Pulmonary arterial hypertension is a chronic disease of the lung blood vessels resulting in constriction and high pressures. Treatment is given with a variety of drugs including the prostanoid class (e.g. epoprostenol, iloprost and the phosphodiesterase 5 (PDE-5) inhibitors (e.g. sildenafil).

Although these drugs are known to be effective alone, little is known about combining them together in various combinations.

In this trial we will add a long-acting PDE-5 inhibitor to the treatment of patients with pulmonary arterial hypertension who currently are receiving only a prostanoid drug.

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.