Effect of body mass index on the ED50 volume of bupivacaine 0.5% for supraclavicular brachial plexus block

The aim of this study was to determine whether the ED50 dose of bupivacaine 0.5% for supraclavicular brachial plexus block increases with increasing body mass index (BMI). This double-blind, non-randomized trial followed an up-and-down sequential allocation design. Adult patients undergoing elective upper limb surgery under ultrasound-guided supraclavicular brachial block were recruited. A preliminary study was used to guide the dosing schedule for the main study in which patients were divided into three groups according to their BMI (Group A, BMI > 27 kg m(-2); Group B, BMI 24-27 kg m(-2); Group C, BMI < 24 kg m(-2)). The study design and analysis followed Dixon's small sample model using a 'nominal' sample size of six per group. Twenty-one patients were recruited to the preliminary study. Using isotonic regression, the ED50 for bupivacaine 0.5% was estimated to be 8.9 ml [95% confidence interval (CI) 7.8-15.9]. In comparison, the ED50 volume was found to be 10.8 ml (95% CI 5.9-19.7) using the Dixon-Massey formula. In the main study, six patients were recruited in each group with mean (range) BMI of 31.5 (27.2-38.8) kg m(-2) in Group A, 25.6 (24.4-26.3) kg m(-2) in Group B, and 21.6 (19.7-23.8) kg m(-2) in Group C. The ED50 (95% CI) for Groups A, B, and C were 8.9 (6.2-12.7), 10.7 (7.5-15.4), and 13.4 (9.3-19.1) ml, respectively (P=0.05 for Group A vs Group C). Our study demonstrates that the ED50 of bupivacaine 0.5% does not increase with an increase in BMI. We found evidence of a possible inverse relationship between ED50 and BMI.