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BJA Advance Access originally published online on December 25, 2008
British Journal of Anaesthesia 2009 102(2):227-233; doi:10.1093/bja/aen356
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© 2008 The Author(s)
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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

A. J. Aho1,3,*, A. Yli-Hankala1,3,{dagger}, L.-P. Lyytikäinen3 and V. Jäntti2,3

1 Department of Anaesthesia
2 Department of Clinical Neurophysiology, Tampere University Hospital, PO Box 2000, 33521 Tampere, Finland
3 University of Tampere, Medical School, Tampere, Finland

* Corresponding author. E-mail: antti.j.aho{at}uta.fi

Background: EntropyTM 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.

Keywords: anaesthetics i.v., propofol; analgesics opioid, remifentanil; measurement techniques, electromyography; monitoring, electroencephalography

{dagger} Declaration of interest. Professor Yli-Hankala is a paid consultant for GE Healthcare Finland.


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D. M. Mathews, A. J. Aho, A. Yli-Hankala, L.-P. Lyytikainen, and V. Jantti
Response entropy-state entropy difference and nociception: a matter of context
Br. J. Anaesth., July 1, 2009; 103(1): 135 - 137.
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The Response Entropy-State Entropy Difference and Nociception: A Matter of Context
Donald M Mathews
British Journal of Anaesthesia, 1 Apr 2009 [Full text]
Re: The Response Entropy-State Entropy Difference and Nociception: A Matter of Context
Antti J Aho
British Journal of Anaesthesia, 21 Apr 2009 [Full text]

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