The aim of this study is to determine the minimum effective concentration of local anesthetic (ropivacaine 40mL) required to produce an effective double-injection USG-SCB for surgical anesthesia in 90% of patients (MEC90) in young group (18-40 years) and in middle aged group (40-65 years) of patients scheduled for elective surgery of one upper limb distal to the shoulder.

The study is based on biased coin design, and the concentration of LA for the next patient is determined by the result of the last one. In the case of block failure, the concentration will be increased by 0.025%. Conversely, block success will result in either a reduction in concentration by 0.025% (probability 11%) or no change in concentration (probability 89%). A blinded assistant will assess sensory and motor blockade in each nerve territory ( the median, radial, ulnar and musculocutaneous nerves) at 10-min intervals up to 30 min after completion of the block. Finally, MEC90 is calculated by isotonic regression.

Inclusion Criteria:

1. age between 18 and 65 years
2. ASA 1-3
3. body mass index between 18 and 35kg/m2

Exclusion Criteria:

1. inability to consent to the study
2. pregnancy
3. allergy to local anesthetics
4. preexisting neuropathy or coagulopathy
5. prior surgery in the supraclavicular region