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Background and Objectives: The interscalene is one of the most common approaches used in brachial plexus block. However, the association of this approach with the ipsilateral blockade of the phrenic nerve has been demonstrated. The resulting diaphragmatic dysfunction causes changes in lung mechanics, which can be potentially deleterious in patients with limited respiratory reserve. The objective of the present study was to evaluate the repercussion of interscalene brachial plexus block on pulmonary function by measuring forced vital capacity (FVC)., Methods: This is a double-blind study with 30 patients, physical status ASA I or II, randomly separated into two groups of 15 patients each; 0.5% ropivacaine (Ropi Group) or 0.5% bupivacaine with epinephrine (Bupi Group) was administered. A peripheral nerve stimulator was used, and 30 mL of the local anesthetic were administered. Four spirometries were done in each patient: before the blockade, 30 minutes, four hours, and six hours after the blockade. Patients were not sedated., Results: One patient in the Ropi Group and three patients in the Bupi Group were excluded from the study due to failure of the blockade. The Ropi Group showed maximal FVC reduction at 30 minutes (25.1%), with a tendency for recovery from this point on. With bupivacaine, the reduction in FVC was less important at the different study moments; an additional reduction was observed between 30 (15.8%) and four hours (17.3%), but it was not statistically significant. A tendency for recovery was observed from four hours on. In both groups, the FVC six hours after the blockade was still below baseline levels., Conclusions: Interscalene block reduces FVC in most cases. Changes were more pronounced in the Ropivacaine group.