The purpose of this monocentric, preliminary study is to assess the evolution of pulmonary capillary blood volume (Vc), measured by the double diffusion nitric oxide/carbon monoxide (NO/CO) method (Guenard et al.; Respir Physiol 1987), from the starting of the anticoagulant therapy to 6 months after, and to search correlations with the evolution of clinical (dyspnea and results of the 6 minutes walking test), biological (BNP; Troponin), echocardiographic (right ventricular dysfunction, pulmonary arterial pressure) and ventilation/perfusion lung scan data.