Lung function in diffuse interstitial lung disease of unknown cause.

To examine the role of lung function tests in diagnosis and management of patients with diffuse interstitial lung disease (DILD) we measured total lung capacity (TLC), gas transfer (Tl), lung distensibility and arterial saturation (SaO2) and ventilation during progressive exercise in 24 patients with DILD including 18 with biopsy confirmation. Tl was decreased in all patients, in 21/24 there was progressive hypoxia with exercise, in 12/24 TLC was greater than 80% predicted and in 6/18 lung distensibility was normal. Tl and the change of SaO2 with work output during exercise were highly correlated (r = -0.65, p less than 0.001). In serial studies, the change of SaO2 with work output altered most with clinical improvement or deterioration. The clinical course and response to treatment was not related to a particular pattern of abnormal function. These results and those of previous studies indicate that in DILD the most sensitive index of abnormal parenchymal function is Tl, that lung volumes and distensibility may be normal so that the term "restrictive lung disease" can be misleading, that the fall of SaO2 with work output and Tl are the most sensitive parameters for assessing severity and following the course of the disease and that lung function alone does not predict outcome.