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LETTER TO THE EDITOR

Electroencephalogram-Based Indexes Indicate the Drug Effect of the Brain, Not the (Un)Consciousness Itself

GE Healthcare Monitoring Solutions; Helsinki, Finland; laura.strandberg{at}ge.com

To the Editor:

Vassiliadis et al.1 presented a case of awareness during propofol– remifentanil anesthesia, despite low spectral entropy values. GE Healthcare Entropy^TM Module was used. In this particular case, an opioid and a muscle relaxant were given and the sedative dose of hypnotic (propofol) was not sufficient to guarantee unconsciousness. The electroencephalogram (EEG) slowed because of the remifentanil, and not because of hypnotic-induced unconsciousness. Similar cases have been reported previously, especially for bispectral index (BIS).2–4

Pharmacokinetic and pharmacodynamic interactions of remifentanil are published.5,6 We have calculated the effect-site concentrations in this particular case. Figure 1 reveals an exceptionally high remifentanil effect-site concentration that is sufficient to suppress EEG activity. EEG (or any index extracted from EEG) never indicates (un)consciousness itself, but rather the drug effect on the brain. When such effect is achieved with nonhypnotic pharmaceuticals, epileptic activity, or with some other non-conventional aid, every EEG-based index might be an inaccurate measurement of unconsciousness.

View larger version (15K): [in this window] [in a new window]   Figure 1. Effect–site concentrations of propofol and remifentanil.

Entropy monitoring is based on acquisition and processing of raw EEG and frontalis electromyography (FEMG) signals by using the entropy algorithm.7 Various studies have shown that entropy parameters, State Entropy (SE) and Response Entropy (RE), correlate well with a patient's anesthetic state. We assume that, in this case, the algorithm has correctly processed the EEG data. The EEG signal was slow because of the large dose of remifentanil and, consequently, Entropy was <60.

Entropy labeling cautions that entropy measurement is to be used as an adjunct to other physiological parameters in assessing the effects of certain anesthetic agents. Interpretation of entropy should always be based on understanding that the parameters are computed from raw EEG and FEMG signals and that any process that influences these signals does influence RE and SE, regardless of the source of increase or suppression.

L.S. and J.K. are paid full-time employees of GE Healthcare.

REFERENCES

1. Vassiliadis M, Geros D, Maria K. Awareness despite low spectral entropy values. Anesth Analg 2007;105:535[Free Full Text]
2. Rampersad SE, Mulroy MF. A case of awareness despite an "adequate depth of anesthesia" as indicated by a Bispectral Index® monitor. Anesth Analg 2005;100: 1363–4[Abstract/Free Full Text]
3. Mychaskiw G, Horowitz M, Sachdev V, Heath BJ. Explicit intraoperative recall at a Bispectral index of 47. Anesth Analg 2001;92:808–9[Free Full Text]
4. Kurehara K, Horiuch T, Takahash M, Kitaguchi K, Furuya H. A case of awareness during propofol anesthesia using bispectral index as an indicator of hypnotic effect. Masui (Japanese) 2001;50:886–9
5. Bouillon TW, Bruhn J, Radulescu L, Andresen C, Shafer TJ, Cohane C, Shafer SL. Pharmacodynamic interaction between propofol and remifentanil regarding hypnosis, tolerance of laryngoscopy, bispectral index, and electroencephalographic approximate entropy. Anesthesiology 2004;100:1353–72[Web of Science][Medline]
6. Egan TD, Minto CF, Hermann DJ, Barr J, Muir KT, Shafer SL. Remifentanil versus alfentanil: comparative pharmacokinetics and pharmacodynamics in healthy adult male volunteers. Anesthesiology 1996;84:821–33[Web of Science][Medline]
7. Viertiö-Oja H, Maja V, Särkelä M, Talja P, Tenkanen N, Tolvanen-Laakso H, Paloheimo M, Vakkuri A, Yli-Hankala A, Meriläinen P. Description of the entropy algorithm as applied in the Datex-Ohmeda S/5 Entropy Module. Acta Anaesthesiol Scand 2004;48:154–61[Web of Science][Medline]

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via Web of Science (1) Citing Articles via Google Scholar Google Scholar Articles by Strandberg, L. Articles by Korhonen, J. Search for Related Content PubMed PubMed Citation Articles by Strandberg, L. Articles by Korhonen, J.