Inclusion Criteria:  Informed consent as documented by signature (Appendix Informed Consent Form) PH class I (PAH) or IV (CTEPH) diagnosed according to guidelines: mean pulmonary artery pressure >20 mmHg, pulmonary vascular resistance ≥3 wood units, pulmonary arterial wedge pressure ≤15 mmHg during baseline measures at the diagnostic right-heart catheterization  Exclusion Criteria:  resting partial pressure of oxygen <8 kilopascal at Zurich at 490 m low altitude exposure to an altitude >1000 m for ≥3 nights during the last 2 weeks before the study inability to follow the procedures of the study other clinically significant concomitant end-stage disease (e.g., renal failure, hepatic dysfunction)