The investigators are beginning a clinical research study here at UCSF to learn more about how hearts with single ventricles work. The investigators hypothesize that myocardial strain , as measured by cardiac MRI,in patients with single ventricles differs from normal hearts. This abnormality is associated with depressed cardiac power output and impaired exercise capacity. The study will utilize cardiac magnetic resonance imaging, exercise testing, and lung function testing to study how the single ventricle heart works so that in the future the investigators can improve how the investigators care for patients with this type of congenital heart disease.