Entropy during propofol hypnosis, including an episode of wakefulness.

Depth of anaesthesia has proved to be a complex process to quantify. Monitors based on bispectral analysis of the electroencephalogram and auditory evoked potential have been available, but only recently has a monitor based on entropy become available. This study determined state entropy and response entropy in nine healthy volunteers during propofol hypnosis with a brief intervening period of wakefulness. Both the calculated entropy indices decreased with increasing levels of sedation (r2 = 0.58 and 0.61, respectively) and they showed a high correlation with each other (r2 = 0.94). However, an overlap was observed in real time indices between different stages of the Observer's Assessment of Alertness/Sedation Scale. Only three of the nine volunteers had explicit memories from the episode of wakefulness. Electroencephalographic entropy monitors seem to have potential for staging clinical hypnotic effects.