Faculty Bibliography

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 (DLCO) with partition into membrane diffusion (DM) and capillary blood volume (VC), and cardiac MRI (n = 24). Alveolar-capillary membrane efficiency was assessed by calculation of DM/VC. MEASUREMENTS AND MAIN RESULTS: Mean age was 45+/-12 years; mean BMI was 44.8+/-7 kg/m2. Vital capacity was 88+/-13% predicted with reduction in functional residual capacity (58+/-12% predicted). Despite normal DLCO (98+/-18% predicted), VC was elevated (135+/-31% predicted) while DM averaged 94+/-22% predicted. DM/VC 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 (R5 and X5) occurred in subjects with lowest DM/VC (r2 = 0.31, p<0.001; r2 = 0.34, p<0.001). Cardiac output and index (cardiac output / body surface area) were directly related to DM/VC (r2 = 0.41, p<0.001; r2 = 0.19, p = 0.03). Subjects with lower DM/VC 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.

We agree that the "holy grail" of pulmonary physiologists is a test that detects early chronic airway disease. While Dr. Enright remains "cautiously optimistic" that FOT can serve this purpose, there are sufficient data to mitigate his caution.

Detection of airway disease by physiologic testing was initially described using spirometry to determine vital capacity and expiratory airflow under maximal effort to distinguish obstructive from restrictive disease processes. Subsequently, Dubois demonstrated direct assessment of airway resistance using plethysmography and in a separate publication described the precursor of the forced oscillation technique to measure respiratory system resistance. This review addresses the question of whether direct assessment of resistance by forced oscillation provides diagnostic information equivalent or superior to standard assessment of airflow rates by spirometry.