Lung capillary blood volume and membrane diffusion in precapillary pulmonary hypertension

Combined diffusion capacity of the lung for carbon monoxide (DLco) and nitric oxide (DLno) measurements allow for the estimation of pulmonary capillary blood volume (Vc) and alveolar membrane diffusion (Dm). The clinical usefulness of these measurements in pulmonary hypertension (PH) is unclear.

Combined DLco and DLno were measured in 290 consecutive patients with precapillary PH (pulmonary arterial hypertension (PAH), n = 153; pulmonary veno-occlusive disease (PVOD), n = 33; and chronic thromboembolic pulmonary hypertension (CTEPH), n = 104). Clinical correlates of Vc and Dm were assessed in a sub-group of PAH patients without comorbidities.

PVOD patients compared with PAH and CTEPH patients displayed the lowest values of Vc (29.4 ± 16.8 ml vs 56.3 ± 26.5 ml vs 56.9 ± 26.2 ml, p < 0.01, respectively) and Dm (27.7 ± 11.6 ml/mm Hg/min vs 43.4 ± 14.8 ml/mm Hg/min vs 44.7 ± 17.7 ml/mm Hg/min, p < 0.01, respectively). The DLno/DLco ratio was highest in the PVOD group (5.82 ± 2.04 vs 4.95 ± 1.31 vs 5.16 ± 1.58, p < 0.05). In a sub-set of 69 PAH patients without comorbidities, Vc and Dm correlated significantly with functional capacity (6-minute walking distance, oxygen consumption) and New York Heart Association Functional Classification but not with invasive hemodynamics. Only Dm was significantly associated with survival. On receiver operating characteristic curve analysis, Vc, Dm, and DLno/DLco were not superior to DLco for discriminating PVOD from PAH or CTEPH.

PVOD patients display higher values of the DLno/DLco ratio compared with PAH and CTEPH, suggesting proportionally greater reduction in Vc relative to Dm. However, partitioning of diffusion failed to be more clinically relevant than conventional DLco for detection of PVOD.