atrium). From the left atrium blood moves in to the left ventricle and is pumped from the heart to all parts of the body. An atrial septal defect (ASD) exists when there is a hole in the septum
that divides the left and the right atrium. The pressure in the left atrium is normally higher than the pressure in the right atrium and blood moves from the left side to the right side. This is not really a problem. Blood that has already been through the lungs is simply pumped through the lungs again. However, for a small part of every cardiac cycle the pressures are
reversed and blood moves from the right side to the left side. This blood bypasses the lungs and is pumped to the body. As it is high in carbon dioxide and low in oxygen, it dilutes the blood coming from the lungs and reduces the amount of oxygen in the blood going to the tissues. This is a small effect however and has little practical significance. When blood is moving from the right to left side of the heart, bubbles
and/or clots can be carried through the ASD and pumped to the tissues. The first branches off the aorta after it leaves the heart are the coronary arteries that supply blood to the heart muscle. However, the openings to these arteries are located behind the leaflets of the aortic valve and clots or bubbles in the blood are unlikely to enter them. The next branches are the carotid arteries. They supply 80 percent of the blood to the brain and it is highly likely that any clots or bubbles in the blood will enter them and be pumped to the brain. The presence of an ASD has always been and remains a contraindication to diving. The hole can be closed by insertion of device through the femoral vein in the leg on a long wire. After the closure device has healed over with tissue it should be safe for the person to dive. As I explained in detail in the 1999 article, an ASD exists in all babies
before they are born as a requirement for the baby to survive. At birth, when the baby takes its first breath, a flap of tissue drops over the ASD blocking it. If the flap fails to completely cover the ASD, an ASD will continue to exist. When the flap first covers the ASD it is loose and with every heart beat
when the pressures reverse, a small amount of blood will leak past the flap from the right to the left atrium. Over time the flap becomes permanently attached to the intra-atrial septum and the hole is completely sealed. However, in many people the flap does not completely seal to the septum leaving a flap valve. This is called a patent foramen ovale or PFO. The size of the opening in a PFO varies from greater than 0.4 of an inch
(1 cm) to minuscule. The frequency of PFOs depends on several factors. They are more common in children as the flap continues to seal down as we age. Small PFOs are much more common than large PFOs. In very general terms, about 10 percent of adults will have a large PFO, 30 percent will have a PFO through which a few bubbles can be shown to pass and up to 80 percent will have a PFO that can pass a wire at autopsy but through which no blood would have passed during life. Using transthoracic echo, about 30 percent of divers can be shown
to have a PFO but only about 10 percent of divers will have a PFO large enough that we are concerned.
Controversy So, what does this mean to divers? This is the controversy. Many studies have shown that divers bubble after diving. Statistically,
many divers should have a PFO and therefore many divers should have bubbles going through a PFO to the brain. However, very few divers develop serious DCS or DCS with symptoms that most likely come from the brain. How is this possible? If you look closely at the data, many divers bubble after diving, but most of
these divers generate only a few bubbles. Certainly after recreational dives, very few divers generate large numbers of bubbles. After dives requiring a lot of decompression divers can generate very large numbers of bubbles. Large PFOs are much more common in divers who have suffered serious
DCS than the general population. Virtually all cases of DCS will be associated with large numbers of bubbles (my interpretation of the data used in my Master of Science theses).
The consensus interpretation of the relatively poor
data in the literature reached at the meeting I attended was that the relative risk of having a large PFO was in the range of 25. This means that the risk of a diver developing severe DCS was 25 times greater if they had a large PFO compared to not having a PFO. The majority of diving medical consultants agree that
it does not matter if a recreational diver has a PFO or not. They should not be screened. Recreational dives are unlikely to generate large numbers of bubbles. The risk of DCS after recreational dives is extremely small. If you multiply an extremely small risk by 25 you still end up with a very small risk. At the same time, if a recreational diver knows
that they have a PFO (research subject, screen for migraines, etc.), especially a large one, I would strongly recommend that they dive conservatively to minimize their risk of developing bubbles after diving. On the other end of the
conservatively to minimize the risk of developing bubbles after diving
If a recreational diver has a PFO I strongly recommend he/she dives
spectrum, many diving medical consultants would agree that divers who routinely conduct dives with a high risk of developing large numbers of bubbles should be screened for a PFO. This would include anyone conducting dives that require significant amounts of
decompression (my personal opinion would be more than 30-60 minutes of deco).
a PFO (especially a large PFO), I personally would recommend that they get if fixed or stop doing this kind of diving. The in-between group (scientific, military, and most
commercial) is contentious. I would base my advice on how much decompression they were doing as per above. Canadian Forces Clearance Divers do very few dives requiring extensive decompression and on that basis the group at the meeting recommended that we continue screening for PFOs but not disqualify anyone. Over the next several years we will see if those with large PFOs develop more serious DCS. The last group is of those divers who develop
serious DCS or repeated cases of DCS. I personally recommend that they be screened for a PFO and if found to have one, get it repaired or stop diving. I know several divers who stopped getting DCS (and one who stopped getting migraines) after their PFO was repaired. I personally do not have a PFO, but I have been bent.
www.divermag.com 57 If they are found to have
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