Pulmonary Vascular Congestion: A Mechanism for Distal Lung Unit Dysfunction in Obesity

RATIONALE: Obesity is characterized by increased systemic and pulmonary blood volumes (pulmonary vascular congestion). Concomitant abnormal alveolar membrane diffusion suggests subclinical interstitial edema. In this setting, functional abnormalities should encompass the entire distal lung including the airways. OBJECTIVES: We hypothesize that in obesity: 1) pulmonary vascular congestion will affect the distal lung unit with concordant alveolar membrane and distal airway abnormalities; and 2) the degree of pulmonary congestion and membrane dysfunction will relate to the cardiac response. METHODS: 54 non-smoking obese subjects underwent spirometry, impulse oscillometry (IOS), diffusion capacity (D(LCO)) with partition into membrane diffusion (D(M)) and capillary blood volume (V(C)), and cardiac MRI (n = 24). Alveolar-capillary membrane efficiency was assessed by calculation of D(M)/V(C). MEASUREMENTS AND MAIN RESULTS: Mean age was 45±12 years; mean BMI was 44.8±7 kg/m(2). Vital capacity was 88±13% predicted with reduction in functional residual capacity (58±12% predicted). Despite normal D(LCO) (98±18% predicted), V(C) was elevated (135±31% predicted) while D(M) averaged 94±22% predicted. D(M)/V(C) varied from 0.4 to 1.4 with high values reflecting recruitment of alveolar membrane and low values indicating alveolar membrane dysfunction. The most abnormal IOS (R(5) and X(5)) occurred in subjects with lowest D(M)/V(C) (r(2) = 0.31, p<0.001; r(2) = 0.34, p<0.001). Cardiac output and index (cardiac output / body surface area) were directly related to D(M)/V(C) (r(2) = 0.41, p<0.001; r(2) = 0.19, p = 0.03). Subjects with lower D(M)/V(C) demonstrated a cardiac output that remained in the normal range despite presence of obesity. CONCLUSIONS: Global dysfunction of the distal lung (alveolar membrane and distal airway) is associated with pulmonary vascular congestion and failure to achieve the high output state of obesity. Pulmonary vascular congestion and consequent fluid transudation and/or alterations in the structure of the alveolar capillary membrane may be considered often unrecognized causes of airway dysfunction in obesity.