Reducing Right Ventricular Failure in Pulmonary Arterial Hypertension (RELIEVE-PAH)

Main Inclusion Criteria: 1. Group 1 PAH (idiopathic, connective tissue disease, HIV, corrected congenital heart disease). 2. WHO Functional Class III or IV symptoms. If WHO Class III, at least 1 High-Risk characteristic or 2 Intermediate-Risk characteristics from the 2015 ESC Guidelines. 3. Receiving maximal available and tolerable pharmacological PAH therapy ≥3 months at a stable dose for ≥1 month. Main Exclusion Criteria: 1. Resting oxygen saturation \<90 % without supplemental oxygen corrected for altitude. 2. Mean Right Atrial Pressure \>20 mmHg. 3. Severe restrictive or obstructive lung disease. 4. Evidence of organ dysfunction other than right heart failure. 5. Left ventricular ejection fraction \<40 %. 6. Anatomical anomaly on transesophageal echocardiography or intracardiac echocardiography that precludes implantation of Shunt across fossa ovalis (FO) of the interatrial septum. 7. Inadequate vascular access for implantation of shunt, e.g. femoral venous access for transseptal catheterization and inferior vena cava (IVC) is not patent. 8. Hemodynamic heart rhythm, or respiratory instability at time of Final Exclusion Criteria.