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Although addition of clonidine to local anesthetics can prolong pain relief after peripheral nerve block, a dose-range effect has not been determined.Fifty-six outpatients undergoing carpal tunnel release were randomly assigned to receive in a double-blind fashion 45 ml of a mixture containing either 400 mg lidocaine plus saline or 400 mg lidocaine plus 30, 90 or 300 micro gram clonidine for axillary nerve block. In each group (n = 14), blocks were evaluated at regular time intervals to determine sensory and motor functions in the five nerve regions of the hand and forearm. Also, adequacy of the block for surgery, postoperative pain intensity, and side effects were evaluated.Compared with saline, each dose of clonidine reduced the onset time of sensory block and extended the field of adequate anesthesia. Ten minutes after injection, 30 micro gram clonidine was more effective than 90 micro gram clonidine in producing sensory blockade. Sedation occurred with clonidine 30 and 300 micro gram. Clonidine reduced the use of supplementary intravenous anesthetic agents for surgery and produced dose-dependent prolongation of analgesia, reaching a mean 770 min (range, 190–1440 min) for the largest dose. Clonidine also produced a dose-dependent decrease in systolic arterial pressure of up to -22.5% (range, -6.0–29.9%) of baseline. With clonidine, 300 micro gram, three patients had mean arterial pressure of < 55 mmHg; four patients had episodes of arterial oxyhemoglobin saturation of < 90%, and two others were not discharged because of hypotension.This study suggests that a small dose of clonidine enhances the quality of the peripheral blocks from lidocaine and limits the classical alpha2 -agonist side effects to sedation.