Reduced diffusing capacity as an isolated finding in asbestos- and silica-exposed workers

From a cohort of 286 patients referred to an Occupational Medicine Clinic because of exposure to asbestos and/or silica, we identified 53 patients with a reduced diffusing capacity (Dco) (< 75 percent predicted) as their only abnormality. Specifically, their clinical evaluation, chest roentgenograms, and remaining pulmonary function test results were all normal. These patients were divided into nonsmokers (n = 13) and smokers (n = 40). The significance of the isolated reduction in diffusing capacity in these patients (n = 53) was explored with graded exercise testing (n = 19) and bronchoalveolar lavage (BAL) (n = 50). The results obtained from the patients with reduced diffusion were compared with those obtained from comparable smoking (n = 35) and nonsmoking patients (n = 37) in the original cohort who had normal chest roentgenograms and normal results of pulmonary function studies, including normal Dco values (≥ 75 percent of predicted value). Patients with low diffusion demonstrated a tendency for elevated alveolar to arterial O₂ differences both at rest and during exercise, and a significant reduction in exercise capacity (V̇₂ max) was observed in the smoking patients with reduced diffusion when compared with their smoking counterparts with normal diffusion. All other exercise testing indexes were normal in the study groups and there was no correlation between the percent predicted Dco value and any of the exercise variables. In contrast, BAL revealed significant differences between patient groups. Both the smoking and nonsmoking patient groups with low Dco values had greater numbers of total BAL cells, alveolar macrophages, neutrophils, lymphocytes, and eosinophils in their BAL fluid than did their comparable controls with normal diffusion values. These differences were statistically significant (p < .05) for total BAL cells and total macrophages in the nonsmoking patients and for total BAL cells, total macrophages, and total lymphocytes in the smoking patients expressed as either the total cell number per BAL or total cells per milliliter of BAL. In contrast to the observed exercise testing results, there was significant and inverse correlation between Dco values and each BAL cell type for all four groups combined as well as nonsmokers alone. The Dco values from smokers were significantly and inversely correlated with BAL cells and total macrophages. These results suggest that the finding of a reduced Dco may be related to an active inflammatory process in the lung caused by occupational dust posure. Patients with both dust exposure and low Dco values should be followed up closely even if the low Dco value is an isolated finding; however, the utility of noninvasive exercise testing in the evaluation of these patients requires further investigation.