Inhomogeneities of ventilation and the diffusing capacity to perfusion in various chronic lung diseases

Although impairment of gas exchange caused by ventilation-perfusion (V̇A/Q̇) mismatch has been extensively analyzed, there have been no systematic studies focused on determining the distributions of diffusion properties in close connection with those of V̇A/Q̇. We attempted to clarify the simultaneous distributions of V̇A/Q̇ and diffusion capacity to perfusion (D/Q̇) in patients with idiopathic pulmonary fibrosis (IPF) or chronic obstructive pulmonary disease (COPD). To assess pathologic determinants causing functional abnormalities, we compared V̇A/Q̇ and D/Q̇ distributions with the findings on high-resolution computed tomography. O₂, CO₂ and CO together with six foreign inert gases were used as indicator gases. We transformed the measured data on indicator gases in arterial blood into a continuous distribution of Q̇ in the V̇A/Q̇-D/Q̇ field. In IPF, active alveolitis or acinitis played a major role in producing low D/Q̇ regions impeding gas exchange via a diffusion limitation, whereas extensive fibrosis with minimal inflammation accounted for low D/Q̇ as well as low V̇A/Q̇ regions. In COPD, no regions with low D/Q̇ ratios were observed, but an abnormality in the V̇A/Q̇ distribution with low or high V̇A/Q̇ ratios was identified. Emphysematous lesions produced high V̇A/Q̇ regions, whereas peripheral airway involvement yielded low V̇A/Q̇ regions. These findings suggest that hypoxemia in patients with IPF is caused by inhomogeneous distributions of D/Q̇ in combination with those of V̇A/Q̇. Hypoxemia in patients with COPD is attributable primarily to inhomogeneities in V̇A/Q̇ rather than in D/Q̇ distributions.