A mixture of dexamethasone 2mg with dexmedetomidine 50mcg will be added to the local anesthetic used to block the cords of the brachial plexus with an infraclavicular approach.

Sum of: 1- the acquisition time of the ultrasonographic image. and 2- the time to perform the block itself (from the skin anesthesia to the end of local anesthetic injection)

Evaluated with the Numeric Rating Scale for Pain. This scale is graduated from 0 to 10 points. A 0-point score represents the absence of pain, and a 10-points score represents the worst imaginable pain. Patients will be asked to rate their pain verbally with this scale. The blinded assessor will register the score reported.

Time required to reach a minimal sensorimotor composite score of 14 points out of a maximum of 16 points. The sensorimotor score is described in outcome 7.

The sensorimotor block will be assessed every 5 minutes until 30 minutes after the end of local anesthetic injection using a 16-point composite score evaluating sensory and motor block of musculocutaneous, medium, radial, and ulnar nerves.

Sensation will be assessed with ice in each nerve territory with a 0 to 2 point scale. 0= no block, patients can feel cold; 1= analgesic block, the patient can feel touch but not cold; 2= anesthetic block, the patient cannot feel cold or touch.

The motor function will be assessed for each nerve with a 0 to 2 points scale where 0= no motor block; 1= paresis; 2= paralysis.

Successful blocks at 30 minutes correlate with a final score ( sum of all individual sensory and motor scores) of at least 14 points out of 16.

Percentage of blocks with a minimal sensorimotor composite score of 14 points out of a maximum of 16 points at 30 minutes after the injection

Ability to proceed with surgery without the need for intravenous narcotics, general anesthesia, rescue blocks or local anesthesia infiltration by the surgeon

Determined by the presence of paresthesia, local anesthetic systemic toxicity, vascular puncture, Horner syndrome or hoarseness after the block.

Presence of low respiratory rates (lower than 8 breaths per minute) or persistent oxygen requirements (pulse oximetry lower than 90% without supplementary oxygen).

The respiratory rate and pulse oximetry are continuously measured intra- and post-operatively (in the post-anesthetic care unit) as a standard of care with a multiparameter monitor. Alarms are set to alert when predetermined levels are reached

Defined as a heart rate lower than 50 beats per minute. The heart rate is continuously measured intra- and post-operatively (in the post-anesthetic care unit) as a standard of care with a multiparameter monitor. Alarms are set to alert when predetermined levels are reached

Mean arterial pressure lower than 60 mmHg. The blood pressure is measured intra- and post-operatively (in the post-anesthetic care unit) as a standard of care with a multiparameter monitor. Alarms are set to alert when predetermined levels are reached

Persistent sedation after surgery using Ramsay sedation scale. The scale has 6 points from 1 to 6. A higher score represents a more sedated patient.

1= anxious, agitated or restless; 2= co-operative, oriented and tranquil; 3= responds to command only; 4= brisk response to light pain or loud auditory stimulus; 5= sluggish response to light pain or loud auditory stimulus; 6= no response.

Presence of persistent sensory or motor postoperative deficit. The patients will be contacted by telephone and inquired about any sensory or motor deficit in the operated extremity.