X-Message-Number: 33379
From:
Date: Mon, 28 Feb 2011 02:43:55 EST
Subject: Melody Maxim's Distorted Reality 14
Content-Language: en
Melody Maxim: Changes not yet implemented:
1. Incorporate parallel tubing paths to permit perfusion with, or without,
flow through the oxygenator.
Mathew Sullivan: Same as the bypass line on the clot filter?
Melody Maxim: As previously discussed and agreed upon, it is best to avoid
flow through the oxygenator during procedures in which oxygenation is not
desirable.
Mathew Sullivan: I'm not aware of this agreement. For my own edification,
are you saying that an oxygenator that is under pressure on the liquid side
and at atmospheric pressure on the gas side will uptake O2? If the answer
is yes, then we could cap off the gas connections.
Melody Maxim: This also provides for continued flow in the event of
oxygenator failure.
Mathew Sullivan: As mentioned in the meeting, the only time I'm aware of
an oxygenator failing is during training when the HEX is permanently wet and
can go for weeks or month between use. Washouts are much shorter then
cryoprotection and I have perfused a patient for I believe up to 12 hours.
Charles mentioned that washouts take approximately two hours. He could be right,
but I'm still a bit surprised based on my experience washing patients out
in the OR. My guess is that it shouldn't take more than 45 minutes to do a
washout and recirculation to 10C or lower, unless the patient is
compromised. I have a vague recollection of core body temperatures dropping to
reasonable levels in about 20 minutes, and bringing the patients temp down
those
last few degrees takes longer as the temperature descent curve flattens
out.
Boon: I don't know too much about the washouts and the cryoprotection
procedures, if the incidence of having to change-out the oxygenator is quite
frequent during bypass, then it is a very good idea to incorporate a shunt
line for your oxygenator. (Please see attached file on PRONTO Procedure)
Mike Darwin: ***It would be nice to skip the oxygenator in simple washout
cases and there was a day when this was possible when the old Sarns
non-disposable heat exchangers were still around (all three of mine never made
it
to SA or were otherwise lost). However, with the advent of modern disposable
oxygenators there is, at least to my knowledge, no free standing heat
exchanger that has the necessary efficiency to allow for this (HEX co-efficient
of ~0.6). The oxygenator is thus serving as the heat exchanger. You must
never bypass the oxygenator while perfusing a patient for this reason.
I have run Sarns, Capiox, Gish, and a variety of other hollow fiber
oxygenators in the dog lab (survival) and on cryonics cases. I have routinely
run
all of these oxygenators in excess of 12 hours without failure. The failure
Melody is referring to here is transudation or weeping of plasma or plasma
ultrafiltrate across the hollow fiber membrane. All current hollow fiber
oxygenators are not true membrane oxygenators (like the Kolobow silastic
membrane). Rather, they consist of a microporous fiber which is wetted by
blood or other high-surface tension liquid. The pores (more like spaghetti laid
out on a plate) don't leak liquid water or plasma because of the surface
tension the water forms over the microporous lattice. It's the same
principle that you see when you wet a piece of fine hardware cloth, like a
window
screen; the water is held in the screen holes by surface tension.
After extended operation plasma proteins and salts begin to accumulate on
the gas side of the fiber and these deposits both decrease the
effectiveness of gas exchange and defeat the surface tension, water-retaining
capacity
of the micropores in the fiber. The result is that water starts flooding
out of the fibers under hydrostatic pressure and the fibers become a plasma
ultrafilter. At that point the gas exchange efficiency is lost.
Under normothermic or mild hypothermic conditions in a clinical situation
this is a serious problem and requires that the oxygenator be switched out
per protocol.
Such failures do sometimes occur early in oxygenator use and they are not
the only failures possible. On the Gallager case I had a Sarns oxygenator
(cosmetic defect reject) housing split into two pieces shortly after going
on bypass! Fibers can also be damaged resulting in gas bubbling into the
blood, but this is usually caught during priming.
