Facial muscle activity, Response Entropy, and State Entropy indices during noxious stimuli in propofol-nitrous oxide or propofol-nitrous oxide-remifentanil anaesthesia without neuromuscular block

Abstract

Background: Entropy is an anaesthetic EEG monitoring method, calculating two numerical parameters: State Entropy (SE, range 0-91) and Response Entropy (RE, range 0-100). Low Entropy numbers indicate unconsciousness. SE uses the frequency range 0.8-32 Hz, representing predominantly the EEG activity. RE is calculated at 0.8-47 Hz, consisting of both EEG and facial EMG. RE-SE difference (RE-SE) can indicate EMG, reflecting nociception. We studied RE-SE and EMG in patients anaesthetized without neuromuscular blockers.

Methods: Thirty-one women were studied in propofol-nitrous oxide (P) or propofol-nitrous oxide-remifentanil (PR) anaesthesia. Target SE value was 40-60. RE-SE was measured before and after endotracheal intubation, and before and after the commencement of surgery. The spectral content of the signal was analysed off-line. Appearance of EMG on EEG was verified visually.

Results: RE, SE, and RE-SE increased during intubation in both groups. Elevated RE was followed by increased SE values in most cases. In these patients, spectral analysis of the signal revealed increased activity starting from low (<20 Hz) frequency area up to the highest measured frequencies. This was associated with appearance of EMG in raw signal. No spectral alterations or EMG were seen in patients with stable Entropy values.

Conclusions: Increased RE is followed by increased SE at nociceptive stimuli in patients not receiving neuromuscular blockers. Owing to their overlapping power spectra, the contribution of EMG and EEG cannot be accurately separated with frequency analysis in the range of 10-40 Hz.

Figures

Fig 1

The behaviour of RE, SE, baseline-corrected RE−SE, and HR [mean (sd)] during the study in patients receiving propofol–nitrous oxide or propofol−nitrous oxide−remifentanil anaesthesia. 1, awake; 2, 90 s after anaesthetic induction; 3, 30 s before intubation; 4, 30 s after intubation; 5, after a 5 min equilibrium period; 6, 60 s before skin incision; 7, 30 s after skin incision; 8, 30 s after setting of the needle of Veress; 9, 30 s after beginning of gas insufflation; 10, at the end of gas insufflation; 11, 30 s after setting of the first laparoscopy trochar. ♣P<0.05, ♣♣P<0.01 within the propofol–nitrous oxide group, respectively. #P<0.05, ##P<0.01, and *P<0.05, **P<0.01 within the propofol–nitrous oxide–remifentanil group, and between the groups, respectively.

Fig 2

(a) The appearance of EMG activity in EEG signal after laryngoscopy (arrow downward) and attempted intubation in a patient not receiving remifentanil. EEG signal remains suppressed, despite strong EMG contamination. (b) Power spectrum of the EEG signal before (black line) and after (grey line) laryngoscopy in the same situation as in (a). EMG activity changes the spectrum after laryngoscopy. The spectrum with EMG impact starts to change already below 20 Hz. Dotted vertical line represents the 32 Hz frequency, which is used in Entropy calculation to differentiate EEG activity (<32 Hz) from EMG activity (>32 Hz). In Entropy calculation, low SE value increases rapidly after appearance of EMG, because the power at <32 Hz area increases. (c) EEG spectrogram (frequency vs time) of the same patient as in (a) and (b). Fast activity disappears in the beginning of anaesthesia (0–30 s). Continuous activity below 30 Hz is merely EEG. Placement of oropharyngeal airway (OA) at 1 min 40 s elicits short EMG response, shown as a vertical bar up to 150 Hz. Laryngoscopy and attempted intubation (IA) at arrow is associated with longer EMG response. Replacement of oropharyngeal airway at 3 min 50 s, and successful intubation (Intub) at 4 min 30 s. Long-lasting EMG activity ensues.

Fig 3

EEG spectrograms presenting the whole study periods in two patients: (a) without and (b) with remifentanil infusion. Fast activity disappears in the beginning of anaesthesia. EEG activity is seen in both recordings as a continuous activity below 30 Hz. EMG contamination is seen as vertical bars up to 150 Hz. AI, attempted intubation; I, intubation; PP, patient positioning; SI, skin incision. Laryngoscopy and intubation are associated with strong EMG activity in both recordings. Thereafter, no EMG is seen in (b), that is, the patient receiving remifentanil. In (a), both noxious and non-noxious stimuli (like positioning the patient) elicit EMG activity. Horizontal bars at 50 and 150 Hz are power line (50 Hz) artifacts and its harmonic (150 Hz). Vertical bars (ImpCh) indicate automatic Entropy sensor impedance checks every 10 min.

Fig 4

Two successive 40 s samples of EEG which show development of EMG after intubation (arrow upward) during EEG burst suppression. The box below is an enlarged 4 s piece of the signal, showing the beginning of EMG activity.