Proceedings Summary | DAN/UHMS PFO and Fitness to Dive Workshop
Prior to birth, oxygenated blood flows from the mother through the placenta to the heart of the fetus via the opening in the wall separating the left and right atrium (foramen ovale) into the fetal circulation. The foramen ovale has a “trap door” feature which opens due to the pressure of blood flow from the mother’s placenta entering the right atrium, and lets the blood pass to the left atrium. At birth, the lungs expand and the pressure in the left atrium increases and “slams shut” the foramen ovale. Shortly after birth the “door” fuses together, but in about 27% of people, it fails to fuse completely and results in a patent foramen ovale also called persistent foramen ovale (PFO).
In people with PFO, if the pressure in the right atrium rises above the pressure in the left atrium, blood can flow from the right to the left atrium. The direct flow of blood from the right to the left atrium which bypasses the lungs is called right-to-left shunt (RLS). The RLS is known to let blood clots pass to the arterial side which can cause a stroke (brain trombo-embolism). Similarly, the PFO in divers may let gas bubbles from the venous blood (venous gas emboli – VGE) pass the arterial side and cause decompression sickness.
Epidemiological studies have shown an association between PFO and certain types of neurological and cutaneous decompression sickness. The DCS risk in recreational divers has been reported at 3.6 cases per 10,000 dives, with 0.84 cases of neurological DCS per 10,000 dives and 4-fold increase in risk with PFO.
The overall risk of neurological DCS is low, even in the presence of a PFO. However, for some individuals, PFO seems to be a greater risk than predicted. Guidelines for PFO testing are aimed at identifying such individuals and managing their DCS risk.
The following guidelines were developed from the joint position statement on PFO and diving published by the South Pacific Underwater Medicine Society (SPUMS) the United Kingdom Sports Diving (UKSDMC), and the DAN sponsored workshop held in conjunction with the UHMS Annual Scientific Meeting in Montreal, Canada, June 2015.
Routine screening for PFO at the time of dive medical fitness assessment (either initial or periodic) is not indicated. Consideration should be given to testing for PFO when there is a history of more than one episode of decompression sickness (DCS) with cerebral, spinal, vestibulocochlear or cutaneous manifestations.
Non-cutaneous manifestations of “mild DCI” as defined in the Remote DCI Workshop Proceedings [Consensus Statements, In: Management of Mild or Marginal Decompression Illness in Remote Locations, Workshop Proceedings (May 24-25, 2004). Mitchell SJ, Doolette DJ, Wachholz CJ, Vann RD, Eds. Divers Alert Network, Durham, NC, 2005, pp. 6-9.] are not indications for PFO investigation. Headache as an isolated symptom after diving is not an indication for PFO investigation.
What does a positive test mean?
Following a diagnosis of PFO considered likely to be associated with increased DCS risk, the diver may consider the following options in consultation with a diving physician:
Following closure of a PFO and before returning to diving, the diver requires a repeat bubble contrast echocardiogram demonstrating shunt closure, a minimum of three months after the closure. Diving should not be resumed until satisfactory closure of the PFO is confirmed, and the diver has ceased potent antiplatelet medication (aspirin is acceptable).
Venous bubbles can also enter the systemic circulation through intrapulmonary shunts, although the role of this pathway in the pathogenesis of decompression sickness is not as well established as PFO. These shunts are normally closed at rest. They tend to open with exercise, hypoxia and beta adrenergic stimulation, and close with hyperoxia. It is therefore plausible that exercise, hypoxia and adrenergic stimulation after a dive could precipitate decompression sickness when it might not otherwise have occurred, while supplemental oxygen is likely to minimize this effect.
Source: Denoble PJ, Holm JR, eds. Patent Foramen Ovale and Fitness to Dive Consensus Workshop Proceedings. Durham, NC, Divers Alert Network, 2015, 146 pp.
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