What is a PFO? A patent foramen ovale, or PFO, results from the incomplete closure of the septal wall between the right and left atria (upper chambers) of the heart. The opening exists before birth to allow oxygenated blood to circulate throughout the fetus without having to pass through its lungs. The foramen ovale usually closes after birth in 75% of adults however, for one in four adults, the PFO does not seal completely and blood can flow directly, under certain conditions, from the right atrium to the left atrium, bypassing filtration by the lungs. In these cases, blood clots and/or other agents present in the blood can bypass the lungs and pass directly into the arterial system. Potential medical complications of a PFO include paradoxical embolism, cryptogenic stroke, and right to left nitrogen embolism in severe decompression illness. An association between PFO and severe migraine headaches has also been reported.