A PFO can allow a small amount of blood to pass from the right side of the heart to the left side of the heart; this is called a "shunt. A patent foramen ovale (PFO) is a hole in the wall between the heart's two upper chambers (atria).Before birth all babies have this opening. Treatment is a catheter procedure or surgery to close the opening in the heart. The foramen ovale is a hole in the wall between the left and right atria of every human fetus. This hole allows blood to bypass the fetal lungs. However, it may cause serious complications under certain circumstances. In roughly 75 percent of cases, the hole, called a patent foramen ovale (PFO), closes on its own and requires no treatment. If you have a PFO and low blood oxygen levels or an unexplained stroke, you may need a procedure to close the hole. University of California, San Diego.