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I lost the thread here a bit so I'll be "brief" and maybe provide more
detail if requested. These topics appear from time to time on Techdiver and
I don't want to bore the old timers with the same old stuff.
>>Well, you don't like my hypothesis - can you come up with an
>>alternative one or do you discount the anecdotal observations it
>>attempted to address?
>I forget what these anecdotal observations were. I was just pointing out
>that aterial bubble trapping is an old theory that is neither established as
>fact or particularly likely to be the case.
Blocking of brain arterioles or capillaries by bubbles produced during
decompression or directly injected into the arterial circulation is only
seen when the heart stops beating, ie (as is well known) after death. If we
perform a craniotomy on a rabbit and produce intra-artial bubbling by
decompression or direct injection of air into the arterial circulation the
bubbles are seen to pass though the artioles and disappear. They can in
fact, be detected in the saggital sinus (using ultrasonic Doppler) exiting
the brain vascular bed. Conclusion? Bubbles do not block the capillaries.
Brain blood flow is not affected for at least 2 hours after this type of
injury. Small doses of air produce a progressive loss of brain function
(flow and function remain coupled), larger doses of air produce a sudden and
profound cessation of brain function (flow and function are uncoupled).
>>#This is not so. There is little (or no?, comments Prime Ra?) evidence that
>>#bubbles trap in the arterial circulation. One workers calculations suggest
>>#that the bubble, which would be tubular in shape, would need to extend
>>#through three generations of arteries before they would trap.
And then they only do so temporarily and in any event collateral
vascularisation means that blood flow is not affected. We know the
secondary effects of intravascular bubbling are the important ones because
if we modify them (remove the white cells or make them slippery) the effects
of cerebral arterial air embolism on brain blood flow and brain function can
be largely prevented.
>>Firstly, if bubbles were to lodge at all I would imagine them to do so
>>in arterioles/capillaries: I didn't mean to imply they could get
>>trapped in the arteries themselves. If this trapping can't happen,
>>then how does the lung normally trap the so-called "silent bubbles"
>>present in the pulmonary artery?
>The pulmonary arterial pressure is much lower than the systemic arterial
>pressure.
Althought the lungs also have an enormous surfcae area and will filter a
certain amount of any intravascular gas load, they can fill up. The cycle
of compression-decompression (ascent-descent) will also cause some of air
trapped in lungs to escape back into the arterial circulation.
>>Secondly, lack of evidence don't always mean something ain't true.
What does it mean then? That more experiments need to be done?
>>#Bubbles are a problem if untrapped, they activate the complement and kinin
>>#systems and damage the blood vessel endothelium.
>>That's why I said simplistically, and I didn't suggest that bubble
>>trapping was the only factor involved.
>>Were your assertion correct, then why would arterial bubbles be
>>considered more of a problem (PFO and the like) than venous ones per
>>se? Are the complement and kinin systems only activated on the
>>passage of bubbles through capillaries?
Complement and kinin systems can be activated by shaking blood in a test
tube. To see the effect of exposing blood to a gas interface, just cut
yourself. Bubbles inside the circulation initiate many of these clotting
processes.
>I suspect these systems are activated by venous bubbles (the first to form)
>and this accounts for the 'flu' like symptoms of mild DCI.
So consider the possible spectrum;
- bubbles form in the venous before the arterial circulation because
it is at a lower pressure (bubbles formation in the arterial circulation
has been observed during normal decompressions)
- bubbles then activate components of the complement and clotting cascade
- these (soluble) intermediary compounds arrive in the artial circulation
- some bubbles arrive in the arterial circulation, either because of a
patent foramen ovale, lung dysfunction, patent ductus arterious or
they simply form there
- bubbles bouncing along the vascular lumen irritate the endothelial cells
- damaged endothelium collected some of these clotting factors (some of
them actually live in the endothelial cells
Outcome depend on the interaction of these (and a whole bunch of other) things.
- none of this stuff affects your central nervous system because the dose
is too small. You do you dive and you feel great ("silent bubbles")
- your central nervous system is partially affected. You do you dive,
but you feel a bit more stuffed than usual. ("not so silent bubbles")
- your central nervous system is affected in a big way. You do
your dive and you now have mild but recognisable symptoms of DCI.
Conservative treatment works. You take up diving again some weeks later.
("noisy bubbles")
- your central nervous system is affected in a major way. You do
your dive and you now have serious DCI with neurological symptoms.
Conservative treatment doesn't work. Aggressive and multiple hyperbaric
oxygen treatment is indicated. After a couple of weeks of this you are
better but the treating doctor says "no more diving". ("very loud bubbles")
- your central nervous system is wrecked. You die. ("positively
deafening bubbles")
Each of these conditions has been modelled using experimental animals and
even sometimes (believe it or not) human beings.
/rat
ps Buy the book "Pathophysiological basis of cerebral arterial gas
embolism (DCI)" [AUD$250 from the Dept.Anaesthesia, University of
Adelaide] and get the latest on scientific research into decompression
illness.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
shelps@ac*.ma*.ad*.ed*.au*|Stephen Helps PhD Ack! ___/|
FAX (08)232-3283 |Anaesthesia & Intensive Care \O.o|
Voice (08)224-5495 |University of Adelaide =(___)=
|ADELAIDE, 5005, South Australia U
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
"It's not denial. I'm just very selective about what I accept as reality."
--- Calvin ("Calvin and Hobbes")
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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