A patent foramen ovale (PFO) is a small opening between the right and left atria that persists from fetal circulation into adult life. During fetal development, the foramen ovale allows blood to bypass the non-functioning lungs. Normally, this opening seals when the lungs expand at birth and pressure in the left atrium rises. In approximately 25 to 30% of adults, the foramen ovale fails to seal completely, leaving a small flap-like passage.
A PFO is usually of no clinical significance in the majority of people who have one. However, it creates a potential pathway for blood clots, air, or other embolic material to pass from the venous (right) circulation directly into the arterial (left) circulation, bypassing the lungs where they would normally be filtered. This is the mechanism by which PFO is associated with cryptogenic stroke (stroke of unknown cause) in younger people, and with paradoxical embolism in general.
In the context of cardiac arrest investigation, a PFO may be identified on echocardiography using a bubble contrast study, where microbubbles injected into a vein are observed crossing to the left side of the heart. This finding is documented but does not by itself explain cardiac arrest; its significance depends on the overall clinical picture and the absence of other identified causes.
PFO closure by transcatheter device (inserting a small closure device via a vein) is recommended in selected patients under 60 who have had a cryptogenic stroke with a PFO and no other identified cause. It is not routinely performed simply because a PFO is identified.
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