Randomized crossover trial in patients with Pulmonary Hypertension (PAH, CTEPH) to assess the acute response to simulated altitude (FIO2: 15.1% = equivalent to 2500m above sea level) on non-invasive cardiac output assessments by Finapres® "NOVA" Technology at rest and under exercise.

Low altitude baseline measurements will be performed in Zurich (470m asl) including Echocardiography, right heart catheterization, six-minute walk test (6MWT), pulmonary function test, clinical assessment and blood gas analysis at rest and under exercise.

Randomly assigned to the order of testing, the participants will be tested under simulated altitude (FiO2: 15.1% with the "AMC Altitrainer") and shamed altitude with the same device.

During the exposure to simulated altitude (FiO2: 15.1%) and shamed altitude of 1 hour each, the participants cardiac output non-invasively assessed will be measured throughout the whole intervention.

Inclusion Criteria:

• Informed consent
• PH diagnosed according to internation Guidelines: mean pulmonary artery pressure (mPAP) ≥ 25 mmHg along with a Pulmonary artery Wedge pressure (PAWP) ≤15 mmHg during right heart catheterization at the time of initial diagnosis
• PH class 1 (PAH) or 4 (CTEPH)
• Stable condition, on the same medication for > 4 weeks
• Patient live permanently at an altitude < 1000m asl.

Exclusion Criteria:

• Resting partial oxygen pressure (PaO2) ≤7.3 kiloPascal (kPA) corresponding to the requirement of long-term oxygen therapy > 16hour daily (nocturnal oxygen therapy alone is allowed)
• Severe daytime hypercapnia (pCO2 > 6.5 kPa)
• Susceptibility to high altitude related diseases (AMS, High Altitude Pulmonary Edema (HAPE), etc.) based on previous experienced discomfort at altitudes.
• Exposure to an altitude >1500m for ≥3 nights during the last 4 weeks before the study participation
• Residence > 1000m above sea level
• Inability to follow the procedures of the study, e.g. due to language problems, psychological disorders, neurological or orthopedic problems with walking disability
• Women who are pregnant or breast feeding

The allocated gas mixture will not be disclosed to the patient since he will breath trough a facemask during both interventions.