Motor-Sparing Surgical Nerve Blocks for Upper Extremity Surgery: Significantly Less Motor Paralysis Using 15 mL versus 30 mL of Mepivacaine 1.5% for Supraclavicular Block - A Prospective Randomized Double-Blinded Study

Background and objectives: We performed a prospective randomized double-blinded study evaluating if a reduced volume of local anesthetic would result in operative limb surgical anesthesia while decreasing motor paralysis during an ultrasound-guided supraclavicular nerve block. Current tendencies in clinical practice towards smaller injectate volumes during ultrasound-guided nerve block placement prompted our investigation on its impact regarding block quality. Methods: 43 patients were consented for this prospective, double-blinded randomized clinical trial. Each patient was randomly assigned. Group HIGH received the conventional injection dose of 30 mL of 1.5% Mepivacaine. Group LOW received the reduced volume dose of 15 mL. An ultrasound-guided supraclavicular nerve block was performed on each patient. Motor block and sensory perception to pin-prick were assessed in the nerve distributions for the ulnar, median, radial, and musculocutaneous branches at 5, 10, 15, 20, and 30 minutes post-injection. Results: Complete motor block in the radial, ulnar, musculocutaneous and median nerve distributions at 30 minutes, was present in 55% of patients in Group HIGH versus 10% in Group LOW and was statistically significant between both groups (p<0.01). The anatomic distribution of the observed motor-sparing was statistically significant in the median (p<0.01) and ulnar (p<0.05) nerve branches among those patients who received 15 mL LA boluses. Conclusions: Our study demonstrated that 15 mL vs. 30 mL injections of mepivacaine 1.5% at the supraclavicular approach provide equivalent surgical anesthesia, while reducing the incidence of motor block. These findings may have implications on early postoperative physical therapy for the subset of patients that present with Galeazzi-type fractures, carpal tunnel syndrome, and minimally-displaced distal radius fractures.