The Hidden Risks of Freediving: Understanding PFO and Stroke Risk

The Golden Rule: Never Dive Alone

Written by Master Freediver Laura Wilson

As Freedivers, we are made aware that there are risks from the first course we attend. One of the biggest take-home messages we are informed of is “never dive alone.”

Shallow water blackout is a potentially fatal outcome while freediving, and if you don’t have a buddy to rescue you, the event will likely end in death.

So, what are some other risks while freediving?

What is a PFO (Patent Foramen Ovale)?

In the adult population, 25–35% of people have a heart defect that they are born with. The defect is known as a patent foramen ovale (PFO). We all have one of these when we are in the womb. It is a small flap between the two upper chambers of the heart known as the left and right atrium.

When we are still in the womb, we need this channel to stay patent so oxygenated blood can pass from our mother and circulate throughout our body. Our lungs are not yet functioning and if this were to close prematurely we would become hypoxic (low oxygen in our blood).

When we take our first breath at birth, the changes in pressure within our heart slam this flap closed for the majority of the population. This keeps the oxygenated blood from our lungs from shunting from the left to the right side of the heart.

How the Heart Circulates Blood

To understand this better:

Blood comes from our lower limbs and abdomen through veins into one major vein — the inferior vena cava. The blood from the head, neck, and upper limbs passes through the superior vena cava. This blood is deoxygenated and is taken to the right side of the heart — from the right atrium to the right ventricle — and pumped into the lungs via the pulmonary artery.

Once oxygenated within the lungs, it flows through the pulmonary veins into the left atrium, then into the left ventricle where it is pumped through the aorta (a main artery) and throughout the body.

The Problem with Having a PFO

The problem with having a PFO is that certain pressure changes can reverse the direction of blood flow. Most people can go their entire life with no problems having a PFO. However, a PFO does increase your risk of having a stroke.

When a stroke is caused by having a PFO, it results in an ischaemic stroke caused by a paradoxical embolism which is passed through the defect.

A stroke can occur when a clot bypasses the lungs by passing through the PFO and crossing over to the left side of the heart. The clot then has a direct path to the brain.

The Role of the Valsalva Manoeuvre

It is a rare event, but divers perform one manoeuvre known as a Valsalva. With freedivers, we perform this multiple times as we descend and often dive multiple times a day.

When taking this into consideration, every time we do a Valsalva our risk of stroke or injury increases.

A Valsalva manoeuvre is a technique which divers use to equalize their ear canals as they descend into deeper water.

When a Valsalva is performed it also causes a change in pressure in other areas of the body — such as the heart.

If a person has a PFO, the shunting direction will change briefly from left to right, to a right to left shunt. This right to left shunt is where deoxygenated blood from around the body can move to the left side of the heart, entering the arterial circulation and flowing to the brain.

Increased Risk with Clotting Conditions

If a person has a history of deep vein thrombosis (DVT) or conditions that increase their venous clot risk, then having a PFO while freediving does increase their chance of having a stroke.

Other Injuries Linked to PFO and Diving

There are also other injuries that can occur from having a PFO while diving.

As the lungs create a barrier between the venous and arterial circulation, having a PFO causes blood to bypass the lungs and enter the arterial circulation directly.

When divers ascend, nitrogen gas comes out of the solution, forming bubbles in the blood. These bubbles can pass through the PFO, entering the arterial circulation and increasing the risk of decompression sickness.

How to Find Out if You Have a PFO

To find out if you have a PFO, a person can get an echocardiogram. This is a test using ultrasound that assesses the structure of the heart.

In addition, a bubble study test can be added to the ultrasound to definitively rule out the presence of a PFO.

The test involves a solution of saline, air, and the patient’s blood, which is injected into their vein in their arm. At the same time on ultrasound, bubbles can be visualised in the right heart chambers.

If the bubbles appear in the left chambers within 4 heartbeats, there is a PFO. However, sometimes a Valsalva needs to be performed to change the pressures and unmask a PFO.

Summary: Know Your Risk

In summary, if a person has an increased risk of having venous clots and has a PFO, their risk of a stroke while diving is increased.

Patent foramen ovales can be closed via a nonsurgical procedure. Speaking to a cardiologist is recommended for further information.

Ready to learn how to freedive safely? Join a course with The Pressure Project and transform your mindset, breath, and body—one dive at a time.

