PDE5A and pulmonary arterial hypertension: Confirmed PAH (mPAP >25 mmHg, PCWP <15 mmHg, PVR > 4 Wood units), WHO functional class III, Stable on inhaled iloprost 5 μg via I-Neb prior to study, Concomitant PAH therapies allowed if stable (≥3 months for ERAs/PDE5 inhibitors).