Background and Objectives Interscalene block has produced appropriate anesthesia for shoulder surgery. Success is partly determined by the quality of motor block achieved. This study was designed to evaluate the influence of alkalinization of mepivacaine on the quality of motor block achieved with interscalene brachial plexus anesthesia.

Methods The study design included a prospective, randomized trial of 2 groups of 20 patients each undergoing shoulder surgery by one surgeon with interscalene block. Both groups received interscalene block with 40 mL 1.4% mepivacaine, 1/200,000 epinephrine, using a 25-gauge, blunt-bevel needle and elicitation of paresthesia in the arm. One group had 1 mL sodium bicarbonate (1 mEq/mL) per 10 mL mepivacaine added and the other had an equal volume of saline. Average pH was determined for each solution in a nonclinical setting. Motor block was graded by a blinded observer after a standard 3-minute injection at 1 and 10 minutes after injection on a 0-5 standard neurologic scale. The surgeon, blinded to group selection, graded the quality of motor block as excellent, good, fair, or poor at preparation, incision, and 15 minutes postincision.

Results Both groups were similar demographically. All blocks provided successful surgical anesthesia. Motor block quality was evaluated as better at all times in the study group (alkalinization) by both the blinded observer and the surgeon.

Conclusions Alkalinization of mepivacaine improves the quality of motor block after interscalene brachial plexus block for shoulder surgery.