PFO and Diving - part 1
Do you have a hole in your heart and if so, does it matter?

By Dr. Sawatzky

In humans (and many other animals), the heart is really just two pumps working side by side. The left side of the heart gathers blood from the lungs and pumps it out to the body. The right side of the heart gathers blood from the body and pumps it out to the lungs. The two sides of the heart could be in two different locations in the body and, in some ways, it might be better if they were! If we follow the blood around the system, it leaves the left heart and goes to all parts of the body. The blood delivers oxygen and nutrients to the cells and collects carbon dioxide and waste products from the cells and returns to the right heart where it is pumps out to the lungs. In the lungs, the blood releases the carbon dioxide and takes up oxygen before returning to the left heart and being pumped back out to the body.

Insert picture of normal circulation

When a fetus is developing in the uterus (before we are born), the circulation is very different. The lungs are full of amniotic fluid and are not involved in the exchange of oxygen and carbon dioxide. The placenta performs that job. In addition, the fluid filled lungs have very little circulation. Therefore, the fetus must have some way for the blood that is returning to the right heart from the body to get to the left heart so that it can be pumped back out to the body. There are two ways this happens.

In the fetus, the wall dividing the left and right atria has a flap valve that allows blood to move from the right heart to the left heart. This opening is called the foramen ovale. In addition, the fetus has a connection between the pulmonary artery (delivering blood to the lungs) and the aorta (delivering blood to the body). This connection is called the ductus arteriosus. Therefore, in the fetus, the blood circulates from the left heart to the body and back to the right heart. Some of the blood moves across the foramen ovale to the left atrium and some enters the right ventricle where it is pumped out into the pulmonary artery, through the ductus arteriosus and into the aorta where it flows to the body. There is very little blood flow through the lungs and they are effectively bypassed. While the blood is circulating through the body, it also circulates through the placenta where it releases carbon dioxide and other waste products and picks up oxygen and nutrients.

Insert picture of fetal circulation

When we are born and take our first breath, several amazing things happen to the circulation (this discussion is a gross simplification of the process). The lungs fill with air and this reduces the resistance to circulation through the lungs. The pressure in the right heart falls dramatically and the right heart starts to pump blood through the lungs. The combination of a fall in the pressure in the right heart and the increased pressure in the left heart from the blood now returning from the lungs, pushes the flap valve closed, sealing the foramen ovale (right atrial pressure averages three mm Hg less than left atrial pressure). Over the next several weeks/months/years, the flap grows together to form a solid wall between the right and left atria. Unfortunately, this seldom happens perfectly. In addition, the increased partial pressure of oxygen in the blood causes the ductus arteriosus to close and eventually degenerate into a simple fibrous band.

In some individuals, the flap does not form correctly and does not completely cover the foramen ovale, leaving an open hole between the right and left atria. This is called an atrial septal defect (ASD) and is relatively rare. It is far more common for the flap to completely cover the opening but not completely seal down. This is called an open or patent foramen ovale (PFO) and varies from a flap that is not sealed at all to a flap that has largely sealed but left a tiny pathway open. In a normal individual, a PFO has absolutely no significance. The pressure in the left side of the heart is almost always higher than the pressure in the right side of the heart so the flap is usually held closed, even if it is not sealed. However, there are some circumstances where the pressure gradient can be reversed. In all hearts, there is a very brief moment during the cardiac cycle where the pressure is reversed and if the heart has a PFO, blood can move from the right side to the left side, bypassing the lungs. If the pressure in the right heart is increased, more blood will move across to the left heart. The most common situation where the pressure in the right heart is elevated is when the person does a valsalva maneuver (to clear your ears) or strains to lift or push a heavy object. The amount of blood that moves through the PFO depends upon the pressure gradient, the size of the opening and the duration of the pressure reversal. In a normal, healthy person, even if 10% of the blood bypassed the lungs, there would be no symptoms and the person would not know they had a PFO. Even with an ASD, often there are no symptoms.

