A brief look at the subject of PFO’s (Patent Foramen Ovale) and the potential implications to the diver. Also covers which types of decompression illness are more likely for divers with significant PFO’s. Options for divers who are diagnosed with a PFO.
What is a PFO?
A PFO, or Patent Foramen Ovale, is a term used to describe an opening that is present between the two upper chambers (atria) in the heart as can be seen above. Other heart conditions that can have similar consequences in divers are Atrial Septal Defects (ASD’s).
During the time that a baby is developing in the mother’s womb, the umbilical cord supplies oxygenated blood to the baby and this is connected at the point of the PFO.
After birth this opening, or flap, usually heals over, sealing the gap between the two atria and separating oxygenated blood from deoxygenated blood.
Figures on how common it is as a condition vary from 1 in 4 to around 1 in 3, quite a significant number. Around 99% of people with a PFO that has not healed have no indication at all of their condition and diving medicals would not pick one up as a specialised test is needed for that.
Some evidence suggests that people who regularly suffer from migraines with visual aura, or those who have a familial background of stroke, may be at greater risk of having a PFO.
The hole, or flap, can vary from insignificantly small at less than a millimetre up to larger openings that are more likely to cause problems at around 25 millimetres.
Of those divers who have PFO’s, not all of them will be at increased risk of decompression illness at all and as a population there seems to be little evidence that screening is cost-effective or necessary. Even commercial divers are not routinely scanned for the presence of a PFO as the risk factor is relatively low.
Why is it relevant to divers?
As we know, when diving we are breathing compressed gases at depth and the unused or inert portion of gas breathed saturates into tissues – how much depending on factors such as depth, times at depth, number of dives etc.
Decompression tables are designed to stage the ascent in such a way that this saturated gas safely passes from tissues back into the bloodstream at which point the lungs generally perform very efficiently at filtering the inert gas from the body.
In divers with a significant PFO there can be an increased risk of inert gas shunting. This describes the movement of some inert gas bubbles from the right side of the upper heart to the left (right to left shunt between atria).
When this occurs an amount of inert gas is circulated around the body again with the net effect that the lungs are not able to filter the gas bubbles as effectively as they would in a diver without a PFO – hence the potential for an increased risk of decompression illness.
Certain types of decompression illness seem to be more likely with a PFO – vestibular or inner ear, skin bends, spinal or cerebral decompression sickness from a dive within tables.
As a medical facility we will sometimes refer a diver on for a PFO scan in cases where a PFO is thought to be a potential contributory factor in their decompression sickness.
In summary, several conditions need to be met to cause the diver a problem:
- A PFO or flap is present that opens and allows gas bubbles to shunt
- The dive is deep and long enough to require decompression stops to remove gas bubbles in venous blood
- Gas bubble shunting is provoked by straining or valsalva manoeuvres
- Shunted gas bubbles that enter the arterial circulation then enter a tissue already supersaturated with gas, therefore increasing the size of exisiting bubbles to the point that decompression sickness symptoms are evident.
PFO’s are detected by a cardiologist performing a specialised test, commonly known as a bubble test:
If a PFO is detected the cardiologist will talk through the options. Giving up diving is one, reducing the gas load of dives by diving shallower for less time and less frequently may be another.
Closure of a PFO can also be a possibility and many divers we have known have had this operation and continued diving for the rest of their diving career with no further problems.