Lung capillary blood volume and membrane diffusion in idiopathic interstitial pneumonia

Diffusing capacity of the lung for carbon monoxide (DLCO) is a good marker of disease severity in patients with idiopathic interstitial pneumonia (IIP). The combined diffusing capacity of nitric oxide (DLNO) and DLCO determines the two components of diffusion: membrane conductance (Dm, CO) and pulmonary capillary blood volume (Vc).

The aim of this study was to evaluate Vc and Dm, CO in patients with fibrosing IIP in order to determine the relative contribution of membrane resistance and vascular resistance to the loss of DLCO.

32 patients with IIP (IPF: n = 22, NSIP: n = 10) were evaluated using MRC dyspnea scale, plethysmography, combined DLNO/DLCO, 6-min walk test (6 MWT), echocardiography and chest computed tomography (chest CT).

DLCO (41.8 ± 11.9%pred), Dm, CO (40.5 ± 12.7%pred) and Vc (41.9 ± 18%pred) were severely and equally reduced. Dm, CO and Vc were related to MRC scale, FVC, maximal desaturation during 6 MWT, and systolic pulmonary artery pressure (sPAP). There was no correlation with the extent of fibrotic changes on chest CT.

Our main results indicate that Dm, CO and Vc contribute almost equally to DLCO reduction in IIP. Dm, CO and Vc are related to functional indicators of disease severity and to sPAP in agreement with the concept of vascular involvement in IIP.