By Dr. Alfred A. Bové and Dr. Richard E. Moon
In the unborn fetus, the lungs are not functioning, and to provide oxygen to the fetal circulation, an opening is present (the foramen ovale) in the wall between the two atria (the atrial septum). The foramen ovale allows blood coming from the placenta to flow through the right atrium directly into the left atrium and then into the body, providing the oxygen needed for growth and development.
At birth, when the baby begins to breathe, a flap valve closes over the opening in the atrial septum and eventually becomes part of the atrial septum from growth of tissue sealing the opening. However, in about 30 percent of all people, the closure of the foramen ovale is incomplete, and the persistent foramen ovale (patent foramen ovale, or PFO; see figure in PDF attachment below) becomes a potential route for blood from the veins to flow directly into the arterial circulation, thus allowing bubbles to enter the arteries and bypass the lung filter.
In a small number of people, the flap valve is missing completely, and a persistent opening occurs in the atrial septum (atrial septal defect, or ASD). Concern for bubbles reaching the brain by this pathway seem to be validated by several reports in the medical literature of unexpected or unexplained decompression sickness (DCS), particularly involving the brain in divers who were subsequently found to have a PFO. A medical report in 1986 suggested that neurological DCS after a 15-minute dive to 125 feet (38 meters) in a recreational scuba diver with an ASD was caused by venous gas emboli (VGE) passing through an undiagnosed ASD.
Divers have enthusiastically latched onto this concept. During fitness-to-dive evaluations, questions about PFO are always near the top of divers' lists. Also, DAN is bombarded with PFO queries.
With the development of devices that can be implanted without surgery to close the PFO called occluders, this interest has gained momentum. In Europe, these devices are commercially available and approved for closure of ASD and PFO. In the United States, these occluders are still undergoing experimental study. They are soon to be approved by the U.S. Food and Drug Administration (FDA) for treatment of certain forms of stroke caused by blood clots, which are thought to come from the veins, then flow to the right atrium and cross a PFO.
To reduce the risk of DCI, some professional divers have received the occluder devices. During a follow-up period of three to 12 months, these divers experienced no further neurological decompression episodes. However, it is difficult to be sure that this represents a true reduction in risk. More information is needed about their diving patterns before and after the procedure.
While 20-30 percent of divers might be expected to have a PFO, decompression illness (DCI) in recreational divers occurs after only 0.005-0.08 percent of dives, clearly much lower than the one in five or six that might be expected if every diver with a PFO and venous bubbles developed DCI. Based on current experience, the estimated risk of a DCI incident characteristic of those correlated with PFO is between 0.002-0.03 percent of dives.
For DCI to occur, there must therefore be other factors, such as bubble load or some other susceptibility factor as yet unknown, possibly involving body tissues. It is also conceivable that a PFO represents a marker for susceptibility (such as red hair and sunburn) but is not involved directly in DCI.
Clinical observations to date have focused on the correlation of PFO with neurological injuries, particularly serious ones, but these represent only a third of DCI incidents. Most DCI cases in recreational diving consist of pain or sensory abnormalities, and no one has yet shown that PFO is related to these cases. The exception is skin bends, but this is uncommon. Only around one third of cases of DCI in recreational divers are considered severe. If 60 percent of these have a PFO, and 25 percent of the remainder have one, then it can be estimated that most bends cases must occur in divers without a PFO.
Associating a common finding (PFO) with an uncommon disease (DCI) is a common mistake, and this often mistaken relationship is likely to be involved in the data regarding DCI and PFO.
More recent articles in medical journals have continued to report complications such as device malposition, device embolization, changes in heart rhythm, perforation of the heart, vein damage, bleeding and both right and left atrial blood clot formation. Late complications have included sudden death.
One of the purposes for which these occluder devices will be marketed is to prevent blood clots from passing through a PFO and into the left side of the heart, from which they could travel to the brain and cause a stroke. However, even for prevention of recurrent blood clots, the effectiveness of transcatheter devices has not been proven. Regarding closure of a PFO to prevent DCI, scientific evidence does not exist at present.
Dr. Alfred Bové is Emeritus Professor of Medicine, Section of Cardiology, Temple University School of Medicine, Philadelphia, Pennsylvania. Dr. Bove is a retured US Navy diving medical officer. He is the author of numerous articles and book chapters about diving medicine and physiology. He is the editor of the textbook Diving Medicine, 4th edition published in 2004. Dr. Bove is an international expert on cardiovascular disorders and diving. He consults for the US Navy, the National Academy of Sciences, the National Oceanic and Atmospheric Administration, and DAN.
(c) DAN - Alert Diver September / October 2004