The Patent Foramen Ovale and Fitness to Dive Consensus Workshop was held June 17, 2015, in Montreal, Canada. The Undersea and Hyperbaric Medical Society (UHMS) and DAN® invited experts to review the current state of knowledge about diagnosis of patent foramen ovale (PFO), evaluation and mitigation of individual risk, and how PFOs affect divers’ safety. DAN published the proceedings of the workshop, including the consensus statement, which are available online at DAN.org/research/workshops.
A PFO is a passage in the wall between the right and left atria of the heart that can be found in about 25 percent of adults. Its size and the degree of blood flow through it vary. In a small percentage of people, a PFO allows for a continuous passage of blood from the right atrium to the left atrium — a spontaneous right-to-left shunt (RLS). In some people, the RLS occurs when pressure in the right atrium exceeds pressure in the left atrium. This may happen after relieving a temporary obstruction to blood flow to the heart such as with a Valsalva maneuver or straining while lifting, sniffing, coughing or defecating. Spontaneous or provoked RLS may be seen in 10-15 percent of adults.
A PFO with RLS has long been suspected for paradoxical embolism, wherein particles carried by venous blood bypass the pulmonary filter, enter arterial circulation and block blood flow in terminal vessels (embolism), which results in ischemic injury of tissue downstream of the obstruction. The most common form of embolism is caused by blood clots from peripheral veins passing through the PFO and causing a stroke. Paradoxical embolism may occur in divers with a PFO if the RLS occurs postdive when a lot of venous gas emboli (VGE) are present. Paradoxical embolism caused by VGE may manifest with symptoms of neurological (spinal, cerebral and vestibular) or cutaneous decompression sickness (DCS).
The overall incidence of DCS in recreational divers is two to four per 10,000 dives, and the incidence of neurological DCS is less than one per 10,000 dives. In the presence of a PFO, however, the incidence of neurological DCS increases fourfold. While the average DCS risk for divers with a PFO seems low, for some individuals the risk may be greater than overall statistics predict. The main question regarding PFOs and diving is how to identify individuals who are likely at an increased risk of DCS and how to mitigate that risk. The workshop’s consensus guidelines provide some answers.
Who Should Be Tested for a PFO?
The consensus maintains that no routine screening for PFOs in all divers is necessary. It recommends, however, that divers with a history of more than one episode of DCS with cerebral, spinal, vestibulocochlear or cutaneous manifestations should be tested for a PFO. On the other hand, divers with mild (other than cutaneous) DCS need not be tested.
How to Test for a PFO
Experts well practiced in the procedures should conduct the test. The most appropriate testing method is transthoracic echocardiogram (TTE) with bubble contrast and a provocative maneuver such as a Valsalva or sniffing. Other methods are suboptimal.
Interpretation of Findings
A spontaneous shunt is a passage of contrast bubbles from the right atrium to the left atrium without a provocative maneuver. This is considered to very likely represent an increased risk for DCS in cases when dives result in a lot of VGE.
A large provoked shunt means that a lot of VGE are passing through after a Valsalva maneuver or sniffing. It is likely to open with any kind of straining and is recognized as a risk factor for the previously listed forms of DCS.
The presence of smaller shunts is associated with lower risk that should be evaluated on a case-by-case basis. It is important to note that the detection of a PFO in divers who suffered an episode of DCS is not proof that the PFO caused that specific episode of DCS.
Risk Mitigation
Divers with a diagnosed PFO that’s likely to be associated with increased risk of DCS should consult a dive physician and consider options that best suit their needs and diving styles with a solid understanding of the risks and benefits of each option. The options are as follows:
- Stop diving.
- Dive more conservatively to reduce occurrence of VGE, and do not strain after diving to avoid opening the PFO and provoking RLS.
- Close the PFO.
Conservative diving includes strategies to reduce the risk of significant venous bubbles postdive and of shunting bubbles through the PFO. Since there is significant variability in VGE occurrence among divers and in the same diver over time, discuss options with a dive medicine expert before making any decision. For more details about conservative diving, see AlertDiver.com/Conservative_Diving.
The closure of a PFO may reduce the risk of DCS, but it is not a guarantee that DCS will not occur in the future.
Deep and long dives may cause DCS without VGE passing to the arterial side. Even in the absence of a PFO, VGE may pass to the arterial side through shunts within the lungs that tend to open with exercise, hypoxia and beta adrenergic stimulation and close with hyperemia.
Return to Diving After a PFO Closure
Diving should not be resumed before full closure is confirmed with another contrast echocardiogram at least three months after the closure. Divers should not return to diving as long as there is a need to take potent antiplatelet medications. If the test at three months or more after closure shows complete closure and the diver is prescribed only aspirin or nothing for clotting prevention, diving can be resumed.
Divers should always remember that the main factor causing DCS is the dive exposure itself — the depth, time and ascent rate. With a significant exposure, anybody is at risk of DCS. Most people who get DCS do not have a PFO. Divers with a complete closure of PFO may avoid DCS episodes that they may have had in the past, but if they engage in extreme diving, their risk of DCS will be commensurate.
© Alert Diver — Q4 Fall 2016