The foramen ovale is an opening that exists between the right and left atria, the two upper chambers of the heart. During the fetal period, this communication is necessary for blood to bypass the circulation of the lungs (since there is no air in the lungs at this time) and go directly to the rest of the body. Within the first few days of life, this opening seals over, ending the link between these heart chambers. In approximately 25-30 percent of individuals, this communication persists as a small opening, called a patent foramen ovale (PFO).
A PFO may be very small, physiologically insignificant, or it may be larger and occasionally a route for the bypass or shunting of blood. Usually, because the pressure in the left atrium exceeds that in the right atrium, no blood crosses the PFO (when patent, or open, there is still a flap of tissue in the left atrium that overlies the opening of the PFO).
Fitness and Diving Issue
As in the case of atrial and ventricular septal defects, under certain circumstances, a PFO can result in shunting of blood from the right side of the heart to the left side. This is much more likely to occur in the atria than the ventricles because of the pressure differences between the chambers. Innocuous bubbles that may develop in the venous side of the circulation after a dive may be shunted to the left side of the heart and then distributed through the arteries. The result is that a paradoxical gas embolism or severe decompression sickness can result from a seemingly innocent dive profile.
Studies of divers with severe decompression sickness have shown a rate of patent foramen ovale higher than that observed in the general population. Special Doppler bubble contrast studies can identify a PFO. The diver with a known PFO should know the potential increased risk of decompression illness. A diver with a PFO who has suffered an embolism or serious decompression sickness after a low-risk dive profile should likely refrain from future diving.
At present, most diving physicians agree that the risk of a problem associated with a PFO is not significant enough to warrant widespread screening of all divers. An episode of severe decompression illness that is not explained by the dive profile should initiate an evaluation for the existence of a PFO.
For more information on cardiovascular conditions, see the complete article by Dr. James L. Caruso on Cardiovascular Fitness and Diving from the July/August 1999 issue of Alert Diver.