In a randomized, sham-controlled trial the investigators will test whether supplemental oxygen given during cardiopulmonary exercise testing will improve exercise performance and physiological parameters in patients with pulmonary arterial or chronic thromboembolic pulmonary hypertension.

Inclusion criteria:

• diagnosis of pulmonary arterial hypertension class I and IV (according to the latest WHO classification scheme of Dana Point 2008)
• on optimized medical therapy and in stable condition for at least 4 week
• desaturate in the 6 minute walking test by at least 4% to values <92%

Exclusion criteria:

• unstable conditions
• pregnant women
• patients with pulmonary venous hypertension
• patients with relevant concomitant lung disease and severe daytime hypoxemia