PFO - possible contributor to freedive blackout?

[Ash]

I was recently reading through a dive medical book when I noticed a chapter on breath hold diving with a paragraph outlining how a PFO could 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 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.”

If the theory above is correct, then someone with a bad PFO may well be increasing their chances of a blackout while freediving in much the same way that a scuba diver with a PFO may be exposing themselves to a greater degree of risk of DCS.

Does anyone on this forum have any thoughts on this?

Thanks

Ash

 

[Saturation]

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.
Does anyone on this forum have any thoughts on this?

Thanks

Ash

I don't think this hypothesis is tenable, Ash. From what book is this from?

The pressure exerted on the right heart is also imposed on the left. Thus there is no relative difference between both, so no reverse gradient. Since this primary idea is not tenable, then the rest follows as untenable, and thus, to avoid too much writing, need not be commented on.

The existing left-to-right shunt is still operant, with the left sided pressures far exceeding right sided pressures.

 

[Ash]

Thanks for that feedback. I really appreciate it.

The book is Safe Diving - A Medical Handbook for Scuba Divers, written by Dr. Allan Kayle. I picked up a copy in South Africa, where it was published as far as I recall (don't have it with me at the moment).

Ash

 

[Dr Deco]

Dear Ash:

If the chest cavity was actually compressed enough to hinder the movement of blood through the lungs, I could imagine that the resistance would cause an increase of pressure in the right atrium. If the blood flow was blocked, then flow into the left atrium would be reduced and the pressure would be lowered. That part seems to make sense, but I do not know if it is correct, that is, has it been measured.

The part about syncope on the surface because of a PFO, however, seems to be assuming a very large role for the PFO and almost nothing for the (oxygen rich) lungs. According to this model, there should be a quick release of oxygenated blood into the arterial circulation.


Dr Deco

 

[Saturation]

Dr. Kayle is well known and respected. Dr. Deco's conception makes sense, so I claim ignorance and will give it more looksee :icoeek:

 

[Ash]

I asked Dr. Simon Mitchell the question about PFOs and freediving. He very kindly sent me the following reply:

“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.”

Best regards

Ash