A Patent Foramen Ovale (PFO) is a small, flaplike opening between the heart’s two upper chambers, the right and left atria. This feature is a remnant of the fetal circulatory system that does not fully close after birth for some individuals. Present in about a quarter of the adult population, a PFO is the persistence of a fetal structure rather than a defect. Most people with a PFO are asymptomatic and unaware they have it.
The Role of the Foramen Ovale Before Birth
During fetal development, the lungs are not used for breathing, so the circulatory system is designed to bypass them. A fetus receives oxygen-rich blood from the placenta, which enters the right atrium. To get this oxygenated blood to the brain and other organs, the fetal heart uses a shortcut called the foramen ovale.
This opening in the atrial septum, the wall separating the atria, allows blood to flow directly from the right atrium to the left atrium. From the left atrium, the blood moves to the left ventricle and is then pumped to the body. This pathway is a necessary component of fetal circulation, ensuring oxygenated blood reaches the body without passing through the high-resistance, fluid-filled lungs.
The foramen ovale functions as a one-way passage. Before birth, the pressure in the right atrium is higher than in the left, which keeps the flap of the foramen ovale open. This allows blood to move through it, directing flow for prenatal growth.
The Atrial Septum and PFO Formation
The formation and closure of the foramen ovale depend on the atrial septum’s anatomy. This wall is not a solid structure but is composed of two overlapping layers: the septum primum and the septum secundum. The septum primum is a thin, flexible membrane that acts as a flap, while the septum secundum is a thicker, muscular rim that forms a rigid border.
These two layers overlap to create the tunnel-like passage of the foramen ovale. Before birth, higher pressure on the right side of the heart keeps the flap of the septum primum pushed open, allowing blood to flow from right to left. This dynamic changes with a baby’s first breath.
At birth, pulmonary circulation increases blood flow to the lungs and back to the left atrium. This raises the pressure in the left atrium, making it higher than in the right. This pressure shift pushes the flexible septum primum against the septum secundum, functionally closing the foramen ovale. In about 75% of individuals, these layers permanently fuse during the first year of life. A PFO results when this anatomical fusion does not occur.
Anatomical Features of a PFO
When a foramen ovale fails to fuse, it persists as a PFO. It is not a hole but a tunnel-like passage between the unfused septum primum and septum secundum. The length and width of this tunnel can vary significantly among individuals.
After birth, the higher pressure in the left atrium keeps the flap of the septum primum pressed against the septum secundum. This holds the PFO closed, creating a seal that prevents blood from flowing between the atria. The structure acts as a one-way valve that is normally shut.
Under certain conditions, the pressure in the right atrium can temporarily exceed the pressure in the left. This can happen during activities like coughing, sneezing, or straining. The pressure change can briefly push the flap open, allowing a small amount of blood to shunt from the right atrium to the left.
PFO vs. Atrial Septal Defect (ASD)
A Patent Foramen Ovale is anatomically distinct from an Atrial Septal Defect (ASD). A PFO is a failure of two overlapping tissues to fuse, leaving a potential, tunnel-like opening. In contrast, an ASD is a true hole in the atrial septum where tissue is physically missing.
An ASD is a congenital heart defect because it represents an abnormal development of the atrial septum. The distinction can be visualized with an analogy: a PFO is like an unsealed envelope flap. An ASD is like a hole punched directly through the envelope.
This structural difference also affects blood flow. A PFO is closed by pressure and only opens intermittently. An ASD often allows for a more constant flow of blood between the atria, usually from the higher-pressure left atrium to the right atrium.