Twenty-four patients with known heart failure will be recruited. They will undergo a cardiac magnetic resonance for ventricular analyses and after this first exam they will be randomized to placebo or Sildenafil. After one hour drug intake they will undergo another cardiac magnetic resonance to evaluate the drug impact on right and left ventricular function.

Twenty-four patients with known heart failure will be recruited. They will undergo a cardiac magnetic resonance for ventricular analyses and after this first exam they will be randomized to placebo or sildenafil. After one hour drug intake they will undergo another cardiac magnetic resonance to evaluate the drug impact on right and left ventricular function.

The first and second exams will be performed by the same staff. All patients will be evaluated by two, three and four chambers view. Left and right ventricle quantification analyses will be performed by Argus software.

Inclusion Criteria: Patients with heart failure in stable clinical condition compatible with New York Heart Association functional class I to III. Eligibility criteria is consent to participate in the study after receiving detailed information about procedures, possible clinical benefits, and risks.

Exclusion Criteria: patients with new diagnosed lesions during the procedure, eg pulmonary mass and important claustrophobia, low blood pressure ( systolic blood pressure <90 mmHg or diastolic blood pressure <60 mmHg) nitrate or nebivolol use over the last 48 hours, metallic non MRI compatible implantable devices.