Hypnosis monitoring during general anaesthesia : with focus on awareness

Abstract

An objective measure of the hypnotic component of general anaesthesia has been sought for decades. The electroencephalogram (EEG) was early recognized as a possible useful intraoperative monitor when the effects of anaesthetic drugs on human EEG recordings were noted. The last few years have brought forward a number of commercialized monitor devices that by an index claim to give the anaesthetist an objective measure of hypnosis. Two diverse monitoring methods, differing in principle, have been developed: 1) passively processed EEG, and 2) stimulated and thereafter processed EEG. The Bispectral Index Scale (BIS) and the A-line ARX-Index (AAI) are two such monitors were BIS represents the first principle and AAI the second.

In several studies the use of BIS and AAI has been shown to reduce the amount of anaesthetics given and expedite postoperative recovery as compared to standard practice. However a number of studies have also shown that the sensitivity and specificity for BIS and AAI are not perfect, and fears have been raised that the incidence of awareness even might increase if the anaesthetist aims at an upper threshold value. In this thesis, it is demonstrated that BIS monitoring during general anaesthesia in a noncardiac, surgical population requiring endotracheal intubation and/or a neuromuscular blocking agent (NMBA) was associated with a significantly reduced incidence of awareness as compared to a historical control population.

Our results suggest that BIS better displays drug related alterations in the level of hypnosis than AAI in an already anesthetized patient, and that there is no difference between BIS and AAI in the response time to a noxious stimulus. We have further seen that profound neuromuscular block attenuate BIS and AAI responses to a standardized noxious stimulation during sevoflurane anaesthesia as compared to a partial neuromuscular block or after neostigmine induced reversal, and that the monitor responses were not due to electromyogram (EMG) contamination of the EEG. It is demonstrated that EEG responses to noxious stimulation, in terms of power and coherence in the γ-band, are affected by profound neuromuscular block during light sevoflurane anaesthesia.

In conclusion we demonstrate that hypnosis monitoring can reduce the incidence of awareness, BIS is superior to AAI to display alterations in the level of hypnosis during aneasthesia and NMBAs affect the level of hypnosis during light general anaesthesia in stimulated patients.