Patent foramen ovale (PFO) is a relatively benign cardiac defect that creates a passage in a wall that normally separates the left and right upper chambers of the heart. PFO is found in about 25 percent of adults, most of whom will never know they have it. The passage may open in certain circumstances and enable venous blood to pass through to the arterial side of the heart; this is a unique concern for divers. If gas bubbles form after a dive, the venous bloodstream will carry them to the right side of the heart. In divers with PFO, the bubbles could pass through the opening, bypassing the lungs and theoretically putting the diver at an increased risk of decompression sickness (DCS).
A number of retrospective studies have established that the incidence of PFO is two to six times greater in divers who experience a neurological DCS hit; however, these studies do not prove PFO is the cause of DCS symptoms. The only prospective study designed to measure how PFO affects the risk of DCS is ongoing. A similar association without causative relation was established for PFO and migraines as well as PFO and coronary heart disease. These conditions may be inherited in a similar way and thus may appear in the same people, but they do not necessarily cause each other. Many divers who get DCS do not have a PFO, and many divers with a PFO do not get DCS. Even if PFO is a risk factor for DCS, the risk is very small due to the low incidence of DCS. The risk can be additionally reduced by avoiding dive profiles that are likely to generate a lot of bubbles.
It is reasonable to expect that closing the PFO would reduce the risk of DCS; however, there are no consensus criteria for PFO closure in divers. Divers have been undergoing transcatheter closures for years, but an evidence-based risk-benefit analysis is not available, and PFO closure as a means to prevent DCS remains controversial. Most diagnosticians recommend the procedure only in cases of repeated “undeserved” DCS — or DCS occurring without clear causative factors — involving the skin, brain or inner ear. To shed light on this poorly understood topic, we asked recognized experts to provide their opinions regarding three questions of interest for divers concerned with PFO testing and PFO closure.
Should divers be tested for PFO?
Dr. Alfred Bove: There is no indication for a diver to have a routine screen for a PFO. A PFO is present in 25 to 30 percent of all people. DCS is caused by excess supersaturation of inert gas with subsequent bubble formation in blood and in tissues, and not by a PFO. Bubbles in the venous blood may cross a PFO, but there are many cases of DCS in divers without a PFO, so other factors need to be considered. Several studies have shown that a PFO will increase the risk of DCS; however, the DCS risk remains miniscule even with a PFO. The most common neurological manifestation of DCS is spinal cord injury, and the correlation of this disorder with a PFO is weak, as is the relation to musculoskeletal DCS.
Dr. Peter Germonpré: No. PFO is so common, and decompression sickness is so rare that the systematic testing for PFO would cause more concern than it would do any benefit. There are other shunts of blood possible (e.g., at the level of the lungs), and there are other factors that may promote DCS. All divers need to be aware that any decompression bubbles, when present, may provoke DCS, and they should dive so as to minimize those bubbles.
Dr. Richard Moon: It is important to keep in mind several issues. First, the relationship between PFO and DCS is an association. While there is a plausible explanation for the apparent connection (i.e., DCS triggered by arterial bubbles), this is not yet proven. Indeed, PFO has not been linked to the most common symptoms such as pain-only bends, numbness, tingling or fatigue. Second, PFO exists in 25 to 30 percent of people, and studies have shown that venous bubbles are extremely common after recreational dives. Therefore, many divers must be experiencing arterial bubbles, yet DCS is extremely rare (especially the more serious variety that is associated with PFO). Third, if the bubble explanation is correct, a PFO could precipitate DCS only after a dive of sufficient depth and duration to generate venous gas embolism. PFO is therefore unlikely to be a factor for mild DCS cases or ones that occur after short exposures or shallow depths (i.e., “undeserved”). Finally, most cases of DCS occur in divers without a PFO. It is difficult or impossible in an individual diver to ascribe a particular DCS occurrence to a PFO, as PFO will exist in nearly one-third of cases even if there is no causal relationship. Therefore, testing for a PFO is useful only in instances where there have been several DCS incidents of a type known to be PFO-associated, and the person cannot modify depth-time exposures or breathing gas.
What is an acceptable indication for PFO closure in divers?
