From: fgz@kodak.pa.dec.com (Federico Genoese-Zerbi)
Subject: Re: Diving and cardiac problems?
Reply-To: fgz@kodak.pa.dec.com (Federico Genoese-Zerbi)
Organization: Digital Equipment Corporation
Date: Mon, 28 Sep 92 19:37:18 GMT


In article <16608@pitt.UUCP>, km@cs.pitt.edu (Ken Mitchum) writes:
> The incidence of an ANATOMICAL patent foramen ovale (one that you
> can pass a probe through at autopsy) is fairly high. However, the
> incidence of FUNCTIONAL patent foramen ovale is much lower, as
> demonstrated by the low incidence of "paradoxical emboli" (clots

it has been theorized that certain hyperbaric environments may increase
the likelyhood that an anatomical PFO may open and allow the exchange
of gas bubbles.

> from the legs and pelvis which bypass the lungs, go through the
> patent foramen ovale to the left side circulation and cause strokes
> or other embolic event. I don't know the exact numbers, and I
> certainly don't know the incidence of paradoxical air emboli in
> scuba divers. 

A BSAC study of people who had suffered DCS hits showed a much greater
incidence (I don't remember the exact numbers and I don't have the
article in front of me) of anatomical PFO than the general population.
This incidence increased even further when only cases of DCS hits within
the current (at the time) BSAC tables were examined.   Additionally,
DAN statistics of post DCS examinations as well as the autopsies of divers 
show pretty convincingly that this is a risk factor.  The question is
how big...

> You would have to know these numbers to figure out
> whether cardiac catheterization in potential scuba divers should
> be done on a routine basis. This is an expensive procedure with a
> definite morbidity and mortality that should not be done without
> good reason.

Major discussion right now as to whether this is advisable or not.  I don't
believe it is, mostly because if you look at the overall numbers, DCS
in active divers is relatively rare (and relatively benign if treated
promptly).  Others disagree.  It may take a while
for the dust to settle (if ever) on whether divers should be screened
in this manner or not.  I think the BSAC may have been pushing for
mandatory screening.  British posters may be able to provide more info
on this.

> In most people, small air emboli cause no problems. This is
> usually discovered early in medical training when a medical student
> or intern doesn't clear a syringe completely and injects a small
> amount of air in a vein. The student is usually panic stricken,
> based on information presented in his basic science courses, as
> he assumes he has just killed the patient! The patient does just
> fine.

Doppler studies of divers found that divers diving close to the edges of
the U.S. Navy tables had significant levels of VGEs.  If I recall correctly,
some were up to level 3 or level 4 and none of the divers in the study had
symptoms of DCS.  The lungs do a great job of screening bubbles,
but it takes quite a while to offgass from there.  Interestingly enough,
when some of these people were recompressed in chambers breathing
100% O2, none of them suffered AGEs (which was a concern of the
scientists performing the experiment).  Another problem with the
accumulation of N2 bubbles in the lungs can be a decrease in lung function
(the chokes).

> It actually takes quite a bolus of air to do damage, 
> although I'm not sure Agatha Christie knew about this. A functionally
> patent foramen ovale would make this more dangerous.
> 
> There used to be some radiologic based studies in which CO2
> was injected to help measure ventricluar size, I believe. The
> CO2 was resorbed and caused no problems.

I would guess it was exchanged in the lungs instead.  Are you sure
about it being reabsorbed, Ken?  I guess it is possible, since blood
is nowhere near saturated with CO2, but that would probably take longer
than for it to get to the lungs and just escape.

> This is an interesting area, and I certainly am not an expert in it.
> This is what sci.med should be for. Anyone else out there have some
> information?

Well, that was some info.  Hope it helps.  I'm no expert either, 
but I try to keep up.  If you're interested, there was a decent 
(had a couple of facts that struck me as wrong) review article in 
the NEJM about a year ago (if you care it was June 1991 and
by some Israeli hyperbaric physicians, if you want more info, 
I'll get the exact details...)

Cheers.

F.

Newsgroups: sci.med