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LETTER TO THE EDITOR

Awareness Despite Low Spectral Entropy Values

Department of Anesthesia; General Hospital of Giannitsa, Greece; makail{at}freemail.gr

To the Editor:

Entropy (Datex-Ohmeda) is a recently introduced depth of anesthesia monitor that compares with BIS (1,2). We describe a case of patient awareness despite low spectral entropy values. A 28-yr-old, 103-kg, woman was admitted for an urgent appendectomy. She was included in a study protocol that included monitoring depth of anesthesia, while giving different IV agents, and was randomized to the propofol group. While awake, state (SE) and response (RE) entropy values were 89 and 96, respectively. Anesthesia was induced with remifentanil 100 µg, propofol 180 mg, and rocuronium 80 mg, and maintained with propofol 0.04–0.06 mg · kg^–1 · min^–1 and remifentanil 0.10–0.30 µg · kg^–1 · min^–1. During the 80-min anesthetic, the patient received 358 mg of propofol and 2.17 mg remifentanil. The entropy values during the procedure were between 35 and 60, with the exception of SE 76/RE 82 immediately after intubation for about 3 min (Fig. 1). Surgery proceeded uneventfully, without any signs of sympathetic stimulation. However, during our postoperative visit, the patient related that she experienced substantial recall during surgery, precisely retelling several comments made by the surgeon. She did not describe any pain during surgery nor any psychological problems postoperatively.

View larger version (16K): [in this window] [in a new window]   Figure 1. Upper trends: RE (top of line), SE (end of line). Lower trends: heart rate (HR). Notice the change during intubation and the maintenance of values below 60.

Despite evidence from our entropy monitoring indicating an adequate hypnotic state as well as an absence of sympathetic hyperactivity, the low spectral and response entropy values did not ensure amnesia. Based upon our experience from this incident, every anesthesiologist in our institution is urged to use benzodiazepines for premedication to avoid the possibility of patient awareness during surgery.

Editor’s note: We were unable to obtain a response from the manufacturer in spite of repeated efforts and contacts with several individuals at GE Healthcare.

REFERENCES

1. White PF, Tang J, Romero GF, Wender RH, Naruse R, Sloninsky A, Kariger R. A Comparison of state and response entropy versus bispectral index values during the perioperative period. Anesth Analg 2006;102:160–7[Abstract/Free Full Text]
2. Soto R, Nguyen TC, Smith RA. A comparison of bispectral index and entropy, or how to misinterpret both. Anesth Analg 2005;100:1059–61[Abstract/Free Full Text]

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