Congenital heart defects are the most common types of birth defects and are responsible for an estimated 300,000 newborn deaths per year. The most severe of these defects can result in a “single ventricle” physiology. Thankfully, over the last 40 years surgeons have pioneered a set of 3 staged surgeries to palliate single ventricle heart defects, which results in a total cavopulmonary connection. Short term outcomes of these “Fontan” patients are very promising, with a 1 year survival rate around 95%. However, as these patients age, long term complications are inevitable. The central purpose of this thesis is to investigate the effectiveness of current, clinically implemented “solutions” for two of the most common modes of Fontan failure including pulmonary arteriovenous malformations (PAVMs) and liver disease. Specific Aim 1 will test if surgical planning can be used to accurately predict post-operative hepatic flow distribution (a factor in PAVM formation), and if Y-grafts can provide more balanced hepatic flow distribution than traditional Fontan connections. Specific Aim 2 will test if the extent of liver fibrosis in Fontan patients is associated with poor hemodynamics, and if ventricular assist devices can decrease Fontan hepatic congestion by augmenting flow and decreasing inferior vena cava pressure.