Hypoplastic left heart syndrome is a congenital heart defect in which the left side of the heart does not form correctly and is unable to pump enough blood to the body. Children with this condition go through a series of operations to create the Fontan circulation aimed to ensure that there is no obstruction of blood flow through the aorta to the body and to supply blood to the lungs. In the final stage of the operations, an artificial extracardiac conduit (ECC) is typically used to divert the venous blood from the inferior vena cava (IVC) to the left and right pulmonary arteries (LPA and RPA, respectively) without passing through the heart. The Fontan circulation is subjected to multiple abnormal physiological features leading to postoperative complications. The abnormal narrowing of the ECC, i.e. stenosis, usually occurs as a result of thrombosis and calcification of the ECC interior and is believed to be one of the most danger￾ous factors resulting in heart failure [1]. In the present study, the effect of varying ECC stenosis on the hemodynamics in the geometrically perfect surrogate total cavopulmonary connection (TCPC) model is investigated using high-fidelity numerical simulations. The efficiency of the Fontan circulation is quantified according to the power loss, relative lung perfusion and the percentage of conduit surface area with abnormally low/high wall shear stress (WSS). Additionally, the impact of respiration and asymmetry in the stenosis geometry to the system is examined. The results show that the flow in the TCPC model exhibits pronounced unsteadiness even under the steady initial boundary conditions. Recirculation patterns are observed near the corners of the junction and these locations were found to strongly correlate with areas of low WSS. The largest power loss was found to correspond to the case with the diffuse ECC stenosis where the left lung perfusion strongly dominates over the right lung perfusion. Finally, it was observed that the impact of respiration and asymmetry in the stenosis geometry leads to increase in the power loss in the system. Results obtained in the study would potentially allow modification of surgery to suit the individual patient constraints.