This study will compare three different doses of perineural dexmedetomidine 0.67 mcg/kg, 1 mcg/kg, and 1.33 mcg/kg as adjuvants to local anesthetics for ultrasound-guided infraclavicular brachial plexus blocks (ICBs). Our research hypothesis is that 1.33 mcg/kg will provide a 15%-longer duration than 1mcg/kg, which in turn will provide a 15%-longer duration than 0.67 mcg/kg. Since analgesic duration and sensory duration can be influenced by intake of pain medications and surgical trauma to small cutaneous nerves, respectively, we will select motor block duration as the main outcome to better target the action of dexmedetomidine on the brachial plexus.

With the approval of Ethics Committee of the McGill University Health Centre, a total of 69 patients undergoing upper extremity surgery (elbow and below) will be recruited. Recruitment will be carried out by an investigator not involved in patient care one day before surgery

All ICBs will be supervised by the coauthors and conducted preoperatively in an induction room.

After skin disinfection and draping, the ICB will be performed with a previously described technique. In each group, a proven 90% effective volume of 35 mL of local anesthetic solution will be injected. As LA solution, it will be used a mixture of lidocaine 1.0%-bupivacaine 0.25% with epinephrine 5 µ/mL plus PN 2 mg dexamethasone. The injectate will be slowly injected through the block needle.

Patients will be randomized to receive the study drug, PN dexmedetomidine 0.67 mcg/kg or PN dexmedetomidine 1 mcg/kg, or PN dexmedetomidine 1.33 mcg/kg mixed with the above-mentioned LA solution.

A research assistant (licensed anesthesiologist) will prepare the local anesthetic solutions and will add the study drug following the randomization order. The operator, patient, and investigator assessing the block will be blinded to group allocation.

The primary outcome will be the duration of the motor block (defined as the temporal interval between the end of LA injection through the block needle and the return of movement to the hand and fingers) for patients with successful ICBs. Patients will be provided with a data sheet and asked to record the time at which motor function returns. An investigator blinded to group allocation will collect this data sheet in person (inpatients) or by phone (outpatients) on postoperative day 1.

Inclusion Criteria:

• patients undergoing upper limb surgery at or below the elbow
• age between 18 and 75 years
• American Society of Anesthesiologists classification 1-3
• body mass index between 18 and 35 kg/m2

Exclusion Criteria:

• adults who are unable to give their own consent
• allergy or contraindication to dexmedetomidine
• exposure to dexmedetomidine during the previous 48 hrs
• pre-existing neuropathy (assessed by history and physical examination)
• coagulopathy (assessed by history and physical examination and, if deemed clinically necessary, by blood workup i.e. platelets ≤ 100, International Normalized Ratio ≥ 1.4 or partial prothrombin time ≥ 50)
• renal failure (assessed by history and physical examination and, if deemed clinically necessary, by blood work-up i.e. creatinine. eGFR < 90)
• hepatic failure (assessed by history and physical examination and, if deemed clinically necessary, by blood work-up i.e. transaminases ≥ 100)
• allergy to LA
• pregnancy (as institutional policy, female patients with childbearing potential undergoing non urgent surgery are tested preoperatively for pregnancy with blood tests)
• breast feeding
• prior surgery in the infraclavicular region
• chronic pain syndromes requiring opioid intake at home

A research assistant (licensed anesthesiologist) will prepare the local anesthetic solutions and will add the study drug following the randomization order. The operator, patient and investigator assessing the block will be blinded to group allocation.