PFO and Scuba Diving: The Science Behind 'Undeserved' Decompression Sickness
Every diver is taught the golden rules from day one: watch your ascent rate, never hold your breath, and stay within the limits of your dive computer. We treat these limits as gospel, believing that if we stay below our "invisible ceiling," we are shielded from the "bends." Yet, every year, experienced divers following conservative profiles find themselves in hyperbaric chambers with what the medical community calls an "undeserved" hit.
The Mystery of the 'Undeserved' Hit
An "undeserved" hit refers to decompression sickness (DCS) symptoms that manifest despite a diver strictly adhering to all safety protocols, depth limits, and decompression schedules 3. In these cases, the diver's computer shows they remained well within their "no-decompression limits" (NDL) or completed all required stops, yet they still suffer from neurological, cutaneous, or joint-related issues.
The root of this mystery often lies in the limitation of mathematical models. As we explored in The Mystery of M-Values: How Your Dive Computer Calculates Your Invisible Ceiling, dive computers rely on theoretical "M-values"—maximum allowable nitrogen pressures within various tissue compartments. These models are based on statistical averages of the general population and do not account for individual physiological anomalies.
One of the most significant physiological factors in these unexplained incidents is a Patent Foramen Ovale (PFO). While traditional decompression theory assumes a closed circulatory loop where the lungs act as the primary filter for nitrogen bubbles, a PFO provides a "shortcut" that bypasses this filter entirely.
Anatomy of the Heart: What is a PFO?
To understand why a PFO is a concern for divers, we have to look back at our time in the womb. The fetal circulatory system is fundamentally different from that of an air-breathing adult. Because a fetus receives oxygenated blood from the mother via the umbilical cord, there is no need to send blood to the non-functional fetal lungs.
The Foramen Ovale
To facilitate this, the fetal heart has a natural opening between the right and left atria called the Foramen Ovale. This opening allows blood to flow directly from the right side of the heart to the left, bypassing the pulmonary circulation 2.
The Transition at Birth
When a baby takes its first breath, the pressure dynamics in the heart shift instantly. The sudden increase in pressure on the left side of the heart typically forces the flap of the Foramen Ovale to close. Within the first year of life, this flap usually fuses shut permanently, creating a solid wall (the atrial septum) between the two chambers 2.
Statistical Prevalence
However, this sealing process is not universal. In approximately 25% to 30% of the adult population, the flap remains "patent" (open) or only partially sealed 2. Under normal circumstances at sea level, this is medically insignificant. Most people with a PFO will live their entire lives without ever knowing it exists. But for a scuba diver, this tiny flap can become a gateway for decompression illness.
The Lung Filter and Venous Gas Emboli (VGE)
To grasp the danger of a PFO, we must first accept a reality of diving: silent bubbles.
Research using Doppler ultrasonic bubble detectors has shown that most divers produce micro-bubbles, known as Venous Gas Emboli (VGE), during or after a standard ascent 3. These bubbles are "silent" because they are usually too small and too few to cause symptoms. They travel through the venous system (veins) toward the heart.
The Lungs as a Biological Filter
In a diver with a "normal" heart, these VGE are pumped from the right atrium into the right ventricle and then into the lungs. The pulmonary capillary bed acts as a sophisticated biological filter. The tiny capillaries trap the bubbles, which then diffuse across the alveolar membrane and are exhaled through the breath 1.
Connecting to Kinetic Asymmetry
The volume of bubbles the lungs must process is influenced by Kinetic Asymmetry: Why Nitrogen Leaves Your Body Slower Than It Enters. Because nitrogen off-gassing is inherently slower than on-gassing, the "bubble load" in the venous system can remain high for several hours after a dive 4. This prolonged presence of VGE increases the window of risk for any diver with a cardiac shunt.
| Feature | Normal Heart Function | Heart with PFO (Shunting) |
|---|---|---|
| Venous Bubbles | Trapped in lung capillaries | Can bypass lungs |
| Blood Flow | Right Atrium → Right Ventricle → Lungs | Right Atrium → Left Atrium (Shunt) |
| Gas Clearance | Exhaled through lungs | Distributed to arterial system |
| Risk Level | Standard DCS risk | Elevated "Undeserved" DCS risk |
The Shunt Mechanism: How Bubbles Bypass the Filter
The real danger occurs during a "Right-to-Left Shunt." This is the physical movement of venous blood—and any bubbles it contains—directly into the arterial side of the heart through the PFO 1.
The Role of Pressure Differentials
Under normal conditions, the pressure in the left atrium is slightly higher than in the right, which keeps the PFO flap pushed shut. However, certain activities can momentarily reverse this pressure gradient 2.
Common "shunting triggers" include:
- The Valsalva Maneuver: Forceful equalization can spike right-atrial pressure 1.
