Letters to the Editor: Letters & Announcements
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).
Matthew T. V. Chan, FANZCA
Tony Gin, MD
Department of Anaesthesia and Intensive Care
Chinese University of Hong Kong
Prince of Wales Hospital
Shatin, Hong Kong
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.
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.
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.