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 (ED50) and 95% effective dose (ED95) 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/mm2 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/mm2 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, ED50 and ED95 were estimated. Volunteers and physicians performing the block were blinded to the volume used.
Results: A total 17 of volunteers were investigated. The ED50 volume was 0.08 mL/mm2 (SD, 0.01 mL/mm2), and the ED95 volume was 0.11 mL/mm2 (SD, 0.03 mL/mm2). The mean cross-sectional area of the nerves was 6.2 mm2 (1.0 mm2).
Conclusions: Based on the ultrasound measured cross-sectional area and using ultrasound guidance, a mean volume of 0.7 mL represents the ED95 dose of 1% mepivacaine to block the ulnar nerve at the proximal forearm.
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This study was funded by the Scientific Fund of the Swiss Society of Anesthesiology and Reanimation, and the Scientific Fund of the Department of Anesthesiology and Pain Therapy, Bern University Hospital, and University of Bern, Switzerland.
Parts of this work have been presented at the Annual Meeting of the American Society of Anesthesiologists; October 13-17, 2007; San Francisco, Calif; and Annual Meeting of the Swiss Society for Anesthesiology and Reanimation; November 1-3, 2007; Interlaken, Switzerland.
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