Aside from the catastrophic failures I note above, failure of the
oxygenator from fiber compromise is the only likely scenario. This is a
non-issue
in cryonics cases as far as Im concerned. It is very unlikely to happen and
if it does you can simply turn off gas flow to the oxygenator and keep
running it. If the drip of fluid out the gas ports is annoying you can put a
short section of A tubing on the ports and clamp them. This assumes that the
patient is in ultraprofound hypothermia (< 10 degrees C) and that you want
to continue recirculation. There are many reasons why you might want to do
this on an ECMO (MARC) cart, but they are beyond the scope of this
commentary. <br>
To summarize, the oxygenator is mostly there because of the heat
exchanger. In most cryonics settings the ATP will be used no more than 2 hours
and
oxygenator failure due to wetting of the fibers is unlikely to be a material
be a material consideration.
Typical (actual) washout and cooling times range from 15 minutes to 60
minutes on CPB. Much depends upon how much the patient has cooled externally
before going on bypass.
In the future, if we ever return to 1980s style cryonics with prompt and
effective CPS and capable personnel, the issues of prolonged gas exchange,
appropriate FiO2, sophisticated monitoring (gases and pH), and oxygenator
failure will need to be addressed. However, IMHO, that time is not now. For
now, primum non nocerum (first, do no harm) should be the goal. That means
pressure alarms, air bubble detectors, and as intrinsically safe a system
possible.***
2. Melody Maxim: Incorporate independent heat exchanger.
Discuss Heat Exchanger Performance Factor
Mathew Sullivan: As mentioned in the meeting, I'm not sure this is
necessary based on the above discussions.
3. Melody Maxim: Replace current oxygenator.
Unfortunately, nearly all currently available oxygenators in the U.S. come
with an integral heat exchanger, with the exception of an independent
silicone membrane oxygenator that is generally only used for ECMO procedures.
The silicone membrane is expensive and difficult to manage, due to the
flexibility of the membrane.
There are several comparable oxygenators available that would be more than
satisfactory for Suspended Animations perfusion circuit, but this decision
is best made after choosing the heat exchanger, for which there are
several recent leads that deserve further investigation.
Mathew Sullivan: As mentioned in the meeting, I think we are better off
sticking with an integrated system. I believe there was universal agreement
from all of us that the product which is most desirable will be one with the
highest cooling efficiency.
Mike Darwin: ***See my comments above. I do not favor scrapping the
oxygenator at this point even though in most cases it functioning only as a heat
exchanger. Ive had extensive experience with the Sci-Med Kolobow and do not
recommend its use in this application.
I strongly suggest you abandon the Sarns oxygenators you are using. These
are no longer manufactured and are not well configured for this use. This
is the third time I have suggested this, and second time in print. You need
to use an oxygenator that is going to be manufactured into the foreseeable
future. The Sarns is history. and if they were stored in Florida under
uncontrolled conditions I would NOT use them to pump a human. Every Sarns I had
(and that you now have) is either a diverted return from a hospital, or a
cosmetic defect reject. I took meticulous care of them and never stored them
in high humidity environments nor did I ever expose the overwrap to
shelves or handling. A small hole in that overwrap and, with varying barometric
pressure, high humidity, and spores, and you have a nonsterile oxygenator
and possibly one with actual microbial overgrowth. These oxygenators were
dispensed in corrugated cardboard boxes to protect the sterile packaging. I
would use these for animal (survival) training runs, but even then only after
careful vetting.
The Monolith is is many ways ideal (size wise), but is being discontinued.
I suggest you look at a format like the Gish or Medtronic which is broadly
similar to the Monolith. If you want a good home for the excess Sarns
oxygenators I can tell you who to send them to.***
Boon: Hi Aschwin,
In your email you wrote, Please focus on what you believe are the most
important issues. Our short term goal is having basic capability, not the most
sophisticated circuit in the world. OK, let me break it down to different
points.
