The investigators aim to study the effect of SOT in participants with pulmonary vascular diseases (PVD) defined as pulmonary arterial hypertension or chronic thromboembolic pulmonary hypertension (PH) who permanently live >2500m on 6-minute walk distance (6MWD) assessed at 2840m.

Participants with PVD diagnosed with precapillary PH with right heart catheterization and classified to groups 1 and 4 (PAH or CTEPH) who permanently live at HA >2500 (PVDHA) will have 6-minute walk distance near their living altitude in Quito at 2840m with and without SOT at 3l/min via nasal cannula according to a randomized cross-over design.

IInclusion Criteria:

• Adult patients 18-80 years old of both genders,
• Residence > 2500m of altitude
• diagnosed with precapillary PH (mean pulmonary artery pressure (mPAP) >20 mmHg, pulmonary artery wedge pressure (PAWP) ≤15 mmHg and pulmonary vascular resistance (PVR) ≥2 wood units (WU) by right heart catheterization) with PH being classified as PAH or CTEPH according to guidelines
• Patients stable on therapy
• New York Heart Association (NYHA) functional class I-III
• Provided written informed consent to participate in the study.

Exclusion Criteria:

• Age <18 years or >80 years
• unstable condition
• Patients who cannot follow the study investigations, patient permanently living < 2500m.
• Patients with moderate to severe concomitant lung disease (FEV1<70% or forced vital capacity <70%), severe parenchymal lung disease, severe smokers (>20 cigarettes/day)
• Severely hypoxemic patients at Quito permanently have persistent oxygen saturation by pulseoximetry (SpO2) <80% on ambient air.
• Patients with chronic mountain sickness (Hemoglobin > 19 g/dl in women, >21 g/dl in men)
• Patient with a non-corrected ventricular septum defect
• Relevant concomitant other disease of the heart, kidney, liver, blood (anemia hemoglobin<11 g/dl)

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