Decrease by ACE-inhibition of the excessive alveolar-capillary membrane resistance to gas transfer in chronic heart failure.

In this study the mechanisms were investigated whereby ACE-inhibitors improve pulmonary diffusion for carbon monoxide (DLco) in chronic heart failure. The two subcomponents of DLco are the alveolar-capillary membrane conductance (DM) and the capillary blood volume (VC). Stress failure of the membrane in chronic heart failure provides a mechanism for reduction of DM and, as a consequence, impairment of DLco. In 27 patients with chronic heart failure in NYHA functional class II to III and in 13 age- and sex-matched normal subjects, we evaluated the pulmonary function and determined DM and VC, according to the classic Roughton and Forster method, while they were given placebo, at 48 hours and 8 weeks after starting enalapril treatment (10 mg bid). ACE-inhibition had no effect in controls at both short- and mid-term. In chronic heart failure patients, a reduction in VC (likely consequence of a decrease in capillary pulmonary pressure) was the only change observed at 48 hours. At 8 weeks, DM was greatly increased even when the effective alveolar volume (VA) was accounted for (DM/VA), resulting in a significant improvement in DLco, despite a decrease in VC. The slow onset DM improvement makes it likely that the modulatory effect of ACE-inhibition on the membrane function emerges gradually, suggesting that it is likely dissociated from changes in pulmonary capillary pressure and VC. Thus, derangements of the alveolar-capillary membrane in chronic heart failure increase gas diffusion resistance; ACE-inhibition restores the diffusive properties of the membrane and gas transfer, and protects the lung when the heart is failing.