Atrial Septal Defect (ASD) is a congenital heart condition where there is an abnormal opening between the upper chambers of the heart, the atria. This opening allows oxygen-rich blood from the left atrium to flow into the right atrium, instead of flowing into the left ventricle and out to the body as it normally should. Over time, this can cause an increase in blood flow to the lungs and potentially lead to other complications if not treated.