Interscalene vs Phrenic-sparing Blocks in Obesity and Effect of Maximum Inspiratory Pressure

Inclusion Criteria: * Age ≥ 18 years (self-report, confirmed on chart). * Body-mass index ≥ 35 kg m\^2 at the pre-operative clinic visit (chart). * Scheduled for elective unilateral shoulder surgery (arthroscopy, arthroplasty, rotator cuff repair, reverse arthroplasty) under general anesthesia at UNC Hospitals * Planned use of a single-injection regional brachial-plexus block for postoperative analgesia (anesthesia record). * Able to perform a maximal-inspiratory-pressure (MIP) maneuver at screening, producing two reproducible efforts (bedside test) * Able to read or understand English and provide written informed consent (consent discussion). Exclusion Criteria: * Emergent or trauma shoulder procedure or case converted to open surgery (schedule/chart). * Pregnancy confirmed by point-of-care urine test on day of surgery (POC test). Prisoner status or legal incapacity to consent (chart/interview). * Severe pulmonary disease: GOLD stage 3-4 COPD, restrictive lung disease unrelated to obesity with FVC \< 50 % predicted, baseline dyspnea Borg ≥ 3, or home oxygen therapy (pulmonary clinic notes, patient interview). * Severe heart disease- Severe valvular heart disease, Congestive Heart Failure NYHA Class III or IV, Obstructive Coronary Artery Disease with Angina * Neuromuscular disorders affecting respiratory muscles (e.g., ALS, myasthenia gravis) or known diaphragmatic paralysis (chart). * Coagulopathy (platelets \< 100 × 10/ L or INR \> 1.5) or local infection at block sites (pre-op labs/assessment). * Anemia and hemoglobinopathies: Hgb \<10 g/dl, clinically significant hemoglobinopathy. * Allergy to bupivacaine, dexamethasone, or ultrasound gel (self-report). * Chronic opioid use \> 100 mg oral-morphine equivalents per day in the three months preceding surgery (medication history, PDMP query). * Prior enrolment in this trial or planned participation in another interventional study that might confound outcomes (research registry). * Anticipated inability to comply with postoperative assessments (e.g., profound cognitive impairment, expected early transfer to outside facility) as judged by the investigator