Fibrosing alveolitis in systemic sclerosis: indices of lung function in relation to extent of disease on computed tomography.

OBJECTIVE: Thin-section computed tomography (CT) provides a sensitive and reproducible method of quantifying the morphologic extent of disease in the clinical management of fibrosing alveolitis associated with systemic sclerosis (FASSc). The aim of this study was to determine which indices of lung function best reflect the extent of disease on CT in FASSc, and to determine the independent influences of smoking history, extent of fibrosing alveolitis, demographic features, and concurrent treatment upon functional impairment in FASSc.

METHODS: Sixty-four patients with FASSc were studied using CT and static and exercise lung function testing. Statistical relationships were determined by multiple regression analyses.

RESULTS: Five patients with overt pulmonary hypertension were characterized by severe impairment in 3 indices of lung function: diffusing capacity for carbon monoxide (DLCO), DLCO adjusted for alveolar volume (KCO), and arterial partial pressure of oxygen. On multiple regression analysis, the major determinant of functional impairment was the extent of fibrosing alveolitis on CT. A history of smoking was independently associated with preservation of total lung capacity and depression of KCO, but did not otherwise influence functional-morphologic correlations. The percent predicted DLCO correlated better with extent of disease on CT (r = -0.70) than did oxygen desaturation on exercise (r = 0.55), the physiologic component of the clinical-radiographic-physiologic score (CRP index) (r = 0.52), or other indices of lung function. Lung volume measures correlated poorly with disease extent on CT.

CONCLUSION: The percent predicted DLCO best reflects the extent of fibrosing alveolitis in FASSc, and therefore should be measured in routine evaluations. Exercise testing may also have a useful role in staging the severity of pulmonary fibrosis, but the CRP index offers no additional advantage over the DLCO and exercise testing.