[Spectral entropy: a new method for anesthetic adequacy.]

Though universally employed, clinical signs to evaluate anesthetic adequacy are not reliable. Over the past years several pieces of equipment have been devised to improve intraoperative handling of anesthetic drugs, some of them directly measuring cerebral cortical activity (hypnosis). None of them, however, has offered the possibility of directly evaluating sub-cortical activity (motor response).

Spectral entropy measures irregularity, complexity or amount of EEG disorders and has been proposed as indicator of anesthetic depth. Signal is collected from the fronto-temporal region and processed according to Shannon's equation (H = - Sp k log p k, where p k represents the probability of a discrete k event), resulting in two types of analyses: 1) state entropy (SE), which evaluates cerebral cortex electrical activity (0.8 - 32Hz) and 2) response entropy (RE), containing both subcortical electromyographic and cortical electroence- phalographic components and analyzes frequencies in the range 0.8 - 47Hz.

Frontal muscles activation may indicate inadequacy of the subcortical component (nociception). Such activation appears as a gap between SE and RE. This, it is possible to directly evaluate both cortical (SE) and subcortical (RE) components providing better anesthetic adequacy.