Go with the hardshell cardiotomy/venous reservoir for its better air
handling and volume capacity.
Stick with the integrated system (oxygenator with integrated heat
exchanger).
Don't put a 3-way stopcock at the arterial cannula connector, instead put
one on top of the arterial line filter as shown in the photo above.
I don't know whether my advice is good enough to help you in deciding how
to proceed.
Thank you for the opportunity to give my 2 cents worth of comments.
Sincerely,
Boon
ATP Circuit
Melody Maxim: Out in the field, are you going to have enough height
differentials between the patient (in supine position) and the inlet of your
venous reservoir to create adequate gravity venous drainage?
Mike Darwin ***I agree with Boon, this is not enough fall. The reservoir
needs to be as low as possible to the ground..This is another reason I don't
like hard-shells because you can't get them as close to the floor. You
need 30 cm of fall on the venous line. Also, you must not snake the venous line
up over the top of the PIB. If you look at the photo below we have
unsnapped the side of the PIB at groin to allow for the venous line to fall
directly from the groin wound to the venous reservoir. In the OR we simply
raised
the table top to get the desired fall (see below). It would have been very
desirable to design the PIB so that it could be raised or lowered. In the
field we achieved this with the old PIB design by placing the PIB on a
mortuary table. All mortuary tables can be raised or lowered either
hydraulically or by a bar-and-ratchet. Thus, we were not constrained by height.
When
the ECMO cart (MALSS) was designed we were stuck with a fixed height due to
engineering constraints. This caused endless problems as the oxygenator and
the venous line literally had to rest on the floor. In order to move the
patient from home or mortuary to the ambulance we had to discontinue CPB and
pull up the Sarns oxygenator holder.
Can the new PIB be adapted so that the ice bath (patient) can be raised up
to a higher level?
As you can see from the two photos below I ran the venous reservoir bag
virtually on the floor in dogs and humans. I kept the reservoir bag nearly
full in this model because the dogs had been in normothermic cardiac arrest
for 16 minutes and vascular tone went from nothing to fully vasoconstricted
in a matter of seconds as various drugs were given. This was the only way to
avoid pumping air. Lower bag volumes are desirable when doing hypothermic
asanguineous recirculation to avoid warming and microbubble formation in
stagnant blood.***
Boon: It looks to me like you are using a Terumo Capiox Cardiotomy. I know
its cheaper than using an integrated cardiotomy-venous reservoir:
However, with cardiotomy-venous reservoir there is a A venous inlet right
on top of the hard-shell, therefore you don't have to use a A x 3/8
reducer. This will create some resistance to your venous flow coming down from A
in tubing to 3/8 in tubing. Your purge line is not connected to the venous
inlet tubing, hence, does not have the venous sampling capability.
Mike Darwin: ***I agree with Boon. If you decide to use a hard-shell use a
proper venous reservoir. Do NOT put flow restrictions in line: it does not
matter that a port is set up to take either 3/8 or A tubing; if you reduce
the diameter, however transiently, you increase back pressure and decrease
flow.***
Boon: This is the integrated cardiotomy-venous reservoir that we currently
use:
Mike Darwin: ***While I don't recommend hard-shells this would be the way
to go. Note all the ports on top of this reservoir! This is a recipe for
confusion and disaster in unskilled hands.***
Boon: Another view of our cardiotomy-venous reservoir during bypass:
Boon: This must be a Peristaltic Pump:
I'm curious how high of the flow it can generate.
Mike Darwin: ***This is a commercial Cole-Parmer Masterflex persistaltic
pump. Youd love the ease of loading the pump shoe. It can easily produce 10
LPM flow with a A ID pump shoe. It is also auto-occluding eliminating the
need for in-field occlusion of the pump after shipment of the pump.
Boon: Do you really need a five-gang sampling manifold?
