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Cervical paratracheal local anesthetic injections (stellate ganglion blocks) are performed to determine the sympathetic contribution to painful and other conditions of the head, neck, and arm. A block is useful for diagnosis only if the desired physiological effect is confirmed, but the frequency with which sympathetic function is successfully blocked is unclear. The goal of this study is to examine the rates of achieving various endpoints of sympathetic interruption by these injections, using commonly available measures of sympathetic change.Retrospective review.Training center.One hundred unselected consecutive blocks in 40 patients.Paratracheal sympathetic block at sixth cervical level.Bilateral hand temperature, ophthalmic changes.Horner's syndrome was successfully produced in 84 blocks and the ipsilateral hand warmed by ≥ 1.5°C in 60 blocks. However, the contralateral hand also warmed in 31 blocks so that ipsilateral warming exceeded contralateral warming in only 27 blocks, with diminished success by this criterion when the hand was warm before the block.We conclude that (a) identifying a Horner's syndrome and ipsilateral warming are not by themselves adequate to confirm selective sympathetic blockade; (b) selective sympathetic blockade of the arm is confirmed only if the temperature increase of the blocked side exceeds that of the contralateral side; and (c) cervical paratracheal blocks frequently fail to produce evidence of sympathetic interruption to the arm. Pathophysiological inferences based on these blocks should be made with caution and only with adequate documentation of physiological evidence of sympathetic blockade.