This prospective comparative study will evaluate the sympathetic effects of ultrasound-guided infraclavicular versus supraclavicular brachial plexus blocks in adults undergoing upper-extremity surgery. Sympathetic tone will be monitored noninvasively with simultaneous bilateral measurements of perfusion index (pulse-oximetry-derived) and skin temperature obtained at baseline and at predefined post-block intervals until return to baseline. The primary outcome is the duration of sympathetic block. Secondary outcomes are between-approach differences in the time course of perfusion index and skin temperature, and the incidence and magnitude of rebound sympathetic activity.