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

Interscalene vs Phrenic-sparing Blocks in Obesity: Effect of Pre-operative Maximum Inspiratory Pressure in a Randomized Trial

The goal of this clinical trial is to learn whether a phrenic-sparing nerve block can lower early breathing problems after shoulder surgery in adults with obesity, and whether a simple breathing-strength test (maximum inspiratory pressure, MIP) helps identify who is at higher risk. The main questions are:

Does the phrenic-sparing approach reduce breathlessness or oxygen need in the recovery room (30-60 minutes after arrival)? Do patients have similar pain control and opioid use compared with the standard interscalene block (ISB)? Are there any breathing-related complications or unplanned admissions within 24 hours? Researchers will compare the phrenic-sparing block (infraclavicular + distal suprascapular) to the standard ISB, both commonly used at UNC.

Participants will:

Have a quick MIP breath test before surgery (and, if age ≥65, a brief thigh muscle ultrasound).

Be randomly assigned to receive either the standard ISB or the phrenic-sparing block (both ultrasound-guided and part of routine care).

Receive usual anesthesia/surgery; have a brief recovery check at 30-60 minutes (breathlessness score, oxygen use, oxygen level).

Have pain medicines recorded from anesthesia start to PACU discharge; the team may review the chart up to 24 hours and make a short follow-up call (24-48 hours).

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