The heart serves as a pumping system, and to understand it, it is best to understand exactly how the heart works. A detailed description can be found here. In short, the heart is divided into four chambers: the left and right atria and the left and right ventricles. A thick wall called the septum separates the left side of the heart from the right. The valves, or one-way doors within the heart, separate the atria from the ventricles. The valves carefully regulate blood flow through the heart’s chambers. The walls of the heart and the valves keep the blood on a “one-way street” through the heart and out to the body and back.

In those with a PFO, a minimal amount of blood usually crosses freely between the right and left atria due to the small pressure difference between these two chambers. However, activities (lifting heavy weights for example) or changes in the chest geometry (after surgery for example) can lead to changes in the pressures inside the heart, leading to more blood passing from the right atrium to the left atrium through the PFO. For many people, the pressure change is usually not felt, and does not lead to a problem in most cases. However, any particles in the blood or any type of a blood clot can pass through the PFO, move out of the heart and travel to the brain, where it may cause a stroke.

The majority of strokes can be traced back to specific risk factors such as atherosclerosis, plaque buildup in the arteries outside or inside the brain, or a blood clot caused by atrial fibrillation. In some cases, the causes of a stroke cannot be identified by doctors, and those types of strokes are called cryptogenic strokes. A small percentage of these strokes may be attributed to the presence of a PFO. Thus, patients who have had a cryptogenic stroke –  and in whom a PFO has been identified –  may be at an increased risk of having a second stroke. The association between cryptogenic stroke and a PFO is stronger in younger individuals (ages 18-60), and in those for whom the septum separating the left and right atria is floppy (aneurysmal septum).