• The primary outcome measure is a fall in the left ventricular filling pressure in association with administration of sildenafil [ Time Frame: The left ventricular filling presure will be assessed at 60 minutes after the administration of sildenafil ] [ Designated as safety issue: No ]
  

• A secondary outcome is the distance walked with the 6-minute walk test after the administration of sildenafil [ Time Frame: Determned at the end of the 6-minute walk test ] [ Designated as safety issue: No ]
  

Sildenafil (Viagra) has been extensively studied in patients with idiopathic pulmonary hypertension. It reduces pulmonary vascular resistance, improves exercise capacity and is now an approved therapy for this condition. Heart failure (HF) patients also have pulmonary hypertension and several recent reports have shown that sildenafil leads to an improvement in their exercise capacity. In HF sildenafil causes a reduction in the pulmonary and systemic vascular resistances, improves pulmonary gas diffusion and perhaps increases cardiac output. It is uncertain if left ventricular filling pressures are reduced and whether there is improvement in left ventricular relaxation. We hypothesize that in HF patients the improvement in exercise capacity associated with sildenafil is related to a significant reduction in left ventricular filling pressures. We propose to study 20 patients with stable, symptomatic HF Class III. The study design is a randomized cross-over trial of the administration of a single dose of sildenafil 50 mg or a matching placebo. Patients will be excluded if walking is impaired due to non-cardiac conditions or if they are taking nitrates. Exercise capacity will be determined before and after the oral administration of sildenafil 50 mg or placebo. The difference in the standardized 6 minute walk test (distance patient is able to walk over a 6 minute interval) will be used to assess exercise capacity. Left ventricular filling pressures will be assessed by Doppler echocardiography and the serum level of B-type natriuretic peptide (BNP). Evaluation of left ventricular relaxation will be determined by Doppler echocardiography techniques. Each patient will have a blood sample for BNP, an initial echocardiogram and perform a 6 minute walk test. They will then be given either sildenafil or a matching placebo in a randomized double-blind fashion. One hr later a blood sample for serum BNP, the echocardiogram and the 6 minute walk test will be repeated.