To Assess The Efficacy and Safety Of Oral Sildenafil in the Treatment of Pulmonary Arterial Hypertension.

• Drug: Sildenafil citrate
  
  oral, 20 mg, tid
• Drug: Sildenafil citrate
  
  oral 1 mg, tid
• Drug: Sildenafil citrate
  
  oral 5 mg, tid
• Drug: Sildenafil citrate
  
  oral 20 mg, tid

• Experimental: Sildenafil High dose
  
  Intervention: Drug: Sildenafil citrate
• Experimental: Sildenafil Low dose
  
  Intervention: Drug: Sildenafil citrate
• Experimental: Sildenafil medium dose
  
  Intervention: Drug: Sildenafil citrate
• Experimental: Sildenafil - Open label Phase
  
  Open label extension from week 12 to week 24.
  Intervention: Drug: Sildenafil citrate

Inclusion Criteria:

• Subjects with PAH (i.e. IPAH or secondary to connective tissue disease or with surgical repair of ASD, VSD, PDA, aorto-pulmonary window) whose baseline six minute walk test distance is >/= 100 m and
• Subjects with a mean pulmonary artery pressure of >/= 25 mmHg and a pulmonary artery wedge pressure of

Exclusion Criteria:

• Subjects whose 6 Minute Walk Distance may be limited by conditions other than PAH related dyspnoea or fatigue, e.g. claudication from vascular insufficiency or significant arthritis.
• Subjects who are currently receiving any forms of chronic treatment for PAH such as prostacyclin, PDE-5 inhibitors, endothelin-receptor antagonists, nitrates or nitric oxide donors (e.g. arginine supplement, nicorandil) in any form, protease inhibitors such as ritonavir and saquinavir, ketoconazole, itraconazole, and alpha blockers. Subjects previously receiving any of these drugs must have stopped use for a period of at least 1 month prior to screening, except in the case of bosentan or prostacyclin (3 months).