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PFO - possible contributor to blackouts?

Thread Status: Hello , There was no answer in this thread for more than 60 days.
It can take a long time to get an up-to-date response or contact with relevant users.


New Member
Nov 5, 2002
I was reading through a dive medical book when I noticed a chapter on breath hold diving.

What really caught my eye was a section where the author details how he thinks that a PFO can be a danger to freedivers. I knew about the possible dangers of a PFO to scuba divers, particularly technical divers but I don't recall hearing of it in reference to freediving.

The extract from the book (Safe Diving – A Medical Handbook for Scuba Divers, written by Dr. Allan Kayle) is as follows:

“Patent foramen ovale – a flap valve between the right and left upper chambers of the heart is present in up to 25-37 per cent of people. If right-to-left shunts such as a PFO are present, this author believes that they may operate with the increased water pressure on the chest with breath hold diving and be an important cause of death in breath hold diving.

As the breath hold diver descends the water pressure around him or her increases. The chest and lungs become compressed. The right side of the heart, a very low pressure system, meets increased resistance to its attempts to pump blood into lung blood vessels. The pressure in the right atrium rises and the PFO opens. Venous blood, high in carbon dioxide and very low in oxygen, is shunted into the left side of the heart and back into the general circulation. Less blood reaches the lungs. This results in a decreased transfer of blood carbon dioxide into alveoli, plus a reduced draw on available alveolar oxygen during the dive. Arterial blood oxygen drops dramatically and carbon dioxide rises sharply. Analysing the gas in the alveoli at this point would show a relatively low carbon dioxide level and ample oxygen! Bottom syncope may then occur due to profound arterial hypoxia and carbon dioxide narcosis. Or a rapid ascent could beat diffusion of oxygen from, an carbon dioxide into, expanding alveoli and result in blackout of ascent.”

So he’s saying that if you have a PFO, your body may be failing to function correctly under freediving conditions, which is obviously not good.

The thought of a possible “smoking gun” that might explain some of the blackouts that plague freediving and spearing is intriguing to say the least.

If Dr. Kayle’s thoughts above are correct, then someone with a bad PFO may well be increasing their chances of a blackout in much the same way that a scuba diver with a PFO may be exposing themselves to a greater degree of risk of DCS. In both cases, they would never know unless they were tested for the presence of a PFO, which is a procedure that ranges from mildly invasive to pretty uncomfortable depending on how it is carried out.

Does anyone have any thoughts on this concept?

Very interesting. Any idea of what depth this PFO valve begins to open? Since I typically dive solo, I've often wondered if I should limit depth or go by time. The number of unexpected blackouts lead me to believe simply "listening to your body" is not sufficient.
Cliff G
PFO is an advantage for FRC freedivers. As an example, crocodiles have a special valve which diverts all blood away from the lungs (which are empty anyway). So, PFO only causes problems for inhale divers, and anyone with PFO would be better off switching to FRC dives since they would have an advantage over the average person in that case.

Eric Fattah
BC, Canada
eric - I think I read somewhere too that seals have a high percentage of PFOs.... do they dive empty lung?

glad I had a PFO check!

Seals dive on empty lungs -- Sebastien Murat thinks he has developed PFO by practicing empty lung dives (and he hopes he has developed it).

Eric Fattah
BC, Canada
Cliff - sorry mate but I have no idea. Even if this theory is true, there would be variables like the size of the PFO and each individual’s physiology, which would make it very difficult to predict.

When you consider that a PFO can open under the pressure of a forceful Valsalva maneuver while equalizing, I imagine that it wouldn’t take much to open one on a freedive.

I have asked a few dive medicine experts for their opinion on the subject as it relates to freediving and will post what responses I get.

I have had 3 responses thus far from medical professionals to my question about PFOs and freediving.

One indicated that if the chest was compressed sufficiently, then it was theoretically possible for a PFO to affect a breath hold diver but he said that he wasn’t sure if this was the case and that as far as he knew this had never been measured. Another indicated that any pressure exerted on right heart is also imposed on the left, so there could be no reverse gradient, which makes the whole concept untenable.

The third (Dr. Simon Mitchell) kindly sent me a reply outlining why he didn’t think the theory was correct. His reply was the most detailed and I have posted it here for anyone who is interested:

“He is right to imply that an increase in pulmonary vascular
resistance will lead to an increase in right atrial pressure, and an
increase in the tendency for right to left shunting to occur. This is how
venous bubbles promote their own shunting: their entry to the pulmonary
circulation increases pulmonary vascular resistance by several means, thus
promoting the shunting of venous blood (containing more bubbles) to the left
(arterial) side of the circulation. However, his physiological explanation
for a similar phenomenon during breathhold diving is flawed.

Resistance to flow through a blood vessel is inversely proportional to the
4th power of its radius, and directly proportional to its length and the
viscosity of the blood. The latter two parameters will not change in any
significant way during a breathhold dive, so we are left with vessel radius.
Thus, for the right heart to encounter increased resistance during a
breathhold dive as claimed by Dr Kayle, there must be a reduction in radius
of the pulmonary vessels. In fact, quite the opposite occurs. As he points
out, increasing ambient pressure causes a relatively negative airway and
alveolar pressure. However, Dr Kayle appears to assume that this is
compensated solely by a change in lung gas volume in accordance with Boyle's
law; that is, everything gets "crushed" to some extent. Were this true, it
would probably cause an increase in pulmonary vascular resistance as he
implies. However, the negative change in relative intrathoracic pressure is
compensated for in large part by the movement of blood from the peripheral
circulation into the core, and into the highly distensible pulmonary
vasculature in particular. Thus, the pulmonary vessels, if anything, tend to
be distended by immersion and breathhold descent. It follows there is no
change in resistance, and no increase of right heart pressure relative to
the left... (not by this mechansim anyway!).

I do have to point out that this is a very complex area of physiology.
Changes in right and left heart pressures DO occur during immersion and
diving, but if anything, the left heart pressures increase first, and
perhaps by a greater amount. In this setting, increased right heart pressure
is secondary to increased left heart pressure and the left side retains its
"pressure predominance" and shunting from right to left is not promoted.

Finally, Dr Kayle inadvertantly points out an epidemiological flaw in his
own argument when he correctly observes that a high percentage of people can
be demonstrated to have a PFO. Were the effects of breathhold diving so
potentially disastrous because of PFO as he describes, I would have expected
there to be far more reports of breathhold diving accidents.”

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