Minimal Local Anesthetic Volume for Peripheral Nerve Block: A New Ultrasound-Guided, Nerve Dimension-Based Method

Background and Objectives: Nerve blocks using local anesthetics are widely used. High volumes are usually injected, which may predispose patients to associated adverse events. Introduction of ultrasound guidance facilitates the reduction of volume, but the minimal effective volume is unknown. In this study, we estimated the 50% effective dose (ED(50)) and 95% effective dose (ED(95)) volume of 1% mepivacaine relative to the cross-sectional area of the nerve for an adequate sensory block. Methods: To reduce the number of healthy volunteers, we used a volume reduction protocol using the up-and-down procedure according to the Dixon average method. The ulnar nerve was scanned at the proximal forearm, and the cross-sectional area was measured by ultrasound. In the first volunteer, a volume of 0.4 mL/mm(2) of nerve cross-sectional area was injected under ultrasound guidance in close proximity to and around the nerve using a multiple injection technique. The volume in the next volunteer was reduced by 0.04 mL/mm(2) in case of complete blockade and augmented by the same amount in case of incomplete sensory blockade within 20 mins. After 3 up-and-down cycles, ED(50) and ED(95) were estimated. Volunteers and physicians performing the block were blinded to the volume used. Results: A total 17 of volunteers were investigated. The ED(50) volume was 0.08 mL/mm(2) (SD, 0.01 mL/mm(2)), and the ED(95) volume was 0.11 mL/mm(2) (SD, 0.03 mL/mm(2)). The mean cross-sectional area of the nerves was 6.2 mm(2) (1.0 mm(2)). Conclusions: Based on the ultrasound measured cross-sectional area and using ultrasound guidance, a mean volume of 0.7 mL represents the ED(95) dose of 1% mepivacaine to block the ulnar nerve at the proximal forearm.