Letters to the Editor: Letters & Announcements
We agree with Drs. Chan and Gin (1), who stated that the effect-site propofol concentrations may not have been at steady-state when we recorded the electroencephalographic measurements in our study (2). We performed our study using target-controlled drug delivery, with the target set at the effect site, which is the current state-of-the-art to minimize plasma effect-site equilibration delay. However, we cannot prove that the effect-site concentrations were constant over the course of the study.
Our study design limits the relevance of their remark. Patients receiving saline had the same protocol as patients receiving atracurium, and so the results are comparable between groups.
Concerning their study (3), the most important difference from our study design is the level of bispectral index (BIS) when the neuromuscular blocking drug is administered. Our BIS was typically around 75 just before administering the muscle relaxant, whereas in their study the BIS was <60. As stated in our report, “in nonparalyzed patients, the effect of neuromuscular blockade on the BIS value depends on the depth of anesthesia. During deep anesthesia (BIS values of approximately 40), a bolus of mivacurium evoked a transient and minor effect on the BIS value. At a light level of anesthesia (BIS values up to 65), a bolus of atracurium decreased the BIS value” (2).
Ngai Liu, MD
Thierry Chazot, MD
Marc Fischler, MD
Department of Anesthesiology
1. Chan MTV, Gin T. Changes of bispectral index after a bolus dose of muscle relaxant. Anesth Analg 2006;103:776–7.
2. 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.
3. Gin T, Chan MTV, Tsang KH. Performance of the bispectral index during electrocautery [abstract]. J Neurosurg Anesthesiol 2005;17:256.