Minimum effective volume of lidocaine for ultrasound-guided supraclavicular block.

BACKGROUND: The aim of this study was to determine the minimum effective volume of lidocaine 1.5% with epinephrine 5 μg/mL in 90% of patients (MEV90) for double-injection ultrasound-guided supraclavicular block (SCB).

METHODS: Using an in-plane technique and a lateral to medial direction, a double-injection ultrasound-guided SCB was performed. A 17-gauge, 8-cm Tuohy needle was initially advanced until its tip was positioned at the intersection of the first rib and subclavian artery ("corner pocket"). Half the volume of lidocaine was injected in this location. Subsequently, the needle was redirected toward the neural cluster formed by the trunks and divisions of the brachial plexus. The remaining volume of lidocaine was deposited in this location. Volume assignment was carried out using a biased coin design up-and-down sequential method, where the total volume of local anesthetic administered to each patient depended on the response of the previous one. In case of failure, the next subject received a higher volume (defined as the previous volume with an increment of 2.5 mL). If the previous patient had a successful block, the next subject was randomized to a lower volume (defined as the previous volume with a decrement of 2.5 mL), with a probability of b = 0.11, or the same volume, with a probability of 1 - b = 0.89. Each increment or decrement was evenly distributed between the "corner pocket" (1.25 mL) and neural cluster (1.25 mL). Lidocaine 1.5% with epinephrine 5 μg/mL was used in all subjects. Success was defined, at 30 minutes, as a minimal score of 14 of 16 points using a composite scale encompassing sensory and motor block. Patients undergoing surgery of the elbow, forearm, wrist, or hand were prospectively enrolled until 45 successful blocks were obtained.

RESULTS: Fifty-four patients were included in the study. Using isotonic regression and bootstrap confidence interval, the MEV90 for double-injection ultrasound-guided SCB was estimated to be 32 mL (95% confidence interval, 30-34 mL). All patients with a minimal composite score of 14 points at 30 minutes achieved surgical anesthesia intraoperatively.

CONCLUSIONS: For double-injection ultrasound-guided SCB, the MEV90 of lidocaine 1.5% with epinephrine 5 μg/mL is 32 mL. Further dose finding studies are required for other concentrations of lidocaine, other local anesthetic agents and single-injection techniques.