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LETTER TO THE EDITOR

BIS and the Electromyogram

Department of Anaesthesia; Royal Brompton Hospital; London, United Kingdom; maurizio.renna{at}imperial.ac.uk

To the Editor:

Liu et al. (1) recently described the effect of neuromuscular blockade on bispectral index (BIS) and entropy values. Their statement that "until now, no study has been conducted during the initial period of anesthetic induction" is incorrect. Several years ago, we reported electromyographic (EMG) effects on BIS during large-dose opioid induction (2), using fentanyl-induced muscular rigidity as a source of EMG "contamination" of BIS. Our findings mirrored those of Liu et al. (1). They found that placebo is associated with a smaller decrease in BIS than atracurium. We found that a larger dose of fentanyl (i.e., more rigidity) was associated with higher BIS values than a smaller dose of fentanyl (i.e., less rigidity), in the absence of neuromuscular blockade. We used version 3.4 of the BIS algorithm in our study, as version 4.0 used by Liu et al. was not available at that time.

REFERENCES

1. Liu N, Chazot T, Huybrechts I, et al. The influence of a muscle relaxant bolus on bispectral and Datex-Ohmeda entropy values during propofol-remifentanil induced loss of consciousness. Anesth Analg 2005;101:1713–18.[Abstract/Free Full Text]
2. Renna M, Wigmore T, Mofeez A, Gillbe C. Biasing effect of the electromyogram on BIS: a controlled study during high-dose fentanyl induction. J Clin Monit Comput 2002;17:377–81.[Medline]

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