A Patent Foramen Ovale (PFO) is a common anatomical variation present in roughly one-quarter of the adult population. It describes an opening between the heart’s two upper chambers, the right and left atria, that failed to seal shut after birth. The term “patent” indicates that the foramen ovale, a normal fetal structure, remains open. For the vast majority of people, this persistent opening is a benign finding that produces no symptoms and requires no medical attention.
Fetal Heart Development and the Foramen Ovale
The foramen ovale is a temporary structure that allows blood to bypass the lungs during fetal development, as the fetus receives oxygen from the placenta instead of breathing air. This opening acts as a shunt, directing oxygenated blood arriving at the right atrium straight into the left atrium. This shortcut efficiently moves blood into the systemic circulation to supply the developing body, particularly the brain.
The change from fetal to newborn circulation depends on a dramatic shift in internal pressures within the heart chambers. When a newborn takes its first breath, the lungs inflate, causing the resistance in the pulmonary blood vessels to drop significantly. This reduction allows a surge of blood flow into the lungs, increasing the volume returning to the heart’s left atrium.
The resulting increase in pressure within the left atrium, which is now higher than the pressure in the right atrium, initiates the closure. This pressure differential pushes a thin, flap-like membrane across the opening, sealing the foramen ovale. While this initial closure is functional, meaning the flow of blood is stopped, complete anatomical fusion of the tissues occurs over the first few months of life, ultimately forming a permanent seal known as the fossa ovalis.
The Components of the Interatrial Septum
The wall separating the two upper chambers of the heart is known as the interatrial septum, a complex structure formed by two distinct tissue components during embryonic development. The thin, flexible membrane that acts as the physical flap or valve is called the septum primum. This tissue grows downward from the roof of the atrium and is positioned on the left atrial side of the septum.
The second component, the septum secundum, is a thicker, crescent-shaped muscular fold that develops to the right of the septum primum. This structure forms the stiff rim or border of the opening from the right atrial perspective. The valve-like mechanism of the foramen ovale is created by the natural overlap between the septum primum and the septum secundum.
After birth, the pressure reversal pushes the septum primum against the sturdier septum secundum rim. This action creates a tight, functional seal that prevents blood from crossing between the atria. If the two septal tissues fail to fuse completely after this functional closure, the result is a Patent Foramen Ovale.
Defining Structural Characteristics of a Patent Foramen Ovale
A PFO is characterized by the persistence of the unfused flap-valve mechanism between the septum primum and secundum, allowing a potential passage for blood flow. The dimensions and physical appearance of this unsealed structure vary significantly between individuals. The size of the opening, measured as the separation or height between the septal edges, is a primary characteristic.
A larger PFO is defined as an opening height of two millimeters or more, while smaller PFOs are less than this measure. Studies analyzing complex PFOs have observed a mean opening length of approximately four millimeters, demonstrating the variable nature of this patent communication. The physical length of the overlapping tissue, known as the tunnel length, describes the distance blood must travel between the two septal components.
A shorter tunnel length is less likely to achieve a tight seal and may be considered a higher-risk structural feature. A long-tunnel PFO is defined as an overlap of eight to ten millimeters or greater, while simpler PFOs have shorter overlaps.
Another structural characteristic is the mobility of the interatrial septum, particularly the septum primum flap. Septal hypermobility describes a mobile or floppy septum that exhibits excessive movement or excursion during the cardiac cycle. This hypermobility is quantified as a movement of five to seven millimeters or more from the midline.
The presence of an Atrial Septal Aneurysm (ASA) is a distinct structural variation frequently associated with a PFO. An ASA refers to a redundancy of the septal tissue, causing it to balloon or protrude significantly into one of the atrial chambers. This condition is defined by a deviation of the septum of ten millimeters or more from the midline, or a total excursion of fifteen millimeters during the heart’s cycle.