The purpose of this study is to investigate the effect of the diameter of an interatrial shunt on patients with heart failure with preserved ejection fraction (HFpEF). HFpEF is characterized by the stiffening of left ventricle heart cells, which results in higher left ventricular pressure and consequently causes a higher pressure in the left atrium as it attempts to force blood through the left ventricle. The study seeks to investigate the effect of placing a small shunt in the septum – the tissue that divides the left and right heart chambers – on reducing the left atrium, left ventricular, and pulmonary pressures that arise as a result of HFpEF. As the shunt diameter increases, more blood is allowed to flow from the left atrium to the right atrium, thereby reducing the pressure buildup in the left atrium at the cost of a rise in pressure in the right atrium. However, large-diameter shunts may be more invasive and cause future complications for the patient, especially by increasing the pulmonary-to-systemic flow rate ratio (Qp:Qs) to dangerously high levels. The goal of this study is to determine the largest possible shunt diameter to reduce the left atrium pressure of a typical HFpEF patient down to a healthy level without increasing Qp:Qs to dangerous levels. The heart atria are modeled by mapping the 3D heart atria into an idealized 2D model of the heart atria. Using the 3D-2D procedure and coupled equations found in Meindertsma, this paper investigates the effect of varying shunt diameter on the target pressures (left atrium, left ventricle, pulmonary system) and flow rate. [8]. Simulations show that increasing shunt diameters from 0 to 12 mm consistently dropped target pressures but increased the Qp:Qs. Using the same criterion as Kemmerling and Meindertsma suggest, our study found the 9 mm shunt to be the largest and most pressure-reducing shunt that still maintains a safe Qp:Qs ratio for patients [7, 8].