Membrane diffusion in diseases of the pulmonary vasculature

SummaryIntroductionWe examined pulmonary diffusing capacity (DLCODLCO) and its partition in pulmonary vascular diseases without evident parenchymal disease to assess the pattern and proportionality of change in membrane diffusion (DmDm) and capillary blood volume (VcVc). Disproportionate reduction in DmDm relative to VcVc (low Dm/VcDm/Vc) in these diseases has been attributed to associated alveolar membrane/parenchymal disease, thus providing a potentially important diagnostic tool.MethodsDiseases included: idiopathic pulmonary arterial hypertension (n=6n=6), chronic thromboembolic disease (n=5n=5), and intravenous drug use (n=14n=14), providing a spectrum of pulmonary vascular diseases. VcVc and DmDm were determined as described by Roughton and Forster.ResultsAll diseases showed a reduced VcVc (59±10, 69±14, 71±21 % predicted, respectively) and DmDm (76±22, 53±19, 63±16 % predicted, respectively) with no differences between groups (p>0.05p>0.05). Disproportionate reduction of DmDm (Dm/VcDm/Vc % predicted <1) was seen in all diseases (range 0.36–1.89). A mathematical analysis is presented to illustrate that changes in vascular geometry may additionally influence the proportionality of changes in DmDm and VcVc. The mathematical analysis suggests that when reduction in patency of some vessels co-exits with compensatory dilatation of the remaining vasculature, a disproportionate reduction in DmDm relative to VcVc may result.ConclusionsThe balance between vascular curtailment and compensatory dilatation may contribute to the variability of the Dm/VcDm/Vc relationship seen in pulmonary vascular disease. Disproportionate reduction in DmDm relative to VcVc may result from this imbalance and need not imply subclinical alveolar membrane and/or parenchymal disease.