In this study, the primary objective was to estimate the minimum effective volume of bupivacaine 0.5% resulting in successful block in 50% of patients (MEV50) for ultrasound-guided retroclavicular approach to infraclavicular brachial plexus block.

Two rules were used to stop the study. First stopping rule was a fixed-sample-size. The investigators used an up-and-down method to estimate the threshold for an all-or-none response for present study design. Therefore, for sample size calculation, the investigators applied the formula by Dixon and Massey, n=2(SD/SEM)2 (SD: standard deviation and SEM: standard error of mean). Second stopping rule included demonstration of minimum of five consecutive up-and-down sequences. On the basis of previous studies with similar binary outcomes, the investigators estimated that a priori a minimum of five negative-positive up-and down deflections was required to calculate MEV50.

Inclusion Criteria:

• patients who received infraclavicular brachial plexus block for elective elbow, forearm, wrist, or hand surgery
• American Society of Anesthesiologists class I, II or III

Exclusion Criteria:

• patients <18 years old
• >65 years old
• body mass index (BMI) <20 or >35 kg/m2
• inability to provide written informed consent
• refusal of regional anesthesia
• pregnancy
• contraindication for regional anesthesia

Adults between 18 and 65 years of age with infraclavicular brachial lexus block for elective elbow, forearm, wrist, or hand surgery , were selected in the study.