In chronic heart failure (HF), the alveolar-capillary membrane undergoes a remodeling process that negatively affects gas exchange. In case of alveolar-capillary gas diffusion impairment, arterial desaturation (SaO2) is rarely observed in HF patients. At play are 3 factors: overall pulmonary diffusing capacity (assessed as lung diffusion for CO, DLCO), global O2 consumption (VO2) and alveolar (A) to arterial (a) pO2 gradient (AaDO2). In 100 consecutive stable HF patients, DLCO, resting respiratory gases and arterial blood gases were measured to determine VO2, paO2, pAO2 and AaDO2. DLCO was poorly but significantly related to AaDO2. The correlation improved after correcting AaDO2 for VO2 (p