The Hidden Risks of Freediving: Understanding PFO and Stroke Risk

The Golden Rule: Never Dive Alone

Written by Master Freediver Laura Wilson

As Freedivers, we are made aware that there are risks from the first course we attend. One of the biggest take-home messages we are informed of is “never dive alone.”

Shallow water blackout is a potentially fatal outcome while freediving, and if you don’t have a buddy to rescue you, the event will likely end in death.

So, what are some other risks while freediving?

What is a PFO (Patent Foramen Ovale)?

In the adult population, 25–35% of people have a heart defect that they are born with. The defect is known as a patent foramen ovale (PFO). We all have one of these when we are in the womb. It is a small flap between the two upper chambers of the heart known as the left and right atrium.

When we are still in the womb, we need this channel to stay patent so oxygenated blood can pass from our mother and circulate throughout our body. Our lungs are not yet functioning and if this were to close prematurely we would become hypoxic (low oxygen in our blood).

When we take our first breath at birth, the changes in pressure within our heart slam this flap closed for the majority of the population. This keeps the oxygenated blood from our lungs from shunting from the left to the right side of the heart.

How the Heart Circulates Blood

To understand this better:

Blood comes from our lower limbs and abdomen through veins into one major vein — the inferior vena cava. The blood from the head, neck, and upper limbs passes through the superior vena cava. This blood is deoxygenated and is taken to the right side of the heart — from the right atrium to the right ventricle — and pumped into the lungs via the pulmonary artery.

Once oxygenated within the lungs, it flows through the pulmonary veins into the left atrium, then into the left ventricle where it is pumped through the aorta (a main artery) and throughout the body.

The Problem with Having a PFO

The problem with having a PFO is that certain pressure changes can reverse the direction of blood flow. Most people can go their entire life with no problems having a PFO. However, a PFO does increase your risk of having a stroke.

When a stroke is caused by having a PFO, it results in an ischaemic stroke caused by a paradoxical embolism which is passed through the defect.

A stroke can occur when a clot bypasses the lungs by passing through the PFO and crossing over to the left side of the heart. The clot then has a direct path to the brain.

The Role of the Valsalva Manoeuvre

It is a rare event, but divers perform one manoeuvre known as a Valsalva. With freedivers, we perform this multiple times as we descend and often dive multiple times a day.

When taking this into consideration, every time we do a Valsalva our risk of stroke or injury increases.

A Valsalva manoeuvre is a technique which divers use to equalize their ear canals as they descend into deeper water.

When a Valsalva is performed it also causes a change in pressure in other areas of the body — such as the heart.

If a person has a PFO, the shunting direction will change briefly from left to right, to a right to left shunt. This right to left shunt is where deoxygenated blood from around the body can move to the left side of the heart, entering the arterial circulation and flowing to the brain.

Increased Risk with Clotting Conditions

If a person has a history of deep vein thrombosis (DVT) or conditions that increase their venous clot risk, then having a PFO while freediving does increase their chance of having a stroke.

Other Injuries Linked to PFO and Diving

There are also other injuries that can occur from having a PFO while diving.

As the lungs create a barrier between the venous and arterial circulation, having a PFO causes blood to bypass the lungs and enter the arterial circulation directly.

When divers ascend, nitrogen gas comes out of the solution, forming bubbles in the blood. These bubbles can pass through the PFO, entering the arterial circulation and increasing the risk of decompression sickness.

How to Find Out if You Have a PFO

To find out if you have a PFO, a person can get an echocardiogram. This is a test using ultrasound that assesses the structure of the heart.

In addition, a bubble study test can be added to the ultrasound to definitively rule out the presence of a PFO.

The test involves a solution of saline, air, and the patient’s blood, which is injected into their vein in their arm. At the same time on ultrasound, bubbles can be visualised in the right heart chambers.

If the bubbles appear in the left chambers within 4 heartbeats, there is a PFO. However, sometimes a Valsalva needs to be performed to change the pressures and unmask a PFO.

Summary: Know Your Risk

In summary, if a person has an increased risk of having venous clots and has a PFO, their risk of a stroke while diving is increased.

Patent foramen ovales can be closed via a nonsurgical procedure. Speaking to a cardiologist is recommended for further information.

Ready to learn how to freedive safely? Join a course with The Pressure Project and transform your mindset, breath, and body—one dive at a time.