Atrial septal defect is the most common congenital heart disease in adults, accounting for 20%-30% of all adult congenital heart conditions. The impact of an atrial septal defect on hemodynamics primarily depends on the amount of shunting. Due to higher pressure in the left atrium compared to the right atrium, a left-to-right shunt is formed. The amount of shunting depends on the size of the defect, as well as the compliance of the left and right ventricles and the relative resistance in the systemic and pulmonary circulations. Diseases that affect the compliance of the left ventricle, such as hypertension and coronary artery disease, can increase the extent of the left-to-right shunt. A continuous increase in pulmonary blood flow leads to pulmonary congestion, increasing the load on the right heart. Pulmonary vascular compliance decreases, progressing from functional pulmonary arterial hypertension to organic pulmonary arterial hypertension. As right heart system pressure continuously increases and eventually exceeds the pressure in the left heart system, the original left-to-right shunt reverses to become a right-to-left shunt, resulting in cyanosis.