The objectives of the RELIEVE-PAH study are to obtain first-in-human experience with the study device in patients with severe pulmonary arterial hypertension, including evidence of initial safety, device performance and possible signals of clinical effectiveness.

This is a multi-national, multi-center, prospective, non-randomized, open label trial of patients implanted with the study device. A total of up to 20 patients will be implanted with the study device and followed at regular intervals for 1 year and then annually for a total of 5 years post implant.

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.