Mike Darwin: ***They probably do not. I used two 5 gang manifolds of the
cerebral ischemia dogs because we needed to give nearly a dozen drugs in
very rapid succession. I have suggested that SA use a 5- or 6-gang manifold in
the IV line during initial external CPS to facilitate rapid administration
of transport meds. Here it is likely to be confusing, and perhaps a 3-gang
should be used.***
Melody Maxim: We find a three-gang sampling manifold is more than adequate
to inject drugs or draw samples.
Mike Darwin: Agreed.
Boon: Why do you need to have two manometers to measure your arterial line
pressure?
Mike Darwin: ***As I understand it, one is back up. These things and the
isolators just creep me out completely. One big pressure excursion and you
have intra-arterial air!
Agreed. I think it is also important to point out here that this set-up is
to monitor filter back pressure (loading), not monitor arterial pressure.
If the pressure in back of the filter begins to rise without an increase in
flow then you know the filter is loading or clotting. This has never been
a problem in asanguineous perfusuion in humans or dogs since the HCT is
always <5%. Obviously, in whole blood clinical perfusion, monitoring pressure
drop across the arterial filter is essential.*** <br>
Boon: It would be nice to reduce the number of connectors you have.
Mike Darwin: ***This is the bypass loop and the A-V loop. I would
substitute the arterial line 3/8 x 3/8 x 3/8 Y connector with a 3/8 x 3/8 x A Y
connector and the A x A x A connector on the venous line with a A x A x/ A
connector. This will let you create your bypass line out of 1/4 tubing with no
reducing connectors. Yes, they do make a A x A x/ A Ys, and they are
configured so that the A luer is parallel to the other (3/8) barb making this
a
very smooth configuration.
Absent this, I would use a solvent bond to unite a piece of A tubing with
a short stub of 3/8 tubing on one end, and the same thing with A tubing on
the other.
There needs to be a 3/8 connector with port and 3-way stopcock on the
arterial line where it will connect to the cannula to debubble the line and the
vessel. Similarly, there needs to be a A connector with port and
3-way-stopcock on the venous line where it will connect to venous cannula.***
Boon: Hi Aschwin,
Here are some of my concerns:
1. Height differentials between the patient and the venous reservoir.
2. Is it feasible to use integrated cardiotomy-venous reservoir instead of
just cardiotomy reservoir?
3. How good is the Peristaltic Pump? Is it going to generate enough flow?
4. Too many connectors in the circuit.
I hope these pictures are helpful. Let me know what you think.
Thank you.
Boon
Mike Darwin: *** 1) Per the above, it would be good if you can adapt the
PIB to be raised up like the top an Ferno-Washington embalming table.
2) I strongly suggest sticking with a compliant bag-type venous reservoir.
3) This pump is a proven workhorse and will deliver more than adequate
flows. It is easy to set up and to operate and vastly safer for the operator
than the traditional Sarns configuration (with the roller on top). You
cannot crush fingers or entangle hair or clothing in this pump.
4) I agree that connectors should be minimized. I cant do this very well
without a hands-on.
5) Arterial and venous temperatures should be monitored on the oxygenator
and not with ports on the lines. A Cu+/C load type thermocouple should be
secured into these ports directly as opposed to using the commercially
available sensors because these sensors leak too much heat down the probe; they
are not designed to run at a delta T of more than 5 degrees C. This also
avoids long runs of TC line, simplifies the operative field, reduces the
possibility of snagging a line and pulling a cannula out, and reduces the risk
of contaminating the sterile field.
6) I can explain how to configure the TC probes so that they fit directly
into the temperature monitoring ports on the oxygenator as opposed to using
manufacturer's probes which will read consistently 3-5 degrees to high
with a large delta T.
Thank you for allowing me the opportunity to comment on this very
interesting set of problems.
Mike Darwin***
END OF COMMUNICATION TO SA
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