For the axillary group, the ultrasound probe will be placed upright in the armpit to obtain a cross section of this region. After visualization of the nerves form the brachial plexus by ultrasound, 5 mL of ropivacaine 0.5% will be injected around each nerve to be blocked (median, ulnar, radial and musculocutaneous). If resistance to the injection of the solution is present or the patient complains of severe pain, the needle will be immediately repositioned.

For the infraclavicular group, the ultrasound probe will be placed in the infraclavicular region (the junction between the clavicle and the coracoid process) to obtain a cross-sectional imaging of the axillary artery. After visualization of the axillary artery by ultrasound, the block will be performed using the technique in plan for visualization of the needle. The needle is placed in position 6-8 hours of the artery, and 20 mL of ropivacaine 0.5% will be injected, observing a dispersal of local anesthetic around the artery.

Ultrasound guided axillary block

All patients will be contacted on the first postoperative day to tell the time of onset of pain postoperatively. The duration of postoperative analgesia is defined as the interval between the end of local anesthetic injection and the onset of postoperative pain reported by the patient.

Venous blood samples will be taken from some patients through a cannula. The samples will be used for the determination of serum levels achieved after ropivacaine blockade. Samples will be taken every 15 minutes during the first hour, every 30 minutes during the second hour and 4 hours after the blockade. These samples will be analyzed using the technique of high performance liquid chromatography coupled with mass spectrometry.