atrial septal defect

atrial septal defect A hole in the wall (septum) between the upper chambers of the heart (atria). Abbreviated ASD. ASD is a major class of heart malformation. Usually, when clots in veins break off (embolize), they travel first to the right side of the heart and then to the lungs, where they lodge. When there is an ASD, however, a clot can cross from the right to the left side of the heart, and then pass into the arteries as a paradoxical embolism. Once a clot is in the arterial circulation, it can travel to the brain, block a vessel there, and cause a stroke. ASDs are surgically closed.