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Paradoxical embolism: Passage of a clot (thrombus) from a vein to anartery. When clots in veins break off (embolize) , they travel first to the right side ofthe heart and, normally, then to the lungs where they lodge. The lungs act as a filter toprevent the clots from entering the arterial circulation. However, when there is a hole inthe wall between the two upper chambers of the heart (an atrial septal defect), a clot cancross from the right to the left side of the heart, then pass into the arteries as aparadoxical embolism. Once in the arterial circulation, 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 is usually recommended that even small atrialseptal defects be repaired. Also called crossed embolism.