Long Acting Phosphodiesterase 5 Inhibitors as Add-on Therapy for Patients With Pulmonary Hypertension Treated With Prostanoids.

Inclusion Criteria: * All patients must satisfy current diagnostic criteria for pulmonary artery hypertension based on their historical right heart catheter data (within 3 years of study enrollment): Mean PAP \>25mmHg at rest or \>30mmHg with exercise, by a PCWP\< 15mmHg and by PVR \>3 Wood Units. * Currently stable for at least 3 months on prostanoid monotherapy (epoprostenol iv or iloprost inhaled). * Willing and able to participate in all study follow-up procedures. * New York Heart Association (NYHA) Class II-IV. * Six minute walking distance between 100-450 meters at the baseline assessment. * Women of child-bearing age must demonstrate adequate contraception or undergo a pregnancy test. * Patients with congenital heart disease are eligible for inclusion. Exclusion Criteria: * Functional Class NYHA Class I. * PAH due to chronic pulmonary thromboembolic disease, left heart disease, chronic lung diseases (VC or FEV1 \< 60% of predicted) or chronic hypoxia. * Acute intercurrent illness requiring hospital admission in the month proceeding screening. * Any non-PAH medical condition likely to interfere with participation in evaluation of study endpoints, e.g. musculoskeletal disorders. * Any uncontrolled or terminal non-PAH medical condition likely to interfere with completion of the study, according to the judgment of the study physician. * Concomitant therapy with drugs known to interact adversely with the study drug. * Chronic renal failure - creatinine clearance \<50ml/min as calculated with the Cockcroft equation. * Current participation in another clinical trial. * Pregnancy or planned pregnancy during the study period.