The meaning of EEG indicators of loss of consciousness during anaesthesia and coma.

Abstract

Raw EEG changes correlate well with the effects of anaesthetics on arousal. The purpose ofthe present paper is to analyse the significance of the EEG spectrograph. The alpha rhythm isa signature of the unconscious and according to the MAB hypothesis (Monoaminergic-Acetylcholinergic Balance) is an index of increased relative inhibitory muscarinic effects onmonoaminergic modulated conscious networks. The source of alpha rhythms varies, as doesthe type of unconscious information: from classical occipital alpha during resting state witheyes closed where no external visual information is processed, to states of unconsciousanaesthesia where alpha shifts to fronto-parietal networks. These are spontaneous oscillatorynetwork responses and the effects of GABA agonist anaesthetics increase the amplitude ofthese intrinsic rhythms. The hypothesis states that the concurrent presence of sustained deltaoscillations and alpha spindles signifies suppressed unconscious and conscious responsesrespectively, and permit neither implicit nor explicit cognition nor recollections. Further, themodel will be used to interpret EEG changes associated with more atypical anaesthetics andprovide reasons for the related cognitive effects. This will be based on the agents’pharmacological profiles and how they interact with these dual neuromodulatory systems.Evidence will also be drawn from EEG states associated with pathophysiology in coma andepilepsy. The pharmacological interpretation of EEG makes predictions of awareness indisorders of consciousness. The difficult circumstances of behaviourally inadequateindicators refers to situations of minimally conscious and in particular, vegetative stateswhere decision making is fraught with uncertainty.