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LETTER TO THE EDITOR

Can Anesthesiologists Trust Anesthesia Depth Monitors?

Department of Toxicology, Hospital Universitario Gregorio Marañón, Madrid, Spain, alma{at}nacom.es

To the Editor:

Spectral entropy, a monitor of anesthetic depth, provides two indices of anesthetic depth: 1) state entropy (SE) that quantifies cortical cerebral activity and 2) response entropy (RE) that quantifies cortical cerebral activity and the surface electromyogram (1). We report a case in which we erroneously diagnosed wakefulness in a patient because surgical drilling introduced artifact into the spectral entropy index.

A 46-yr-old ASA physical status I woman, scheduled for tympanoplasty, was anesthetized with propofol and remifentanil. We adjusted the propofol and remifentanil infusions to maintain moderate hypotension and spectral entropy values less than 55. Thirty minutes after surgery started, the SE value increased from 50 to 80, and the RE value increased from 52 to 90. Signal quality was good. We interpreted the increased SE and RE as a light hypnotic state, and increased the propofol and remifentanil infusions accordingly. However, the SE and RE values remained elevated. The anesthesiologist suspected that the high entropy values might be caused by mastoid drilling and asked the surgeon to stop. Immediately, the SE value decreased to 11 and the RE value to 14. The anesthesiologist proceeded to ignore the entropy values during drilling. The patient emerged from anesthesia uneventfully and did not report recall.

Entropy monitors can identify artifacts, including electrocautery, electrocardiograms, pacemakers, and movement artifacts (1). However, the entropy software did not reject drill-generated noise. With the widespread use of bispectral index (BIS), several artifacts have been reported (2). Nitrous oxide and ketamine produce paradoxical BIS changes, also seen with entropy indices (3,4). IV epinephrine produces an arousal effect in BIS during sedation (5). BIS can also be affected by hypothermia, pacemaker interference, and forced-warm-air therapy (6–8).

We have shown that entropy indices are influenced by mastoid drilling. Depth of anesthesia monitors must be carefully scrutinized for artifact rejection, particularly when the reported index does not agree with the dose of drug delivered and clinical assessment of anesthetic depth.

References

1. Viertiö-Oja H, Maja V, Särkelä M, et al. 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]
2. Dahaba AA. Different conditions that could result in the bispectral index indicating an incorrect hypnotic state. Anesth Analg 2005;101:765–73.[Abstract/Free Full Text]
3. Hans P, Dewandre PY, Brichant JF, Bonhomme V. Comparative effects of ketamine on Bispectral Index and spectral entropy of the electroencephalogram under sevoflurane anaesthesia. Br J Anaesth 2005;94:336–40.[Abstract/Free Full Text]
4. Puri GD. Paradoxical changes in bispectral index during nitrous oxide administration. Br J Anaesth 2001;86:141–2.[Abstract/Free Full Text]
5. Shin HW, Ban YJ, Lee HW, et al. Arousal with IV epinephrine depends on the depth of anesthesia. Can J Anaesth 2004;51:880–5.[Web of Science][Medline]
6. Mychaskiw G, Heath BJ, Eichhorn JH. Falsely elevated bispectral index during deep hypothermic circulatory arrest. Br J Anaesth 2000;85:798–800.[Abstract/Free Full Text]
7. Gallagher JD. Pacer-induced artifact in the bispectral index during cardiac surgery. Anesthesiology 1999;90:636.[Web of Science][Medline]
8. Grocott HP. False increase BIS values with forced-air head warming. Anesth Analg 2003;96:1230.[Free Full Text]

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