The investigators aim to study the effect of SOT in Swiss residents with pulmonary vascular diseases (PVD) defined as pulmonary arterial hypertension (PAH) or chronic thromboembolic pulmonary hypertension (CTEPH).

Participants with PVD diagnosed with precapillary PH with right heart catheterization and classified to groups 1 and 4 (PAH or CTEPH) who live in Switzerland will have 6-minute walk distance (6MWD) test with and without SOT approximately 3l/min via nasal cannula according to a randomized cross-over design.

The trial aims to test that the 6MWD with SOT is non-inferior to a 6MWD in the same participant under ambient air.

The effect size was estimated with a difference of less or equal to 35m compared under SOT compared to ambient air.

Inclusion Criteria:

• Minimum 18 years of age
• Written informed consent
• Patients with diagnosed pulmonary arterial hypertension (PAH), distal chronic thromboembolic pulmonary hypertension (CTEPH)
• Former desaturation under exercise defined as Spo2-decrease >3%
• Treated with a stable drug therapy (with no changes for at least 14 days prior to screening)

Exclusion Criteria:

• Severe hypoxemia needing supplemental oxygen therapy defined as partial pressure of O2 (PaO2) < 6.9 Kilopascal (kPa)
• Pregnancy
• Unability or contraindications to undergo the investigated intervention
• Unability to follow the procedures of the study, e.g. due to language problems, psychological disorders, dementia, etc

Data will be provided upon request and based on a clear intention reviewed by an ethical review board