Introduction: Positional therapy is increasingly prescribed as adjunct therapy for patients ranging from sleep disordered breathing to proning in acute respiratory distress syndrome.  Patients with severe obesity are a high risk group for these breathing disorders.  We examined the effects of different positions on respiratory mechanics in patients with severe obesity.

Methods: Pre-bariatric surgery patients in sleep clinic were enrolled.  Respiratory mechanics assessed using balloon catheters with oesophageal and gastric pressures during stable tidal breathing volume measured with pneumotachograph.  Patients assessed in sitting, supine, prone, left lateral and right lateral position.

Results: 9 patients enrolled (4 female; age 55.1+/-3.4 years; BMI 49.7+/-6.9kg/m2) with 2 diagnosed with obesity hypoventilation syndrome (OHS).  Mean dynamic compliance in OHS was 46.5+/-2.6mL/cmH2O compared to 89.5+/-37.8 in eucapnic cohort.  Trans-diaphragmatic pressure was 25.3+/-0.9 cmH2O in OHS vs 15.7+/-0.4 cmH2O in eucapnic.  Compliance decreased with supine positioning to 66%+/-15 of upright baseline.  No significant change in compliance in response to all non-upright positioning in OHS patients.  In eucapnic cohort, optimal compliance to positioning was seen when proning in 4 patients (32.8%+/-3.8 increase over supine) and left lateral position in 3 patients (54.4%+/-61.3).

Conclusion: Patients with OHS have a fixed low dynamic compliance in all tested postures likely reflective of a fixed compartment with poor elastic response.  Different responses to positioning in the non-hypercapnic patients may be reflective of differential fat distribution and intra-abdominal pressures offering different mechanical advantages.