When the door is pushed open, oxygen-poor blood from the right heart (venous) side may mix with oxygen-rich (arterial) blood from the left heart side. Unfortunately, there are usually many small blood clots in the venous system arising from smaller injuries as well as low flow conditions when patients do not move for several hours (e.g. sitting in an airplane, etc.). These blood clots are normally transported with the blood stream into the lungs, filtered there and finally resolved. If however pressure in the lungs rises, a clot can then come from the right atrium into the left atrium, which would otherwise remain stuck in the filter system of the lungs. This is called a paradoxical embolism. Such clots can be the cause an occlusion of a small blood vessel, and if in the brain, can cause a stroke.
As many as 20% of all people have a PFO. Infants with PFOs have no symptoms if they do not have other heart defects. Unless there are other defects, there are usually no complications from a PFO. The situation in adults is similar. Adults or older people however may suffer more frequently from clotting disorders based on risk factors typical for older age (i.e. smoking, obesity, hormonal contraception (the pill), high blood pressure, arteriosclerosis, etc.) Therefore adults with a PFO may have a higher risk of getting certain types of strokes, namely a stroke via a paradoxical embolism. In these circumstances, medication to prevent blood clots can be advantageous.
Once a person has had a stroke caused by a clot in the brain, and a PFO is diagnosed with the likelihood of a paradoxical embolism, a closure of the PFO is recommended to avoid future strokes.