Germonpré: If a diver has suffered DCS, a systematic search for possible contributing factors should be undertaken. This involves analyzing the dive and circumstances around it and also medical factors such as PFO or pulmonary abnormalities. If any of these factors are found, a suitable preventive strategy may be advised. Current statistics indicate that the possible risks of PFO closure are much larger than the risk for DCS when diving within no-decompression limits. In a few years we may have precise figures when the prospective Carotid Doppler Study from DAN Europe is concluded. There are measures that can be taken to decrease further the DCS risk in recreational diving such as diving nitrox on air tables. Therefore, we recommend PFO closure only in specific cases:
- professional divers, who are required to dive with imposed dive profiles and cannot adapt their diving behavior
- sport divers where no other contributing factors for DCS could be identified and who [experienced DCS after] a “low-risk” dive profile
In any case, the decision to close a PFO must be taken only after thorough information review, analysis of the risks and explicit consent.
Moon: A commercial diver (e.g., offshore worker, dive instructor) who has experienced several DCS incidents of a type known to be associated with PFO (e.g., neurological), and who is not in a position to increase the oxygen percentage in his/her breathing gas or modify depth-time exposures [might consider PFO closure].
Bove: I would not recommend PFO closure in any sport diver, whether or not the diver had a DCS incident. The noted incidence of DCS in recreational divers is between two and five cases per 10,000 dives (0.02 percent to 0.05 percent). The serious complication rate for PFO closure, as noted in Dr. Douglas Ebersole’s comments, is 3-4 percent, more than 100 times the risk of experiencing a DCS event. It is clear that the risk-benefit ratio for PFO closure in divers strongly favors not closing the PFO. Further, the presence of a residual PFO after closure is significant, up to 30 percent in some studies.
If divers consider PFO closure, what risks and benefits do they have to take into account?
Moon: The benefit cannot be absolutely predicted, as in a particular diver it is not possible to ascribe DCS to a PFO with certainty. PFO is only one of probably several individual factors that may confer an increased risk of DCS, although most such factors have yet to be discovered. No invasive medical procedure is entirely without risk, and PFO closure is no exception. Even though the risk is low, a diver must balance it against an uncertain benefit. Other methods, such as reducing depth or bottom time, or using enriched oxygen mixtures, are safer.
Bove: Most DCS cases with or without a PFO are related to excess exposure to depth and time. Diving within the no-decompression limits will have a much greater effect on reducing the incidence of DCS than will closure of a PFO. The risks of closure are well explained in Dr. Ebersole’s comments. There is little knowledge of the long-term effects of implanting the device. Some cardiologists feel that the presence of a PFO occluder would prevent the use of a very effective method for treating atrial fibrillation (catheter ablation). There have been several case reports of commercial divers who experienced repeated incidents of DCS following deep saturation dives, and the frequency of symptoms was reduced after PFO closure. So there are likely to be occasional cases where PFO closure would be warranted in a diver, but this would not apply to recreational divers.
Germonpré: After PFO closure, one of the possible shunts from right to left has been eliminated. There is, however, no guarantee that closure is successful, and even if no shunt remains, some divers keep having recurrences of DCS, probably because other shunts open. In our experience, minor complications (cardiac rhythm disturbances, migraine attacks, bleeding) are not so rare. The long-term outcome of these devices has not been defined; most of the current devices have been on the market only a number of years. The presence of a “foreign body” inside the heart wall may, with age, cause other and more serious cardiac troubles. Finally, it is rarely possible to pinpoint one single cause for DCS; remedying one single factor does not “immunize” a diver from DCS.
Meet Our Panel
Alfred A. Bove, M.D., Ph.D., is the former chief of cardiology at Temple University Medical Center in Philadelphia, Pa., emeritus professor of medicine and the immediate past president of the American College of Cardiology. He has authored texts on coronary disease, exercise medicine and dive medicine. He maintains an active clinic cardiology practice, with emphasis on heart failure and cardiac transplantation, sports cardiology and undersea and hyperbaric medicine.
Peter Germonpré, M.D., is the medical director of the Centre for Hyperbaric Medicine of the Military Hospital in Brussels, Belgium. He is also the DAN Europe (Benelux) medical director and senior researcher of the DAN Europe Research department, where he coordinates the DAN Europe PFO studies.
