Acute Exposure to High Altitude on Blood and Tissue Oxygenation (ABGA, NIRS)

Inclusion Criteria: * Informed consent * PH diagnosed according to internation Guidelines: mPAP ≥ 25 mmHg along with a 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 PaO2 ≤7.3 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, 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