What is it, and what are its implications for scuba divers?
By Richard E. Moon, M.D., DAN Medical Director, & John Rorem, DAN Communications
As scuba divers, we are aware of the pressures our bodies are subjected to at depth. We know the dangers of decompression illness (DCI) - that's why we strive to dive conservatively, abiding by the limits set by dive tables and dive computers. And that's why we read, with great interest, accounts of divers and DCI. Who gets bent and why?
We know the lungs and all body tissues are involved in DCI, but what other factors are there to consider? In answering this, researchers have found that the heart is one important place to start.
Recently, there has been interest in the anatomic variant of normal in the human heart: It is termed patent foramen ovale (PFO - pronounced PAY-tent fore-A-men O-val-eh), and through research, DAN is exploring its possible importance for scuba divers.
What is patent foramen ovale? The foramen ovale is an opening, or communication, between the right atrium and left atrium in the heart. (Foramen is Latin for opening, or aperture; ovale is, appropriately, Latin for oval, indicating the shape of the aperture.) The normal passage of blood in the adult human is from the great veins of the body, through the right atrium into the right ventricle, and then via the pulmonary artery to the lungs. Blood then returns via the pulmonary veins into the left atrium. It is then pumped into the left ventricle and via the arteries to the various tissues of the body.
In the developing fetus, however, the lungs are not functional. Blood, therefore, bypasses the lungs predominantly through the foramen ovale, directly from the right atrium to the left atrium. After birth, the foramen ovale closes, allowing blood to be pumped through the lungs for oxygenation.
The foramen ovale is initially closed by a "flap valve," similar to a spring-loaded door. The valve remains closed because the pressure in the left atrium is slightly higher than the pressure in the right atrium. In most people, the flap valve actually seals over, and the foramen ovale completely disappears. In a small percentage of individuals, however, there is incomplete sealing of the valve.
This incomplete seal is what is termed patent foramen ovale, although it remains closed because of the pressure differential between the two atria. (Patent is Latin for open, indicating the incompleteness of the seal.) In rare instances, the foramen ovale remains completely open - this condition is more pronounced and is called atrial septal defect.
Why should the presence of patent foramen ovale be important? It isn't to a nondiver. People with patent foramen ovale are completely unaware of it and will go through life completely free of any symptoms related to it. For a diver, it could theoretically be important. The reason is that some divers, depending on depth and duration of their dives, produce bubbles in their venous blood during and after decompression (ascent from the dive). These bubbles are small, usually 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.
Recent evidence suggests that after decompression, many normal scuba dives, with no symptoms of decompression sickness, may produce bubbles in the venous blood. Why is this important for a diver with patent foramen ovale? It has been found that some people with patent foramen ovale may actually pump leak quantities of blood from the right atrium to the left atrium. This suggests that divers who have patent foramen ovale who also have venous gas bubbles during decompression may pass gas bubbles directly into the left atrium, bypassing the lungs. Gas bubbles in the left atrium are then carried to the body tissues, where they could cause symptoms.
Do we know if this actually happens? We have no direct evidence of this yet. What we do have is some preliminary data suggesting that patent foramen ovale may result in the bends in some individuals. The evidence is this: We have made use of a diagnostic test which can provide a very accurate image of the beating heart (two-dimensional echocardiography). Using this technique, the flow of blood within the heart can be demonstrated by injecting a tiny quantity of microscopic bubbles suspended in saline solution into a vein. These tiny bubbles are then carried to the heart and can be visualized on the echocardiographic image.
We have examined with a two-dimensional echocardiogram 91 patients evaluated and/or treated for decompression sickness at Duke University Medical Center. Of these 91, 39 had PFO. Sixty-four of the 91 patients had more serious symptoms (weakness, dizziness or symptoms of brain abnormalities) and 32 of these 64 had patent foramen ovale (50 percent). This percentage is higher than one would expect in a normal population (10 to 20 percent).
Does this mean that the presence of patent foramen ovale may cause decompression illness? No, it does not. The data we have are merely suggestive. The number of patients we have checked is too small to draw any firm conclusions. One issue which needs explanation is that whereas 10 to 20 percent of the normal population may have patent foramen ovale, less than 0.1 percent of divers get the bends. In order to form firm conclusions, we must examine many more patients.
Also, the normal populations with which we are comparing our data have been obtained from other laboratories. In order to be absolutely certain of the relationship between patent foramen ovale and the bends, we must investigate many more patients with decompression sickness and also a group of normal volunteers in the same age range as our divers.
What should be done in the meantime? Nothing yet. Diving is a relatively safe sport. We recommend safe diving practices to make the sport even safer. If specific recommendations need to be made about testing divers for patent foramen ovale, we will make this known to you as soon as possible.
(c) March/April 1995 Alert Diver
DAN Gets to the Heart of Things
In the pursuit of more data on diving and PFO, two studies on seven men and four woman were recently conducted at the F.G. Hall Hyperbaric Center and Echocardiography Laboratory of Duke University Medical Center. These studies looked at the effect of both exercise and immersion in water upon right-to-left shunting (the passage of blood from the right side of the heart into the left) through a patent foramen ovale.
The conclusion was that while immersion in water may cause the heart to enlarge, neither immersion nor exercise affected the right-to-left shunting through a patent foramen ovale. DAN will continue to expand its database in its efforts to better understand the relationship between PFO and diving.