Approximately two out of every one thousand live births have a complex congenital heart defect (CHD) that creates a univentricular physiology. As a result of the altered physiology, the overloaded single ventricle pumps both oxygenated and deoxygenated blood to parallel pulmonary and systemic circulations. Since 1971, when Fontan and Baudet were first successful in treating patients with tricuspid atresia with an atriopulmonary (AP) repair, operations that place the pulmonary and systemic circulations in series with the single pump have been termed "Fontan Repairs". The current repair of choice bypasses the right side of the heart, connecting the superior and inferior cava directly to the pulmonary arteries. Such repairs are termed total cavopulmonary connections (TCP) and have various modifications: graft size, connection angles, materials used, etc. Though most Fontan patients lead relatively normal lives, most also suffer some complications, ranging from exercise intolerance to life threatening GI disorders, whose causes are yet to be determined. One prominent characteristic of the Fontan circulation is an increased influence of respiration, which also is not well understood, but may be related to other complications. This research investigated various Fontan modifications not available via computer simulation by implementing surgical designs in-vivo in lambs and collecting pressure and flow data in various vessels and chambers. Data were obtained in four models, the AP connection and three models of the TCP connection: total cavopulmonary connection without a synthetic graft (TCPC), extracardiac total cavopulmonary connection with graft (TCPX), and the total cavopulmonary connection using a Y-shaped graft (TCPY). The overall hypothesis is that a fundamental understanding of modified Fontan repairs will lead to improved surgical planning and designs and thus the potential for long term outcomes in patients. Specific Aims were to: 1) Investigate the effects of ventilation parameters on hemodynamics under normal and various Fontan modifications, 2) Determine power losses of various Fontan circulations under varying physiological parameters, 3) Determine time offsets regarding positive pressure ventilation (PPV) for implementation in an electrical computer simulation model and 4) Determine feasibility and establish protocol at UNC for geometry and velocity mapping via MRI.