One in four people has a patent foramen ovale — a small opening between the two atria of the heart that most carriers never detect. For a diver, that opening can be the difference between a clean dive and an unexplained decompression illness episode. The question is not whether everyone needs testing, but who specifically does.
The foramen ovale is an opening present in every fetus. It allows blood to bypass the lungs — which are not yet functioning — and flow directly to the brain. At birth, with the first breath, the pressure shift seals that opening within hours or days. In 75 % of people it closes completely. In the remaining 25 % it stays open: patent foramen ovale, or PFO.
On dry land, a PFO is completely irrelevant. Left atrial pressure exceeds right atrial pressure, so no venous blood crosses to the arterial side. People live eight decades with a PFO and never know it. The problem surfaces under pressure: nitrogen microbubbles dissolved in venous blood can cross through the opening and enter the arterial system without passing through the pulmonary filter. That drives systemic DCS — brain, spinal cord, joints.
The numbers are stark. Studies of divers with unexplained DCS — profiles within NDL, no risk factors, no obvious cause — find PFO in 60 to 80 % of cases. The general population rate is 25 %. That correlation is hard to dismiss. When a diver develops DCS for no apparent reason, a bubble study echocardiogram to check for PFO is now the first investigation.
One important nuance: having a PFO does not mean you will get DCS. The vast majority of divers with PFO complete entire careers without a single incident. PFO raises risk; it does not make DCS inevitable. The annual DCS probability for a diver with a large PFO and no precautions is roughly 0.5 %; without PFO it is 0.05–0.1 %. A fivefold to tenfold difference, not a hundredfold one.
Who should get tested? Current guidance from DAN and equivalent organisations: anyone who has had an unexplained DCS episode, anyone who suffers migraines with aura (which are linked to PFO), tech divers planning trimix dives, and optionally any diver who simply wants to know. The standard test is a transthoracic bubble study echocardiogram using agitated saline — inexpensive and non-invasive.
If a large PFO is found, there are three paths. First: dive more conservatively — NDL at 50 %, extended safety stops, no exertion, avoid extreme cold. Second: close the PFO with an interventional device, an outpatient procedure that takes about 30 minutes and is now routine cardiology. Third: stop diving, an option almost nobody chooses. Closure is generally recommended for tech divers with a large PFO.
Ignoring the finding is the one option worth avoiding. A diver I know — a physician — had two mild DCS episodes before anyone thought to look for a PFO. He now carries minor neurological sequelae that a 200-euro, 20-minute test could have prevented. Diving sports medicine has advanced considerably over the past 15 years; that progress only benefits you if you access it.
The unfiltered conclusion: PFO is not a reason to stop diving, but it is a reason to get informed. If you have logged more than 50 serious dives and have never had a proper diving medical, a session with a hyperbaric medicine specialist is time well spent — not because a problem is likely, but because one hour of testing gives you actionable information for the next 30 years underwater.