Effect of local anesthetic volume (15 vs 40 mL) on the duration of ultrasound-guided single shot axillary brachial plexus block: a prospective randomized, observer-blinded trial.

Background and Objectives: One of the advantages of ultrasound-guided peripheral nerve block is that visualization of local anesthetic spread allows for a reduction in dose. However, little is known about the effect of dose reduction on sensory and motor block duration. The purpose of the present study was to compare the duration of sensory and motor axillary brachial plexus block (ABPB) with 15 or 40 mL mepivacaine 1.5%.
Methods: Thirty patients were randomly allocated to receive ultrasound-guided ABPB with either 15 (group 15 mL, n = 15) or 40 mL (group 40 mL, n = 15) mepivacaine 1.5%. Onset, efficacy, and duration of sensory and motor block were compared.
Results: Two patients in group 15 mL needed an additional rescue block before surgery and were excluded from subsequent analysis. The overall median duration of sensory and motor block was significantly shorter in group 15 mL (225 [148-265] mins vs 271 [210-401] mins and 217 [144-250] mins vs 269 [210-401] mins, respectively; P < 0.01). Duration of sensory and motor block of individual nerves was significantly shorter in group 15 mL (20%-40% reduction for sensory and 18%-37% for motor block). Time to first request of postoperative analgesia was also significantly reduced in group 15 mL (163 [SD, 39] vs 235 [SD, 59] mins, respectively, P < 0.05). There were no differences in the other block characteristics.
Conclusions: In ABPB with mepivacaine 1.5%, reducing the dose from 40 mL to 15 mL (62.5%) shortens the overall duration of sensory and motor block by approximately 17% to 19%, reduces sensory and motor block duration of individual nerves by 18% to 40%, and decreases the time to first request of postoperative analgesia by approximately 30%.