Effects of Normobaric Hypoxia During Exercise in Patients With Pulmonary Hypertension With/Without Sildenafil

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 Zürich altitude on ambient air * 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 * patients who take nitrates * other clinically significant concomitant end-stage disease (e.g., renal failure, hepatic dysfunction)