Diffusing Capacity Is an Independent Predictor of Outcomes in Pulmonary Hypertension Associated With COPD

Background: Patients with COPD who experience pulmonary hypertension (PH) have worse mortality than those with COPD alone. Predictors of poor outcomes in COPD-PH are not well-described. Diffusing capacity of the lung (DLCO) assesses the integrity of the alveolar-capillary interface and thus may be a useful prognostic tool among those with COPD-PH. Research Question: Using a single center registry, we sought to evaluate DLCO as a predictor of mortality in a cohort of patients with COPD-PH. Study Design and Methods: This retrospective cohort study analyzed 71 COPD-PH patients from the Johns Hopkins Pulmonary Hypertension Registry with right-sided heart catheterization (RHC)-proven PH and pulmonary function testing data within one year of diagnostic RHC. Transplant-free survival was calculated from index RHC. Adjusted transplant-free survival was modelled using Cox proportional hazard methods; age, pulmonary vascular resistance, FEV₁, oxygen use, and N-terminal pro-brain natriuretic peptide were included as covariates. Results: Overall unadjusted transplant-free 1-, 3-, and 5-year survivals were 87%, 60%, and 51%, respectively. Survival was associated with reduced DLCO across the observed range of pulmonary artery pressures and pulmonary vascular resistance. Severe DLCO impairment was associated with poorer survival (log-rank χ² 13.07) (P < .001); adjusting for covariates, for every percent predicted decrease in DLCO, mortality rates increased by 4% (hazard ratio, 1.04; 95% CI, 1.01-1.07). Interpretation: Among patients with COPD-PH, severe gas transfer impairment is associated with higher mortality, even with adjustment for airflow obstruction and hemodynamics, which suggests that DLCO may be a useful prognostic marker in this population. Future studies are needed to further investigate the association between DLCO and morbidity and to determine the utility of DLCO as a biomarker for disease risk and severity in COPD-PH.