Dosage of Mepivacaine in Ultrasound Axillary Block

The use of ultrasonography as an adjunct to regional anesthesia has significantly increased in recent years. Brachial plexus blockade by an axillary approach is amenable to the use of ultrasound guidance. Real time sonography of nerve structures ensures an optimal distribution of the block solution. When compared to other methods of nerve localization, sonography decreases: failure rate procedure time and the onset time for blockade. Furthermore, the use of ultrasound for peripheral nerve blockade demonstrates decreased procedure related complications such as nerve injury and unintentional vascular puncture. Traditional axillary block techniques relying on surface anatomical landmarks require large volumes of local anesthetic, generally 40mL and greater. Utilizing the increased accuracy offered by ultrasound, some studies have shown that low volumes of local anesthetic can yield successful axillary plexus blockade. Therefore, the tradition of using large volumes of local anesthetic for axillary blocks, even without ultrasound, may not be warranted. Although recent investigations support using a low volume of local anesthetic for brachial plexus blockade, there is a lack of outcome data from blinded randomised trials. The primary objective of this study was to evaluate 2 different volumes of local anesthetic for axillary blockade: 1) 20mL or 2) 30 mL. For the 2 different volumes used in this study, a 1.5% solution of mepivacaine was chosen due to its widespread clinical use in axillary blocks, which is secondary to: rapid onset of action, intermediate duration of effect, and relative low cost. The primary outcome was block success rate for outpatients undergoing distal upper limb surgery. Secondary objectives included comparing the 2 volumes with respect to: time required to perform the block, and onset of sensory and motor blockade.