How common are PFOs and how do we detect them? The gold standard is to examine the heart after the person has died and check for a PFO. Even in this situation, the findings are highly variable. Nine studies, dating from 1897 to 1984 with a total of 8,762 hearts examined, reported an incidence of PFO varying from 17 to 36%. There are several reasons for the inconsistent results. The most important is probably how small an opening they called a PFO. I clearly remember my anatomy professor in medical school telling us that if you worked at it, you could find an opening large enough to pass a small probe through in 80 to 90% of hearts (probe patent foramen ovale). The second factor seems to be the age of the hearts that were examined (PFO is more common in children). Autopsy findings are fascinating but as divers, we are far more interested in how often blood actually moves through the PFO while we are still alive.

There are several ways to look for PFOs in living hearts. The most common is to use echocardiography where sound waves are bounced off the heart to give a picture. The probe can be placed in two locations. The most common (and comfortable) is to place the probe on the chest wall (transthoracic echocardiography, TTE). The most accurate is to place the probe in the esophagus (immediately behind the heart) and this is called transesophageal echocardiography (TEE). Smaller PFOs can be detected by injecting millions of small bubbles into a peripheral vein and watching them move through the heart and/or the PFO (bubbles are an excellent reflector of ultrasound and show up clearly on the echo). Finally, even smaller PFOs can be detected if the person performs a valsalva maneuver or coughs while the bubbles are moving through the right heart to momentarily increase the pressure in the right heart. Newer echocardiographic machines have the ability to use colour doppler to detect blood flow and this will sometimes help to detect PFOs. Therefore, the sensitivity of the investigations roughly varies from transthoracic echocardiography (TTE, least sensitive), through TTE with bubble contrast, TTE with contrast and valsalva, TEE, TEE with bubble contrast, to TEE with contrast and valsalva (most sensitive). If a PFO is detected then you have a PFO. If a PFO is not detected, you either do not have a PFO or the PFO is too small to detect with the investigation used (no false positives but many false negatives for those of you with a scientific background).

Up to 50% of PFOs detected by TEE (with contrast and valsalva) are not detected with TTE (also with contrast and valsalva). Unfortunately, having the probe in the esophagus is less than pleasant and has some risks (esophagealinjury, laryngospasm, aspiration, hypoxia, bronchospasm, and heart arrhythmias). An alternative way to detect PFOs is to use transcranial doppler (TCD). The bubbles are injected in to a peripheral vein, the person performs a valsalva maneuver (TCD can detect the effective ness of the valsalva by watching for reduced cerebral blood flow) and TCD is used to detect any bubbles that get into the cerebral circulation. These bubbles either move from the right to the left heart through a shunt in the heart (most commonly a PFO) or in the lungs (some people have arterial/venous shunts in their lungs). TCD is not quite as sensitive as TEE with contrast and valsalva (60 to 100% depending on the study with most being 90-95%) but any shunt detected will have significance, and it detects lung shunts. In addition, TCD is safe, easy and relatively inexpensive.

So why do we care if there is a PFO or a shunt in the lungs of divers? When we dive, we take up inert gas in accordance with Henry's Law of solubility. If we finish the dive with a relatively small amount of additional dissolved inert gas, it slowly diffuses into the blood and exits the body through the lungs. If we have a greater inert gas load, it tends to form bubbles in the veins. These bubbles are washed back to the right heart and pumped out to the lungs. The lungs are usually excellent filters. They trap the bubbles until the gas in the bubbles diffuses into the alveoli and the bubbles disappear (about 45 minutes). In the diver who has a PFO or a shunt in the lungs however, the bubbles can move through the PFO or move through the shunt and enter the arterial circulation (bypassing the lung filter). Bubbles are then pumped out to the body and the first major arteries are the carotids, supplying the brain. If you send bubbles to the brain it is the same as suffering arterial gas embolism and that is often very serious or fatal. In spite of the theoretical seriousness of this situation, it has proven exceedingly difficult to determine its practical significance.

I am out of room in this column so we will have to continue this discussion next issue. In the meantime, dive conservatively to reduce the likelihood of your having bubbles after a dive!