embolism, paradoxical

embolism, paradoxical Passage of a clot (thrombus) from a vein to an artery. When clots in veins break off (embolize), they travel first to the right side of the heart and, normally, then to the lungs, where they lodge. The lungs act as a filter to prevent the clots from entering arterial circulation. However, when there is a hole in the wall between the two upper chambers of the heart (atrial septal defect), a clot can cross from the right to the left side of the heart, and then pass into the arteries as a paradoxical embolism. When a clot enters arterial circulation, it 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 is usually recommended that even small atrial septal defects be repaired. Also known as crossed embolism.