The aim of the study is to investigate the role of the non-invasive assessment of nitric oxyde (NO) diffusion capacity (DLNO) in the differential diagnosis of different forms of pulmonary hypertension (PH). DLNO alone and in combination with the measurement of carbon monoxide (CO) diffusion capacity (DLCO) may help to differentiate between pre- and postcapillary forms of pulmonary hypertension. This hypothesis is based on the fact that NO has a much higher affinity to hemoglobin as compared to CO and may therefore its decrease may more specifically identify the limiting factors between the alveolar space and the lung capillaries including left heart failure (cardiac edema).