Each year 8 out of 1000 babies are born with congenital defects of the right heart. Right heart bypasses, known as "Modified Fontans", are the most common methods used to establish a viable pulmonary circulation in the infant. Fontan has become a generic term applied to numerous surgical procedures that separate the systemic and pulmonary circuits in patients with only one functional ventricle. Two popular options are the total cavopulmonary repair (TCP=the inferior and superior vena cava are connected directly to the pulmonary arteries), and an atriopulmonary shunt (AP=the right atrium is attached to the main pulmonary artery via a shunt). The advantage of the AP is thought to be the inclusion of the pumping power of the right atrium in the modified circulation. Whereas, the advantage of the TCP is the exclusion of chambers which set up areas of flow recirculation and stagnation (i.e., the right atrium). Our research focuses on determining the optimal configuration for long-term use specifically in these two procedures. We are using computational methods to model arterial blood flow pre- and post-Fontan to assist surgeons in determining the optimal configuration.