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 axillary block (AXB).

Methods All subjects received a double-injection ultrasound-guided AXB with lidocaine 1.5% and epinephrine 5 μg/mL. A 17-gauge, 8-cm Tuohy needle was initially advanced until its tip was positioned next to the musculocutaneous nerve. Volume assignment was carried out using a biased coin design up-and-down sequential method, where the 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 1.0 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 1.0 mL), with a probability of b = 0.11, or the same volume, with a probability of 1 − b = 0.89. After injection of the musculocutaneous nerve, the needle was redirected toward the dorsal aspect of the axillary artery. For this second injection, volume assignment was carried out in a similar fashion; however, volume increments/decrements were 3.0 instead of 1.0 mL. Using a composite scale encompassing sensory and motor block, success was defined, at 30 minutes, as a composite score of 4 points (out of 4 points), and 10 points (out of 12 points) for the musculocutaneous and perivascular injection, respectively. Patients undergoing surgery of the forearm, wrist, or hand were prospectively enrolled until 45 successful musculocutaneous blocks or 45 successful perivascular injections were obtained.

Results Fifty patients were included in the study. Using isotonic regression and bootstrap confidence interval (CI), the MEV₉₀ was estimated to be 5.5 mL (95% CI, 3.0–6.7 mL) and 23.5 mL (95% CI, 23.1–23.9 mL) for the musculocutaneous and perivascular injection, respectively.

Conclusions For double-injection ultrasound-guided AXB, the MEV₉₀ of lidocaine 1.5% with epinephrine 5 μg/mL is 5.5 and 23.5 mL for the musculocutaneous nerve and perivascular injection, respectively. Further dose-finding studies are required for other concentrations of lidocaine, other local anesthetic agents, and other techniques for ultrasound-guided AXB.