Alex in L.A.:
Hmmmm -- I've read that sentence several times, and am still having trouble parsing it. If it turns out there isn't a PFO, what is it that's more so?
Sorry for being terse.
I did a search and found little is discussed about this item elsewhere. So here is a summary.
The sources for shunts to cause AGE or DCS2 that are treatable to an extent are: PFOs and "lung shunts". Shunts are various sized arterial to venous connections, commonly as fistulas, and these can occur in any organ. As blood from all organs converge to the lungs, lung fistulas are more severe, as they are basically large defects in the lung as a bubble filter. Note, bubbles can also squeeze passed the lung if the volume of bubbles is large after a dive or the lung has defects in its structure due to acquired disease, such as damage from smoking. Thus, the generic name of 'lung shunt,' its a term for all bypasses of the lung as bubble filter.
http://www.yorkshire-divers.co.uk/forums/showthread.php?t=17827
Severity depends on the size of shunts.
PFO are least risky if 4mm or less in diameter.
Lung shunts and large PFOs offer a higher risk, but its not quantitative at this time. There is a theoretical risk for intra-dive AGE in these situations, and potential drowning at worse case.
To enter arterial circulation via a PFO, bubbles need to traverse the right atrium to the left atrium via an increase in right atrial pressure usually caused by coughing or lifting. Lung shunts push bubbles to the arterial side without such action. Therefore, sources for lung shunts are much worse, and are not evaluated very thoroughly after an unexpected hit on patients.
'More so if there is no PFO...' is because if the PFO evaluation is negative and one had an unexpected hit, it could need more workup and more bubble reducing measures to dive safely.
Without knowing the source of emboli, its not possible to logically plan a protective action, unless we blanketly take ALL possible steps to minimize bubbling. If diver cannot cope with all these added issues, diving is over for them or they dive in very risky conditions for simple recreational dives.
The approach most dive docs are using is simply to forego the workup and just do the conservative measure and see what happens. Typically that is dive to P02 of 1.2 or 1.3 with nitrox, reduce bottom time and longer safety stops both mid depth and 20' variety.
Additional actions are: reducing intra-dive motion as much as possible, and breath 40% or more 02 at the safety stops as a kind of 'deco' stop to wash out as much N2 as possible, increasing fitness, and weight at BMI <= 25.
Diving deeper is a problem for high risk folks as more inert gas is uptaken, and there is a risk of bubbling with long ascent alone as large shunts do not allow bubbles to be decompressed during stops, a worse case scenario is embolization in-water leading to unconsciousness and drowning, such as in the case of Paul Thomas, M.D. who embolized while on ascent possibly through a PFO. His story is all over Scubaboard.
To diagnose these issues requires a TCD and a TEE. TCDs are positive in arterial to venous shunts of any type. TEE is for PFO only. If the TEE is negative and TCD positive, then the patient has a non-PFO shunt which is presumed from the lungs. Shunts in other organs may not pass enough bubbles fast enough to be heard, but it can be the source too. For large PFOs, there is the option of repair for to continue to dive. If fistulas are found, the treatment is a specialized.
Shunts of small dimension that cause diving problems often do not cause other problems, but fistulas can be life threatening as a source of bleeding. They can occur anywhere, and if the TCD is positive the difficult step finding the fistula. And alas, there is still much more to it.
Thus, the "more so" if a PFO is negative as a source for gas emboli: lung shunts that aren't a PFO are the most risky and undefined.
Finally, lung shunts are not static structures, they can be acquired or grow as one ages or worsen from other diseases: a pneumonia can create a lung defect that is otherwise not an issue unless you dive. Its another reason PFO screening for diving is not warranted, even if you are negative, you can still develop another shunt elsewhere that is just waiting for an opportunity to open up. This is often the case when one has dove for years without issue then suddenly gets an unexpected hit.
Luckily, the vast majority of emboli from unexpected hits are not fatal, as note here and elsewhere, the symptoms are startling, obvious, gets the diver to see a dive doctor and trigger a need to decide whether to do something about it or stop diving.
