Hypnosis monitoring during general anaesthesia : with focus on awareness

An objective measure of the hypnotic component of general anaesthesiahas been sought for decades. The electroencephalogram (EEG) was early recognized as a possible usefulintraoperative monitor when the effects of anaesthetic drugs on human EEGrecordings were noted. The last few years have brought forward a numberof commercialized monitor devices that by an index claim to give theanaesthetist an objective measure of hypnosis. Two diverse monitoringmethods, differing in principle, have been developed: 1) passivelyprocessed EEG, and 2) stimulated and thereafter processed EEG. TheBispectral Index Scale (BIS) and the A-line ARX-Index (AAI) are twosuch 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 theamount of anaesthetics given and expedite postoperative recovery ascompared to standard practice. However a number of studies have alsoshown that the sensitivity and specificity for BIS and AAI are notperfect, and fears have been raised that the incidence of awareness evenmight increase if the anaesthetist aims at an upper threshold value. Inthis thesis, it is demonstrated that BIS monitoring during generalanaesthesia in a noncardiac, surgical population requiring endotrachealintubation and/or a neuromuscular blocking agent (NMBA) was associatedwith a significantly reduced incidence of awareness as compared to ahistorical control population.Our results suggest that BIS betterdisplays drug related alterations in the level of hypnosis than AAI in analready anesthetized patient, and that there is no difference between BISand AAI in the response time to a noxious stimulus. We have further seenthat profound neuromuscular block attenuate BIS and AAI responses to astandardized noxious stimulation during sevoflurane anaesthesia ascompared to a partial neuromuscular block or after neostigmine inducedreversal, and that the monitor responses were not due to electromyogram(EMG) contamination of the EEG. It is demonstrated that EEG responses tonoxious stimulation, in terms of power and coherence in the γ-band, areaffected by profound neuromuscular block during light sevofluraneanaesthesia.In conclusion we demonstrate that hypnosis monitoring can reduce theincidence of awareness, BIS is superior to AAI to display alterations inthe level of hypnosis during aneasthesia and NMBAs affect the level ofhypnosis during light general anaesthesia in stimulated patients.