Explaining Entropy responses after a noxious stimulus, with or without neuromuscular blocking agents, by means of the raw electroencephalographic and electromyographic characteristics

Background: Entropy™, an anaesthetic EEG monitoring method, yields two parameters: State Entropy (SE) and Response Entropy (RE). SE reflects the hypnotic level of the patient. RE covers also the EMG-dominant part of the frequency spectrum, reflecting the upper facial EMG response to noxious stimulation. We studied the EEG, EMG, and Entropy values before and after skin incision, and the effect of rocuronium on Entropy and EMG at skin incision during sevoflurane–nitrous oxide (N₂O) anaesthesia. Methods: Thirty-eight patients were anaesthetized with sevoflurane–N₂O or sevoflurane–N₂O–rocuronium. The biosignal was stored and analysed off-line to detect EEG patterns, EMG, and artifacts. The signal, its power spectrum, SE, RE, and RE–SE values were analysed before and after skin incision. The EEG arousal was classified as β (increase in over 8 Hz activity and decrease in under 4 Hz activity with a typical β pattern) or δ (increase in under 4 Hz activity with the characteristic rhythmic δ pattern and a decrease in over 8 Hz activity). Results: The EEG arousal appeared in 17 of 19 and 15 of 19 patients (NS), and the EMG arousal in 0 of 19 and 13 of 19 patients (P<0.01) with and without rocuronium, respectively. Both β (n=30) and EMG arousals increased SE and RE. The δ arousal (n=2) decreased both SE and RE. A significant increase in RE–SE values was only seen in patients without rocuronium. Conclusions: During sevoflurane–N₂O anaesthesia, both EEG and EMG arousals were seen. β and δ arousals had opposite effects on the Entropy values. The EMG arousal was abolished by rocuronium at the train of four level 0/4.