Patients with systemic sclerosis frequently have pulmonary function abnormalities, and recent evidence suggests that pulmonary vascular involvement is a common manifestation. To test the hypothesis that patients with systemic sclerosis have impaired ability to recruit or distend the pulmonary vascular bed, the postural change in the coefficient of carbon monoxide diffusing capacity was measured in 11 patients with systemic sclerosis, and the results were compared with results from age-, smoking-, and sex-matched control subjects with rheumatoid arthritis and with results from healthy subjects. In normal subjects and patients with rheumatoid arthritis increased, the coefficient of diffusion by 9.4 percent (p < 0.005) and 8.4 percent (p < 0.01), respectively, when they moved from the sitting to the supine position. In contrast, patients with systemic sclerosis did not show a significant increase in coefficient of diffusion, even those who had otherwise normal pulmonary function. Regression analyses showed that the change in coefficient of diffusion decreased with increasing age (r = -0.57) in normal subjects, and that the change in coefficient of diffusion was a function of the percent predicted forced vital capacity, both in patients with systemic sclerosis (r = 0.59) and in those with rheumatoid arthritis (r = 0.70). Thus, these findings indicate that patients with systemic sclerosis have a nondistensible pulmonary capillary bed and that the absence of positional change in the coefficient of diffusion in systemic sclerosis is a subtle indicator of pulmonary involvement.
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