Monitoring consciousness: the current status of EEG-based depth of anaesthesia monitors

Direct and indirect inhibitory effects of anaesthetic agents on cortical activity are reflected in the electroencephalogram (EEG) as: (i) a shift from low-amplitude, high-frequency EEG, to high-amplitude, low-frequency activity (indicative of cortical depowering) and; (ii) the appearance of spindles and K-complexes (indicative of thalamocortical hyperpolarisation and sensory blockade). Existing EEG monitors use cortical activity as a proxy measure for consciousness. However the state of the cortex at any given moment does not accurately predict the state that it will enter in response to a noxious stimulus, and EEG monitors do not differentiate well between different levels of rousability. Also the literature reveals many instances where the EEG pattern is dissociated from conscious state (e.g. an awake-looking EEG, but an unresponsive patient; or a slow-wave EEG in an awake patient). Fortunately, a slow-wave EEG (even in the presence of a responsive patient) usually indicates profound amnesia for explicit memory.