Definition: PFO is a congenital heart defect resulting from failure of the foramen ovale to close shortly after birth – Dictionary.com

To a non-diver a PFO (Patent Foramen Ovale) is not important, most people who have it are unaware of it and will go through their entire life without a problem. For a diver, however, theoretically, it could be a problem. Some divers (depending on the depth and the duration of the dives) produce bubbles in their venous blood during and after decompression. These bubbles are relatively few in number and do not give rise to any symptoms since they are trapped by the small blood vessels in the lungs and filtered out of the circulation. But recent evidence shows that after decompression many normal divers, with no symptoms of decompression sickness, may produce bubbles in venous blood. Why is this relevant to a diver with a PFO? Some people with PFO may actually pump quantities of their blood from the right atrium to the left through the hole. This suggests that divers who have PFO who also have venous gas bubbles during decompression may pass gas bubbles directly into the left atrium, bypassing the lungs meaning the small bubbles are not filtered out of the circulation. Gas bubbles are then carried from the left atrium to the body tissues where they could cause symptoms of a DCI.

Diagram showing a PFO and blood flow direction from the right atrium through the interatrial wall and into the left atrium 

Diagram showing a PFO and blood flow direction from the right atrium through the interatrial wall and into the left atrium

As a foetus every person has a connection in their heart which runs between the left and right atriums (the two chambers of the heart). This passage allows the blood to pass through the foetus’ heart without passing through the lungs. Since all oxygen comes into the foetus’ body from the placenta (not from breathing air) the foetus has no need to use its lungs until it is born. As the child grows up the hole in the heart usually seals closed with a small flap of tissue, preventing the blood from passing between the right and left chambers directly. Patent Foramen Ovale (PFO) results from the incomplete closure of the hole. PFO is relatively common. Studies indicate that as many as 30% of adults have a PFO.

Most adults who have a PFO are unaware of it as it usually has no effects on daily life. PFOs range in size, from a tiny opening to an opening of 10mm or more. Individuals who have a PFO have reported symptoms such as a shortness of breath, or faintness during strenuous exercise which may be indicative of a PFO and are sometimes misidentified as simply being ‘out of shape’. In some individuals the hole between the two sides of the heart does not close at all, this is called an Arterial Septal Defect (ASD). In a diver without a PFO, blood flows from the veins into the right side of the diver’s heart. The right side of the diver’s heart then pumps this venous blood into the lungs where it picks up oxygen. From the lungs the blood enters the left side of the diver’s heart where it is pumped back into the arteries. The blood flow in a person without a PFO is as follows:


In a person with a PFO, normal blood flow may be disrupted. Some small amount of blood may ‘leak’ through the PFO. In a person with a PFO some blood might flow in the following way: Notice how the blood has skipped the lungs completely.


Why might a PFO increase a diver’s chance of DCS?

One of the many causes of decompression illness is arterial gas embolism. An arterial gas embolism occurs when a tiny bubble of nitrogen enters an artery, causing cell death, numbness, pain and a variety of other symptoms. Divers commonly have microscopic nitrogen bubbles in their veins (not arteries) after diving. A DAN (Diver’s Alert Network) study found that 91% of divers had venous nitrogen bubbles after a series of repetitive dives well within the recreational dive tables. These venous nitrogen bubbles are normally harmless because venous blood is eliminated by the routine gas exchange. The presence of a PFO can disrupt the normal flow of blood through the heart. Venous blood with tiny, normally harmless, bubbles can leak directly from the right side of a diver’s heart to the left side, where it is pumped back into arterial circulation without passing through the lungs. If nitrogen bubbles enter the arteries, there is now a risk of an Arterial Gas Embolism (AGE). A PFO may allow nitrogen to pass from veins, where they are harmless, to arteries, where they are potentially dangerous.

DAN currently say a PFO is not considered as inadvisable for recreational diving, but this may change in the future. DCI is relatively rare, even for divers with a PFO. However, research suggests that a PFO may increase the small chance of a diver getting DCI by as much as 4-5%. Some research may also indicate that the larger the PFO hole, the greater the risk of DCI. PFO however can be inadvisable for technical diving.

Concerned divers should speak to a cardiologist knowledgeable in diving medicine. Many divers continue to dive with a PFO and do not get decompression illness. Decompression is caused by nitrogen bubbles not by the PFO itself, for divers who choose to continue to dive with a PFO, doctors have given a list of suggestions to minimise the formation of venous nitrogen bubbles and reduce the risk of decompression illness. These are:

  • No decompression diving
  • Adopt conservative dive profiles
  • No exercise after diving
  • No Valsalva manoeuvres (equalising by pinching the nose and blowing through the nose) on ascent
  • Make extremely slow ascents and long safety stops

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