Background: Although addition of clonidine to local anesthetics can pr
olong pain relief after peripheral nerve block, a dose-range effect ha
s not been determined, Methods: Fifty-six outpatients undergoing carpa
l tunnel release were randomly assigned to receive in a double-blind f
ashion 45 ml of a mixture containing either 400 mg lidocaine plus sali
ne or 400 mg lidocaine plus 30, 90 or 300 mu g 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 ne
rve regions of the hand and forearm, Also, adequacy of the block for s
urgery, postoperative pain intensity, and side effects were evaluated.
Results: Compared with saline, each dose of clonidine reduced the ons
et time of sensory block and extended the field of adequate anesthesia
, Ten minutes after injection, 30 mu g clonidine was more effective th
an 90 mu g clonidine in producing sensory blockade, Sedation occurred
with clonidine 30 and 300 mu g, Clonidine reduced the use of supplemen
tary intravenous anesthetic agents for surgery and produced dose-depen
dent prolongation of analgesia, reaching a mean 770 min (range, 190-14
40 min) for the largest dose, Clonidine also produced a dose-dependent
decrease in systolic arterial pressure of up to -22.5% (range, -6.0-2
9.9%) of baseline. With clonidine, 300 mu g, three patients had mean a
rterial pressure of < 55 mmHg; four patients had episodes of arterial
oxyhemoglobin saturation of <90%, and two others were not discharged b
ecause of hypotension. Conclusion: This study suggests that a small do
se of clonidine enhances the quality of the peripheral blocks from lid
ocaine and limits the classical alpha(2)-agonist side effects to sedat
ion.