What brain waves indicate conscious response during anaesthesia?

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Aim To evaluate electroencephalographic (EEG) markers of behavioural responsiveness during anaesthesia, providing insights into the mechanisms of loss and return of behavioural responsiveness during anaesthesia induction, noxious stimulation and emergence from anaesthesia Significance Awareness under anaesthesia is a rare but important complication of anaesthesia associated with significant negative psychological sequelae. The incidence of awareness with recall is well established at approximately 1 case per 1000 anaesthetic events. The ability of currently used depth of anaesthesia monitors to prevent awareness with recall is contentious, and whilst processed values correlate well with pharmacological levels or effect site concentrations of anaesthetic agents, their ability to detect behavioural responsiveness during anaesthesia as revealed by isolated forearm testing is poor. This is perhaps due to their reliance upon frontal EEG measurement and analysis. Recent studies in healthy volunteers using functional magnetic resonance imaging (fMRI) and multichannel EEG have identified key changes in regions of the cortex associated with loss of behavioural responsiveness during anaesthesia. In particular there is growing evidence that a key marker of consciousness is the electroencephalographic connectivity between frontal and parietal regions of the brain. Hypothesis Loss of fronto-parietal connectivity as measured from the EEG results in loss of behavioural responsiveness to command during anaesthesia. Return of behavioural responsiveness during noxious stimulation or emergence from anaesthesia requires return of fronto-parietal connectivity. Objectives 1. To evaluate multichannel EEG markers of behavioural responsiveness to command 2. To evaluate multichannel EEG markers of spontaneous motor response to noxious stimuli 3. To explore the relationships between cortical arousal and autonomic nervous system responses Methods Prospective observational cohort study of 60 patients undergoing microlaryngoscopy and airway surgery. Key features will be a slow controlled induction of anaesthesia to maintain spontaneous ventilation. Multichannel EEG and haemodynamic recordings will be taken continuously. Behavioural responsiveness will be checked by command at multiple time points during induction, emergence and during noxious stimuli. Spontaneous motor responses will also be recorded. The EEG recordings will be analysed using MATLAB software including spectral analysis, entropy and connectivity measures and relationships between EEG markers and behavioural responsiveness and non-responsiveness will be assessed using appropriate statistical tests. Benefits Ultimately we hope our findings will contribute to the development of improved depth of anaesthesia monitoring. We will obtain a rich dataset which can be further analysed to gain insights into mechanism of anaesthesia, interactions between hypnotic and analgesic agents and relationships between cortical arousals, EEG patterns and autonomic responses in the presence and absence of verbal and noxious stimuli.

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