In this study, the investigators will evaluate the effect of sildenafil on exercise tolerance
in patients with a single cardiac ventricle who have undergone the Fontan operation. The
investigators will also evaluate echocardiographic measures of ventricular function and
measure quality of life changes using two validated quality of life measures. The hypothesis
is that sildenafil will result in increased exercise tolerance in patients who have had the
Fontan operation as compared to placebo.