A paradoxical embolism, also called a crossed embolism, refers to an embolus which is carried from the venous side of circulation to the arterial side, or vice versa. It is a kind of stroke or other form of arterial thrombosis caused by embolism of a thrombus (blood clot), air, tumor, fat, or amniotic fluid of venous origin, which travels to the arterial side through a lateral opening in the heart, such as a patent foramen ovale,  or arteriovenous shunts in the lungs.
Paradoxical embolisms represent two percent of arterial emboli.
Passage of a clot (thrombus) from a systemic vein to a systemic artery. When clots in systemic veins break off (embolize), they travel first to the right side of the heart and, normally, then to the lungs where they lodge, causing pulmonary embolism. On the other hand, when there is a hole at the septum, either upper chambers of the heart (an atrial septal defect) or lower chambers of the heart (ventricular septal defects), a clot can cross from the right to the left side of the heart, then pass into the systemic arteries as a paradoxical embolism. Once in the arterial circulation, a clot can travel to the brain, block a vessel there, and cause a stroke (cerebrovascular accident).
Therapeutic preventional anticoagulation is first line, if correction of the underlying anatomical pathology is not already done, by using Heparin or other anticoagulative agents. This approach is also used in the treatment of a present embolism, albeit at higher doses. Once these are exhausted or the use of therapeutic anticoagulation is contraindicated (for example, in a cooccurring traumatic hemorrhage), mechanical thrombectomy may be used to physically remove the embolism from the blocked artery.
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- Tang CE (January 2004). "Paradoxical embolism: a rare life- and limb-threatening emergency". CJEM. 6 (1): 40–4. PMID 17433144.[dead link]
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