The assisted bidirectional Glenn (ABG) procedure was proposed to address the drawbacks of the modified Blalock-Taussig shunt (mBTS) and the bidirectional Glenn (BGD) procedures, as an alternative to first and second stage palliation for neonates with single-ventricle physiology. This thesis introduces a modified ABG (mABG) design to address previous ABG drawbacks of high SVC pressure. The performances are compared with realistic 3-dimensional multiscale simulations. Two simulation methods developed to improve accuracy and reduce costs are introduced: an adaptive mesh refinement method and an aortic pressure regulatory method. A parametric optimization study shows that the optimal nozzle area is around 16% of the SVC cross-sectional area, the optimal nozzle width and the optimal shunt diameter are both 3.5 mm. Simulations show the mABG was able to preserve all advantages of the original ABG procedure while reducing the SVC pressure to 4.3 mmHg at normal PVR. There is no significant improvement in the SVC pressure for high PVR, motivating more extensive paramedic studies.