Congenital Disorder of the Cardiovascular System (CVS) Explain/Define Atrial Septal Defect (ASD) Atrial septal defect is a congenital heart disease that usually results in a defect in the wall (septum) of the upper chamber of the heart i.e. the left atria and the right atria in which an abnormal opening is present at birth in the wall (septum) separating the left atria and right atria. . Due to this abnormal