Median Effective Volume of 0.5% Ropivacaine for Ultrasound-Guided Single-Injection Sub-arterial Axillary Brachial Plexus Block in Extreme Arm Abduction: A Dose Finding Study

Ultrasound guided Axillary Brachial plexus Block serves as an effective anaesthetic technique for surgeries below the elbow, facilitating postoperative pain management, reducing recovery duration, and promoting early discharge of patients who undergo outpatient procedures. The close proximity of musculocutaneous nerve to axillary artery in extreme arm hyperabduction presents the opportunity to block all four nerves(radial, ulnar, median, and musculocutaneous) with a single sub arterial injection (at 6 o’clock position with respect to axillary artery), as opposed to the previously utilized conventional methods. A single sub arterial injection is known to reduce the number of needle passes, the block performance time, and the risk of nerve needle contact, resulting in decreased potential for neural injury and paraesthesia. Literature is sparse with regards to the median effective dose of local anesthetic (0.5% ropivacaine) required to produce an effective ultrasound guided axillary brachial plexus block, with the arm in extreme abduction, using a single sub-arterial injection, thus enabling blocking all four nerves. We thus conceptualized this dose finding study. The volume of the drug to be injected will be determined by up and down sequential allocation study design of binary response variables. An initial single sub arterial injection of 24 mL of 0.5% Ropivacaine (1 dose level below the presumed target) will be administered in slow increments while keeping axillary artery in the center. If the first person receiving the initial volume (24mL) of local anaesthetic achieves a successful block, then the volume of local anaesthetic to be given to the next patient will be decreased by 2mL, whereas block failure will result in an increase of volume by 2mL for next patient. In order to avoid local anesthetic toxicity, we did not exceed the volume of local anesthetic to more than 40ml of 0.5% Ropivacaine.