoring device must be economic. If costs are considerably increased by measuring depth of anesthesia in today's climate of cost consciousness, the monitoring has to be restricted to special high risk groups of patients. If monitoring depth of anesthesia will become simple, safe, and economic, each anesthesia should be monitored for its depth, as today each patient needs pulseoxymetry. We try to give an orientation about the available devices for monitoring depth of anesthesia. Since the introduction of BIS interest in measuring depth of anesthesia is growing very dynamically. Due to the dynamic growth we hope that we were able to present an actual and complete overview about the monitoring systems for measuring depth of anesthesia. Until today no monitoring system has proven to fulfill the mentioned criteria. Monitoring of the depth of anesthesia will be based on any processing of the spontaneous EEG or its evoked potentials. Which type of monitoring, entropy, BIS, PSI, or MLAEP will be used in clinical routine will be shown in the future. All available monitors are no predictors, whether depth of anesthesia is sufficient for the next painful surgical stimulus. They can only monitor the anesthetic state at the time of measurement. There is no "golden number" predicting absolutely safely that the patient is in adequate anesthesia. The anesthetist must consider any technique for monitoring of the depth of anesthesia as an additional help in improving care for his patient.