Richard Moon, M.D., has been associated with DAN since its formation, first as medical director and then as senior medical consultant. He is a professor in the departments of anesthesiology and medicine at Duke University Medical Center in Durham, N.C., and medical director of the Duke Center for Hyperbaric Medicine and Environmental Physiology. In 1989 he published the first analysis demonstrating a link between serious decompression illness and PFO.
PFO: Your Questions Answered
Q: What is a PFO?
A patent foramen ovale (PFO) is an incompletely sealed wall between the right and the left atria (see illustration). Prior to birth, this wall develops from the septum primum, which grows upward, and septum secundum, which grows downward. These septa overlap, providing a “trap door” (the foramen ovale), which allows oxygenated blood from the mother’s placenta entering the right atrium to pass to the left atrium. At birth, the lungs expand and the pressure in the left atrium “slams shut” the foramen ovale. Shortly after birth, the “door” fuses in most people, but in about 27 percent of people it fails to fuse completely and results in a PFO.
The prevalence of PFO in divers with DCS is two times greater than in the rest of the population. In divers with neurological DCS, it is four times greater. The risk of DCS seems to increase with the size of the PFO. Based on these statistics, it is assumed that divers with PFO are at greater relative risk of DCS than divers without PFO; however, the only prospective designed to measure directly the relative risk for DCS in divers with PFO is ongoing.
Q: How is a PFO diagnosed?
The diagnosis is made by injecting a small amount of air into a vein and observing its passage through the heart by echocardiography. An echocardiogram is performed by placing the ultrasound probe on the chest. Transthoracic echocardiography (TTE) is easy and noninvasive, but it detects a PFO in only about 10-18 percent of the population, around half of those who may have it. Transesophageal echocardiography (TEE), where the probe is passed into the esophagus after local anesthesia and intravenous sedation, detects PFOs in 18 to 33 percent of the population. Even if TEE is more sensitive than TTE, there are many “false-negative” results with both techniques, and it is important to realize that a well-conducted TTE with a proper technique may be more reliable than a TEE.
Q: What is transcatheter PFO closure?
Transcatheter PFO closure is the implantation of an occluder device across the PFO through a catheter inserted via the femoral vein in the groin. The occluders come in various shapes and forms, but most act like a double umbrella that opens on each side of the atrial wall and seals it. With time, tissue grows over and completely covers the occluder surface. The implantation procedure is performed in patients under local anesthesia and intravenous sedation while the patient remains conscious. It takes less than one hour and can be performed on an outpatient or one-night-stay basis. Most patients can return to normal activity in two days, but they must take anti-coagulation medication and blood thinners for three to six months. Other restrictions include: no elective dental care (such as cleanings) for three months, no contact sports for three months, and no heavy lifting for one week. After implantation, divers must abstain from the sport for three to six months.
Q: What are the outcomes of transcatheter PFO closure?
Note: Due to the lack of data in divers, we present outcome data in patients undergoing PFO closure for the prevention of stroke. These patients have underlying medical conditions that may contribute to adverse outcomes.
- Complete closure of shunt: 95 percent
- Incomplete closure: 4 to 5 percent
- No improvement of shunt: 1 percent
- Overall mortality: 0.093 percent
- Rescue operation for device adverse events: 0.83 percent
- death, stroke, infection, bleeding, blood vessel injury: 0.2 percent
- device movement or dislodgement: 0.25 percent
- clot forming on device: 0.3 percent
- major periprocedural complications: 1.2 percent.
- minor midterm complications: 2.4 percent
Researching PFO: Divers Wanted
In partnership with Dr. Douglas Ebersole, an interventional cardiologist at the Watson Clinic in Lakeland, Fla., and the director of the Cardiac Catheterization Laboratories at Lakeland Regional Medical Center, DAN recently began the first-ever prospective study of PFO in divers. Divers who already tested positive for PFO and continue diving with or without closure may participate. They will be asked to record their dives and report outcomes for the next five years. At the end of the study it will be established if there was any difference in the incidence of DCS between the divers with and without PFO.
© Alert Diver — Q2 Spring 2010