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Atrial septal defect ( ASD): A hole in the septum, thewall, between the atria, the upper chambers of the heart. Commonly called an ASD. ASDsconstitute a major class of heart formation abnormalities present at birth (congenitalcardiac malformations). Normally, when clots in veins break off, theytravel first to the right side of the heart and, then to the lungs where they lodge as an obstruction (embolus). Thelungs act as a filter to prevent the clots from entering the arterial circulation.However, when there is an ASD, a clot can cross from the right to the left side of theheart, then pass into the arteries as a paradoxical embolism. Once in the arterialcirculation, a clot can travel to the brain, block a vessel there, and cause a stroke ( cerebrovascular accident). Because of the risk of stroke from paradoxical embolism, it isusually recommended that even small ASDs be closed (repaired).