The 90% minimum effective volume of 0.5 ropivacaine for ultrasound-guided supra-clavicular brachial plexus block

Background: Ultrasound-guided supraclavicular brachial plexus block is widely used in upper limb surgery; however, it requires a higher dose (20-30mL) of local anesthetic. In this study, we aimed to determine the 90% minimum effective volume for ultrasoundguided supraclavicular brachial plexus block. Methods: All patients received an ultrasound-guided two-point injection of 0.5% ropivacaine at a starting volume of 0.18mL/mm(2) cross-sectional nerve area. In cases of a successful block, the next patient had the same volume with a probability of 0.89, and the volume was reduced by 0.04mL/mm(2) cross-sectional nerve area with a probability of 0.11. When the block failed, the dose was increased by 0.04mL/mm(2) cross-sectional nerve area. After 45 cases of successful blocks, the 90% minimum effective volume of local anesthetic was calculated using the centered isotonic regression function. Results: Centered isotonic regression analysis resulted in a 90% minimum effective volume and a 95% confidence interval of 0.189mL/mm(2) and 0.176-0.225mL/mm(2) for the supraclavicular brachial plexus block. Conclusion: A good blocking effect can be achieved with 0.189mL/mm(2) of 0.5% ropivacaine with more precise dosing, thereby reducing the risk of local anesthetic poisoning.