Pulmonary arterial hypertension (PAH) is a disease characterised with significant morbidity and poor prognosis. Dyspnoea and impaired exercise capacity are very common manifestations of the disease, and result in significant impairment of patients' quality of life. Although hypoxemia is common among subjects with PAH, published data on the effects of supplementary oxygen therapy on specific clinical outcomes among these patients are currently few, while the existing data on the potential benefits of oxygen supplementation to treat exercise-induced hypoxemia, in this patient population, are even more controversial. Based on the aforementioned, the purpose of this prospective, crossover clinical trial is to investigate the acute effects of supplemental oxygen administration on the: a) exercise capacity, b) severity of dyspnea, c) cerebral oxygenation, b) muscle oxygenation, and e) hemodynamic profile, as compared to delivery of medical air (sham oxygen), in a group of patients with PAH, during steady state cardiopulmonary exercise testing (CPET)

Inclusion Criteria:

1. Group I PAH patients
2. Stable disease (no hospitalisation, disease deterioration or change in regular PAH medication during the last month)
3. Presence of exercise-induced hypoxemia

Exclusion Criteria:

1. Major contraindications for CPET conduction
2. Not providing informed consent -