Foramen ovale (and fossa ovalis)
Foramen ovale cordis
Also known as: fossa ovalis, patent foramen ovale, PFO
The foramen ovale is an oval opening in the fetal interatrial septum, guarded by a flap of septum primum. It lets oxygen-rich blood from the IVC pass from the right atrium to the left atrium. After birth it closes, leaving the fossa ovalis.
Overview
Development. In weeks 4–5, the septum primum grows down toward the endocardial cushions. Before it reaches them, programmed cell death opens a new hole in its upper part, the ostium secundum. Meanwhile the thicker septum secundum grows down on the right side of the septum primum, but it never completes. The oval gap left in it is the foramen ovale. Blood can pass from right to left through the foramen ovale and then the ostium secundum, but the flap of septum primum stops it flowing back.
After birth. Once left atrial pressure exceeds right atrial pressure, the flap is held shut (functional closure). Over the following months the two septa usually fuse (anatomical closure). In the adult, the fossa ovalis floor is the old septum primum, and its raised rim (limbus) is the edge of the septum secundum.
Patent foramen ovale. In about 25% of adults the flap never fuses, leaving a probe-patent foramen ovale (PFO). Blood can cross it whenever right atrial pressure rises above left atrial pressure, for example during a Valsalva manoeuvre or coughing.
Function
- In the fetus, sends oxygenated IVC blood to the left heart and on to the brain and heart.
- After closure, separates the systemic and pulmonary venous returns.
Clinical significance
Paradoxical embolism
A venous clot can cross a PFO into the systemic circulation and cause a stroke. In selected patients under about 60 with an otherwise unexplained (cryptogenic) stroke and a PFO, closing the PFO with a device reduces the risk of another stroke. A PFO is also linked to decompression illness in divers and to migraine with aura.
Bubble contrast echocardiography
To detect a PFO, agitated saline containing tiny bubbles is injected into a vein during echocardiography. Bubbles appearing in the left atrium within a few beats, especially after a Valsalva manoeuvre, show a right-to-left shunt.
Compiled from standard anatomy and physiology references — see the Cardiovascular System sources. For education only; not medical advice.