What is a PFO (Patent Foramen Ovale)?

Picture this: you're ascending from a beautiful dive, following all the rules, staying within no-decompression limits, making your safety stop. Everything's textbook perfect. Yet hours later, you develop strange symptoms—tingling in your limbs, a persistent headache, or unexplained fatigue. Welcome to the puzzling world of decompression illness linked to a condition you might not even know you have: a Patent Foramen Ovale, or PFO.

What Exactly is a PFO?

To understand a PFO, we need to take a brief journey back to before you were born. In the womb, babies don't use their lungs to breathe—they get oxygen from their mother through the placenta. To accommodate this, the developing heart has a small opening called the foramen ovale that allows blood to bypass the lungs, flowing directly from the right atrium to the left atrium.

At birth, when a baby takes their first breath, the lungs expand and pressure changes in the heart cause this opening to slam shut like a trapdoor. In most people, it fuses closed permanently within the first few months or years of life. But here's the surprising part: in about 27% of the population—that's more than one in four people—this opening never fully seals. When it remains open, it's called a Patent Foramen Ovale, with "patent" meaning "open" in medical terminology.

For most people, a PFO causes absolutely no problems throughout their entire lives. Many people live to old age never knowing they have one. But for divers, this little hole can become a significant concern.

The diving connection: why should divers care?

When we dive, our bodies absorb nitrogen from the compressed air we breathe. During ascent, this nitrogen needs to be released gradually through our lungs. Even when following proper procedures, tiny nitrogen bubbles—called venous gas emboli—can form in our bloodstream. Normally, these bubbles travel to the lungs where they're filtered out and harmlessly expelled when we breathe.

But a PFO creates a shortcut. If pressure in the right atrium exceeds pressure in the left—which can happen during activities like clearing your ears, lifting heavy gear, or even just exerting yourself—that trapdoor can swing open. When it does, those nitrogen bubbles can bypass the lung's filtration system entirely, crossing directly into the arterial circulation. From there, they can travel anywhere in the body, potentially reaching the brain, spinal cord, inner ear, or skin.

The statistics are sobering. Divers with a PFO face a 2.5 times greater risk of developing decompression illness compared to divers without one, and a 4 times greater risk specifically for neurological decompression illness—the most serious type.

Size matters: understanding your risk

Not all PFOs carry the same risk. The danger increases dramatically with the size of the opening. While 27% of people have some form of PFO, only about 6% have what's considered a "large" PFO—typically a centimeter or more in diameter. These larger openings pose the greatest concern because they allow more bubbles to cross over.

Research has shown that several factors must align for a PFO to contribute to decompression illness. You need a sufficiently large opening, venous gas bubbles must form during the dive, something must trigger the PFO to open (like a Valsalva maneuver), and those bubbles must reach vulnerable tissues while they're still supersaturated with nitrogen. This explains why many divers with PFOs never experience problems, while others may have unexplained incidents despite conservative diving practices.

What can divers do?

If you're diagnosed with a PFO—or suspect you might have one—you essentially have three options, each with its own considerations:

Stop diving entirely. This is the safest option, though understandably difficult for those passionate about diving.

Dive more conservatively. This means adopting stricter safety protocols: limiting depth to 20 meters or less, avoiding decompression diving, extending safety stops, using enriched air nitrox to reduce nitrogen loading, avoiding strenuous exertion before and after dives, and steering clear of activities that might increase pressure in the right atrium.

Consider PFO closure. A minimally invasive procedure can close the opening using a small device inserted through a catheter. However, this isn't without risks—complication rates range from 5-7%, and the decision should only be made after thorough consultation with a cardiologist experienced in diving medicine. Closure is generally not recommended for divers who've never experienced symptoms.

Should you get screened?

Routine PFO screening for all divers isn't recommended. The absolute risk of decompression illness remains relatively low, even with a PFO, and screening isn't cheap. However, testing should be seriously considered if you've experienced repeated episodes of decompression illness—particularly with neurological, inner ear, or skin manifestations—especially when these occurred after conservative dive profiles that shouldn't have caused problems.

The most effective screening method combines transthoracic echocardiography with a bubble contrast test, ideally performed at a center experienced in this technique.

The bottom line

A PFO doesn't have to end your diving career, but it does require awareness and often adjustment of diving practices. Many divers with PFOs continue diving safely for years by following conservative protocols. The key is making informed decisions based on accurate information about your individual anatomy and risk factors.

If you've experienced unexplained decompression symptoms, have recurrent episodes of DCI despite safe diving practices, or have a history of migraines with aura (which has been linked to PFOs), talk to a diving medicine physician about whether screening makes sense for you. Knowledge is power, especially when it comes to diving safely for the long term.

This article is not written by medical professionals! If you suspect you have a PFO, consult a doctor.