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To the Editor.—
A paradoxical embolus arises in a vein but lodges in a systemic rather than a pulmonary artery; its entrance into the arterial circulation must take place through an arteriovenous communication. The usual explanation of these cases is that the embolus has passed through a patent foramen ovale; however, such a mechanism is difficult to prove, and the presence of a patent foramen ovale or even a septal defect within the heart is not conclusive evidence that paradoxical embolism has occurred. Before an embolus can pass from right to left within the heart, reversal of the normal intracardiac pressure gradients must occur. If emboli arising in the systemic venous system travel to the lungs and obstruct pulmonary blood flow, pressure within the right ventricle and ultimately the right atrium may be raised so that blood will be shunted from right to left atria with consequent potential for paradoxical
Rosier RP, Lefer LG. Paradoxical Embolism. JAMA. 1977;238(18):1911-1912. doi:10.1001/jama.1977.03280190013005