Isolated reduction in single-breath diffusing capacity in the evaluation of exertional dyspnea

STUDY SETTINGS AND INTERVENTIONS: Sixty individuals complaining of dyspnea on exertion, but with normal spirometry and lung volumes and normal chest roentgenograms were reviewed for this study. These individuals were selected from a large group of outpatients (552 individuals over a seven-year period) who were referred to our laboratory for exercise testing to determine the cause of their exertional dyspnea. They were grouped according to the single-breath diffusing capacity, with those less than 70 percent of predicted in the low DCO group (group 1) and those greater than 70 percent of predicted in the normal DCO group (group 2). Both study groups underwent an incremental exercise test.

Twenty-three individuals had a DCO less than 70 percent of predicted. During exercise, seven of these (30 percent) had an abnormal PaO2 and five had an abnormal P(A-a)O2. Thirty-seven people had a normal DCO. Thirty-six of these (97 percent) had a normal PaO2 and P(A-a)O2 during exercise. Overall, eight individuals had an abnormal PaO2 or P(A-a)O2 during exercise; seven of these had an abnormally low DCO at rest.

Based on this selected group of a subpopulation, we conclude that the DCO is an important determinant of the diagnostic approach to a patient with dyspnea who is otherwise normal. If all pulmonary functions, including DCO are normal, an exercise study will fail to reveal abnormal PaO2 or P(A-a)O2 in 97 percent of the cases. However, a low DCO has a poor predictive value with respect to abnormal gas exchange during exercise. Therefore, when investigating exertional dyspnea, based on this selected subpopulation, if the spirometry, lung volumes, and DCO are normal, one may forego additional invasive gas exchange evaluation. However, an abnormal DCO warrants further physiologic testing.