A patent foramen ovale or PFO is a defect in the wall between the two upper chambers of the heart. This defect is actually an incomplete closure of the atrial wall that results in a flap or valve-like opening in the atrial wall. A PFO is present in everyone before birth but seals shut in most people.
If the flap does not seal, an individual can open, and blood can flow in either direction between the right and left heart chambers. If the blood moves from the right chamber to the left, it can bypass the filtering system of the lungs, which dissolves tiny blood clots. If clots remain in the blood and pass through the left chamber, they can lodge in the brain, heart, eyes or kidneys, causing a stroke.