- Coughing or Sneezing: Sudden thoracic pressure changes.
- Heavy Exertion: Lifting heavy tanks or climbing a boat ladder immediately after a dive 3.
Paradoxical Gas Embolism
When bubbles bypass the lung filter, they enter the arterial circulation. This is known as a Paradoxical Gas Embolism. Once on the arterial side, these bubbles can be transported to highly sensitive areas, such as the brain or spinal cord, where they can cause immediate blockages or trigger inflammatory responses 1.
Why PFO Challenges Traditional Decompression Theory
Most modern decompression algorithms, including those utilizing Gradient Factors, are designed to manage the "invisible ceiling" and minimize the formation of VGE that the lungs must filter. They do not account for a scenario where bubbles simply "take a shortcut" past the filter 1.
Specific Symptoms
PFO-related DCS is frequently neurological or cutaneous (skin). Because the arterial system feeds the brain and skin directly, bubbles shunted through a PFO often manifest as:
- Confused thinking or "brain fog"
- Visual disturbances
- Unusual fatigue 3
- Cutis Marmorata: A marbled, itchy skin rash often associated with PFO-related hits 3.
Bubble Volume vs. PFO Size
It is important to note that not all PFOs are the same. Some are tiny "pinholes," while others are large, significant openings. The risk of an "undeserved" hit is generally proportional to the size of the PFO and the volume of VGE produced during the dive. This is why any PFO means you can't dive is a myth—many divers with small PFOs dive for decades without issue. However, for those with a large shunt, the risk profile changes dramatically.
Diagnosis and the 'Bubble Study'
If you have experienced one or more "undeserved" hits, medical professionals often recommend testing for a PFO. The gold standard for diagnosis is the Contrast Echocardiogram, commonly known as a "Bubble Study" 2.
How the Bubble Study Works:
- A cardiologist injects a solution of agitated saline (which contains microscopic salt bubbles) into the patient's vein.
- An ultrasound (Echo) monitors the heart.
- The patient is often asked to perform a Valsalva maneuver to see if the saline bubbles cross from the right atrium to the left.
Expert Insight: There is currently no medical consensus supporting universal PFO screening for all entry-level divers. The test is specialized and can be expensive 2. Most experts suggest screening only for those who have a history of unexplained DCI or those entering high-risk technical diving careers.
Mitigation Strategies: Diving with a Known PFO
If you discover you have a PFO, it doesn't necessarily mean your diving days are over. There are two primary paths: medical intervention or conservative management.
Medical Intervention: PFO Closure
A cardiologist can perform a minimally invasive procedure to "plug" the PFO using a small device inserted via a catheter. Many divers have successfully returned to diving after a closure, though this requires a significant recovery period and clearance from a diving medical officer 2.
Conservative Diving Profiles
For those who choose not to undergo surgery, the goal is to minimize VGE production. If there are no bubbles in the venous system, there is nothing to shunt.
- Lower your Gradient Factors: Use a "low" GF (e.g., 30/70) to stay further away from your M-values.
- Use Nitrox: Breathing a higher oxygen percentage reduces the total nitrogen load 4.
- Extend Safety Stops: Give your body more time to off-gas in the shallowest zone.
- Avoid Shunting Triggers: Do not perform a forceful Valsalva; use gentler equalization techniques. Avoid heavy lifting for at least two hours post-dive 3.
- Stay Hydrated: Dehydration can increase bubble formation and impair circulation 3.
Conclusion: Knowledge as Your Best Safety Margin
The link between cardiac anatomy and decompression safety highlights a critical truth in diving: we are not just following a computer; we are managing a complex biological system. While 25-30% of us may have a PFO 2, the vast majority of divers will never suffer an "undeserved" hit if they dive conservatively and understand their body's limits.
As diving medicine evolves, we are moving away from "one-size-fits-all" tables and toward a more personalized understanding of risk. By staying informed about topics like PFOs, Kinetic Asymmetry, and Gradient Factors, you can build a safety margin that goes beyond the numbers on your screen.
Listen to your body, dive within your comfort zone, and remember that the best dive is the one where you return to the surface feeling as healthy as when you went down.
Are you curious about other aspects of diving theory? Check out our deep dive into The Deep Stop Debate to see how modern research is changing the way we ascend.
Further Reading
- Increased Risk of Decompression Sickness When Diving With a Right-to-Left Shunt: Results of a Prospective Single-Blinded Observational Study
- Return to Diving - StatPearls - NCBI Bookshelf
- Frontiers | Increased Risk of Decompression Sickness When Diving With a Right-to-Left Shunt: Results of a Prospective Single-Blinded Observa
- PFO and Decompression Illness in Recreational Divers - Divers Alert Network