Surveillance de la profondeur de l'anesthésie générale

Although depth of anaesthesia increases when increasing anaesthetic drugs doses, the same level is not induced by the same dose in all patients: age, weight, medical history, and drug interactions may widely influence the drug requirements. Consequently, assessing depth of anaesthesia is a key issue to adjust drug dosing in order to avoid both underdosage (awareness) and overdosage (possible side-effects). Clinical assessment is focused on 2 main components: loss of consciousness: identified by loss of verbal response and retrospectively by lack of memory; response to noxious stimuli as movement, haemodynamic changes, ... Pharmacological assessment estimates drug concentrations related to the clinical state, based on the assumption that the relationship between effect-site (CNS) concentration and clinical effects is time independent. Neurophysiological assessment examines some measurable effects induced by anaesthetic drugs on CNS, either on spontaneous EEG, evoked potentials, or autonomic nervous system activity (heart rate, pupils, oesophageal sphincter...). Today, only automatic analysis of EEG and mid-latency auditory evoked potentials have been developed enough to reach clinical usefulness. However, both are poor predictors of response to stimulation.