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

Background: 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. Methods: 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. Results: 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. Conclusions: 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.