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LETTER TO THE EDITOR

Changes of Bispectral Index After a Bolus Dose of Muscle Relaxant

Department of Anaesthesia and Intensive Care; Chinese University of Hong Kong; Prince of Wales Hospital; Shatin, Hong Kong; mtvchan{at}cuhk.edu.hk

To the Editor:

Liu et al. (1) reported a decrease in bispectral index (BIS) after a bolus dose of atracurium in patients receiving target-controlled infusion (TCI) of propofol. We believe their observations are confounded because the effect-site propofol concentrations were not at steady-state when the measurements were recorded. This is demonstrated by the significant decrease in BIS readings in the saline control group, suggesting model mis-specification of the TCI pump. The study's conclusions would have been more definitive if a few minutes of equilibration had been allowed before the injection of atracurium.

We recently reported the performance of BIS during electrocautery (2). In this experiment we also recorded BIS measurements before and after administering muscle relaxant. Forty-five patients undergoing elective surgery received TCI of propofol and remifentanil, with target effect-site concentrations of 3 µg/mL and 4 ng/mL, respectively. After steady-state conditions were achieved, all patients received an IV bolus dose of rocuronium 0.9 mg/kg. Figure 1 shows the BIS readings and electromyographic (EMG) power 1 min before and 3 min after patients received this dose. The mean (±sd) BIS readings before rocuronium 56.4 ± 6.7 were similar to those after rocuronium 57.0 ± 5.7 paired Student's t-test, P = 0.40. Similarly, there was no significant change in EMG activity over the forehead (P = 0.60). The signal-quality index was above 95 throughout the measurements. Our data are consistent with previous reports that demonstrated that neuromuscular blockade did not affect BIS monitoring in anesthetized patients (3,4).

View larger version (15K): [in this window] [in a new window]   Figure 1. Box plots of bispectral index () and electromyographic power (•) 1 min before and 3 min after a bolus dose of rocuronium in patients receiving propofol/remifentanil anesthesia.

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;101:1713–8.[Abstract/Free Full Text]
2. Gin T, Chan MTV, Tsang KH. Performance of the bispectral index during electrocautery [abstract]. J Neurosurg Anesthesiol 2005;17:256.
3. Dahaba AA, Mattweber M, Fuchs A, et al. The effect of different stages of neuromuscular block on the bispectral index and the bispectral index-XP under remifentanil/propofol anesthesia. Anesth Analg 2004;99:781–7.[Abstract/Free Full Text]
4. Greif R, Greenwald S, Schweitzer E, et al. Muscle relaxation does not alter hypnotic level during propofol anesthesia. Anesth Analg 2002;94:604–8.[Abstract/Free Full Text]

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