Purpose: It is likely that the relationship between lung volume changes and gas exchange in patients with idiopathic pulmonary fibrosis (IPF) and patients with sarcoidosis is different since the two conditions vary widely in histopathology and prognosis. Few studies, however, have examined this relationship. The goal of this investigation was to measure diffusing capacity and gas exchange in patients with IPF and sarcoidosis in whom the reduction of lung volume was equivalent. Patients and methods: In 21 patients with IPF and 20 patients with pulmonary sarcoidosis with comparable reductions in lung volume, the single breath diffusing capacity for carbon monoxide and gas exchange at rest and during exercise were compared. Results: The relationship between lung volume and gas transfer differed in the two groups of patients. Resting and exercise gas exchange tended to be relatively normal and the diffusing capacity was higher in patients with sarcoidosis than in those with IPF. These differences could not be attributed to disparities in race, age, smoking habits, or the radiographic stage of sarcoidosis. Conclusion: The preservation of gas exchange in sarcoidosis as opposed to IPF, despite equivalent degrees of volume restriction, suggests that different pathophysiologic mechanisms underlie the volume loss and gas exchange defects seen in these disorders. Furthermore, these findings suggest that diffusing capacity may not be a sensitive indicator of pulmonary pathology in sarcoidosis since lung volume can be altered independently of abnormalities in the diffusing capacity.
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