Response of the Conox quantitative electroencephalographic monitor to neuromuscular block in awake volunteers

Abstract

The Conox monitor analyses the frontal EEG to generate two indices of anaesthetic effects: qCON, intended to indicate the level of consciousness, and qNOX, designed to reflect responsiveness to noxious stimuli. Two similar quantitative EEG devices, BIS and Entropy, have been shown to require muscle activity (EMG) to generate accurate index values in awake individuals. Without EMG, these devices produce misleadingly low values and incorrectly suggest sedation or anaesthesia despite the cortical EEG showing the subjects are awake. As EMG affects frequency bands used by Conox, it too could be incorporating muscle activity to generate high values in awake individuals. We replayed EEGs recorded during awake paralysis to the Conox monitor via an electronic playback system to test whether it requires EMG to generate accurate values in awake subjects. Both qCON and qNOX decreased after neuromuscular block to values consistent with sedation or anaesthesia, despite subjects being fully awake. qCON decreased below 60 in 15 of 19 trials, and qNOX decreased below 60 in 11 of 19 trials. Overall, 42% of qCON values during paralysis were <60, the level supposedly indicating anaesthesia. Conox requires muscle activity to generate accurate values in awake individuals. Consequently, it might be an unreliable indicator of awareness in patients who have received neuromuscular blocking drugs. Studies conducted without neuromuscular block can provide misleading guidance when applied to Conox use in paralysed patients. Clinicians should approach manufacturer guidelines with caution and not rely solely on index values to guide dosing of anaesthetic drugs.