[Monitoring the depth of anaesthesia: why, how and at which cost?]

The precise titration of anaesthetic agents is necessary to avoid the consequences of a too light depth of anaesthesia such as unexpected intraoperative awareness, as well as a too deep level of anaesthesia, which can be deleterious in terms of postoperative morbidity and mortality. The clinical evaluation of the depth of anaesthesia is poorly sensitive and specific. It does not permit to distinguish between pharmacodynamic components of anaesthesia. Several paraclinical depth of anaesthesia indices are currently available. Most of them are mainly designed to monitor the depth of the hypnotic component of anaesthesia. Their calculation is mostly based on the mathematical analysis of the electroencephalogram. They are efficient at reducing the incidence of unexpected intraoperative awareness, adjusting anaesthetic depth at an individual scale, predicting the time needed for recovery, allowing early extubation of patients, reducing their length of stay in the post anaesthesia care unit, and limiting the number of episodes of peroperative over and under dosage of anaesthetic agents. The knowledge of conditions that may impede the accurate interpretation of those indices is mandatory for an optimal use. Although undoubtedly beneficial for the patients, the use of those monitors is frequently responsible for supplementary' costs, particularly when the procedure is short.