X-Message-Number: 33375
From:
Date: Mon, 28 Feb 2011 01:56:27 EST
Subject: Melody Maxim's Distorted Reality 10
Content-Language: en
Mathew Sullivan: This isn't necessarily an argument against filtering
venous return, but to the extent that a clot may have gotten past the arterial
pump, the clot filter prevents it from getting into the patient. I'm not
aware of any cases where the clot filter clogged and had to be swapped out.
Aside from any potential concern of the arterial filter getting clogged, are
there any other reasons to take into consideration? Could you provide more
information on the filtering of the venous return? The reason why I ask is
because I learned at Mechanisms that the cardiotomy reservoir has become
the scourge of perfusionists because it is a major source of micro bubbles,
so I'm curious to know if it is an internal filter or separate from it, and
to what extent it might introduce micro bubbles. At a minimum, I recommend
that the waste dump line be split off prior to reaching the filter to help
ensure that the filter does not become clogged during washout, similar to
the way you currently have it setup on the training pack.
Mike Darwin ***Clotting should not be a problem in any cryopatient who is
a candidate for closed-circuit perfusion (recirculation). I have
occasionally seen cast clots of the femoral artery or vein in patients who were
cutdown within 45 minutes of arrest and who were also promptly heparinized,
medicated, and given closed-chest CPS and cooling. These clots appear to be
antemortem due to profound shock and accompanying peripheral hypostasis.
Despite this ominous finding, Ive never seen macro clotting beyond this. In
cases
where such a clot is found it is simply extracted with clot forceps; in
all cases these have been well formed, dark, retracted clots.
If visible clotting (as opposed to cold agglutination) is observed during
blood washout, then the patient is not a candidate for recirculation
barring the presence of a skilled perfusionist.
Mathew is right on target about air bubbles and debris from cardiotomy
reservoirs and hard-shell venous reservoirs and there are additional
considerations which are unique to cryonics cases:
1) The higher (beyond safety constraints) that you run the perfusate level
in a reservoir the more microbubbles you will typically form. This is
especially true in hardshell reservoirs because they have a large column of
poorly stirred fluid with a debubbler column of open cell urethane foam
covered with nylon tricot. The big column of poorly stirred fluid is the first
problem because it is cold. Because it is both cold, and far below ambient
temperature, it will out-gas, or fizz. Unlike solids, gas solubility in water
is directly related to temperature: the colder the perfusate is, the more
dissolved gas it will hold. Any uninsulated standing reservoir of cold fluid
will warm up and will generate microbubbles. Soft drinks go flat very fast
at ambient temperature for this reason.
2) An added problem is that the urethane and nylon tricot serve as
fantastic nucleating sites for bubbles. Thus, they actually favor microbubble
formation. You can see the same effect in soft drinks if you note that most of
the bubbles form on the interior of the glass and on the ice cubes; the
latter being far colder than the glass.
3) Compared to plasma the MW of the colloid used in current washout
solutions is astronomical (250,000 to (preferably) 500,000 KD versus 40,000 KD
for plasma proteins, principally albumin). In the near future I expect MHP-2
to be replaced with a solution containing polyglycerol, polyvinyl alcohol,
and possibly polyethylene glycol (the latter in MWs as high as 100,000 KD).
These molecules inhibit cellular edema in hypothermia and ischemia (cold
and warm) but are also profoundly effective at both facilitating the creation
of, and stabilizing foam.
The air-liquid interface of the large surface area, open cell urethane
foam column, will facilitate the generation of bubbles as the liquid level
rises and falls dynamically. In blood, antifoam-A is used to inhibit this
effect, and indeed, the large surface area of the urethane foam is designed to
serve as a matrix for the Antifoam-A and maximize the surface area of blood
foam exposed to Antifoam A. However, perfusate is not blood and the
kinetics of air bubble movement and the ability of Antifoam-A to uniformly
reverse
foaming are untested at best, and in doubt at worst. Even with blood under
clinical conditions near normothermia, the presence of the de-foaming
column is not completely effective and we all know that cardiotomy and hard
shell reservoirs are a source of microbubbles during conventional bypass.
Beyond these unusual problems, there is currently an inviolable rule in
CPB which has been in effect since I started using extraporeal equipment in
the mid-1970s; more surface area equals more debris. Prebypass filters were
invented to deal with the ever-present load of plastic debris that comes out
of even the most carefully washed extracorporeal circuit. I have seen
oxygenators/tubing/filters being cleaned at Gish prior to sterilization and
even washing with hundresof liters of ultrapure water leaves a lot of debris.
While not a major issue in cryonics (given the other horrors) I dont like
the idea of adding yet more debris.
Finally, there is yet another problem unique to cryonics which really
demands discussion on its own. Cryonics started out relying heavily on expired
products. Those of us in the vanguard of this use were very concerned about
this: actually, terrified would be a better adjective.
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