This is a prospective, randomized, assessor-blinded, three-arm, non-inferiority clinical trial. The study aims to compare the effectiveness and safety of an individualized dosing strategy for local anesthetic in brachial plexus blocks against two standard methods. The experimental intervention uses ultrasound to measure the cross-sectional area (CSA) of the brachial plexus nerves to calculate a patient-specific dose of 0.5% ropivacaine. This is compared to a standard weight-based dosing regimen (2.5 mg/kg) and dosing based on the anesthesiologist's clinical experience. A total of 350 adult patients scheduled for elective unilateral upper limb surgery will be randomly assigned to one of the three groups. The primary outcome is the success rate of the nerve block 30 minutes after administration. Secondary outcomes include assessments of diaphragmatic function, postoperative pain scores, the incidence of complications (such as nerve involvement or systemic toxicity), and patient satisfaction. The hypothesis is that the CSA-based dosing method will be non-inferior to the conventional methods in achieving successful anesthesia while potentially optimizing drug dosage.