Diffuse Myocardial Fibrosis in Fontan Patients

The purpose of this protocol is to measure the relaxation of the heart in subjects with single ventricles who have undergone the surgical Fontan procedure. We will do this by measuring relaxation with MRI, echocardiography, and cardiac catheterization and compare to blood levels that measure heart scarring. We will also measure relaxation before and after boluses of intravenous (IV) fluids to see if the relaxation changes when there is more fluid in the heart. Measurements of heart relaxation will be obtained from the MRI, echocardiogram, and cardiac catheterization for each patient and compared to blood markers of heart scarring. We aim to compare all of these measurements to see if we can accurately identify heart scarring and, if present, how much it correlates with impaired heart relaxation.