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To add further confusion to the issue of PFO's in diving is the "issue" 
of *flow* patency vs *probe* patency. When the septum secundum grows down 
over the endocardial cushion reminant (I think it's the septum primum 
(please no flames from the anatomists in the audience) it's *suppose* to 
fuse (hence no PFO) but in a significant percentage of folks it doesn't 
fuse but does act as a flap valve. Relatively higher pressures in the 
left atrium keeps that flap closed so that there's no *flow* across the 
PFO (a bad thing) but you can snake a probe under the flap or pass a 
catheter (probe patent). But who goes around with with a catheter under 
their septum? If the flap remains closed off, there is little 
*functional* difference between such a patient and one where the septum 
secundum fused solid.

Only if the septum is attritic and badly malformed so that the "flap" 
doesn't fully cover the foramen is it patent to *flow* (functionally a 
very different situation).

Therefore all PFO's are *NOT* created equal (thank heavens!). To the best 
of my knowledge, it has not been proven that those with *probe* patent 
(but not *flow* patent) PFO's are at any higher risk of DCI than those 
with fully fused FO's. In all likelihood, a sizeable number of probe 
patent FO's are walking around totally oblivious to their anomally.

We see this sort of thing many times in medicine. Old study techniques 
were "invasive" therefore risky. Hence you only did them on symptomatic 
patients that you planned to "surgerize". You therefore got the 
impression there was only a smaller instance of a medical condition since 
you were only looking at a small symptomatic subset of patients. As new 
"non-invasive", less risky techniques become available (like sonography 
for cardiac studies) you can safely look at everyone (not just the sick 
folks) and you start to see anomalies in higher numbers that you never 
noticed before (like PFO's?). I'm sure that in the past many probe patent 
PFO divers were diving totally oblivious to their "problem".

I'm not saying we ignore the issue of PFO's in diving as a contributor to 
DCI but just as in everything else in medicine the whole issue has to be 
taken in context.

As always, standing by to muddy the waters...   ;-)

Robb Wolov  

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