Frontal alpha-delta EEG does not preclude volitional response during anaesthesia: prospective cohort study of the isolated forearm technique.

Abstract

The isolated forearm test (IFT) is the gold standard test of connected consciousness (awareness of the environment) during anaesthesia. The frontal alpha-delta EEG pattern (seen in slow wave sleep) is widely held to indicate anaesthetic-induced unconsciousness. A priori we proposed that one responder with the frontal alpha-delta EEG pattern would falsify this concept. Frontal EEG was recorded in a subset of patients from three centres participating in an international multicentre study of IFT responsiveness following tracheal intubation. Raw EEG waveforms were analysed for power-frequency spectra, depth-of-anaesthesia indices, permutation entropy, slow wave activity saturation and alpha-delta amplitude-phase coupling. Volitional responses to verbal command occurred in six out of 90 patients. Three responses occurred immediately following intubation in patients (from Sites 1 and 2) exhibiting an alpha-delta dominant (delta power >20 dB, alpha power >10 dB) EEG pattern. The power-frequency spectra obtained during these responses were similar to those of non-responders (P>0.05) at those sites. A further three responses occurred in (Site 3) patients not exhibiting the classic alpha-delta EEG pattern; these responses occurred later relative to intubation, and in patients had been co-administered ketamine and less volatile anaesthetic compared with Site 1 and 2 patients. None of the derived depth-of-anaesthesia indices could robustly discrimate IFT responders and non-responders. Connected consciousness can occur in the presence of the frontal alpha-delta EEG pattern during anaesthesia. Frontal EEG parameters do not readily discriminate volitional responsiveness (a marker of connected consciousness) and unresponsiveness during anaesthesia. NCT02248623.