A patent foramen ovale (PFO) is a relatively common and often asymptomatic heart defect that is present at birth (congenital). During fetal development, the heart’s two upper chambers (atria) are connected by a hole through the wall of muscle that separates them (septum). This hole, along with a blood vessel connecting the pulmonary artery (artery going to the lungs) and the aorta (the main artery going to the body), must be present in the fetus in order to detour blood away from the lungs.  Without these two connections the fetus would not survive and a miscarriage often occurs. This hole is called the foramen ovale, and it normally closes shortly after birth.  When it remains open after birth, it is said to be “patent,” which is another away of saying “open.”

In about 15-20 percent of the general population, however, the foramen ovale remains open (patent), allowing blood to potentially or actually mix between the two atria. Many people do not know they have PFO because it does not give any symptoms. PFO does not have adverse effects on most people and therefore, it does not require treatment.

However, even a small, untreated patent foramen ovale can cause heart-related difficulties such as labored breathing or recurrent respiratory infections. Other medical conditions, such as migraine headaches, have also been associated with a PFO. If closure is recommended as treatment, the hole may be closed with an inserted device or, rarely, by surgery.

The cause of patent foramen ovale is unknown but health experts believe it may be related to genetic and hereditary factors. PFO also has been associated with other cardiac conditions, such as septal or valve defects.

PFO is considered a possible risk factor for stroke and systemic embolism because of the potential formation of blood clots. These clots may form in veins and subsequently pass into circulation through the PFO, without being filtered in the lungs. The relationship between PFO and stroke remains somewhat controversial and it continues to be studied by researchers. 

Because of the pressure of the water and its effect on the circulation especially during surfacing, scuba divers also seem to face a slightly higher risk of stroke if they have a PFO.