X-Message-Number: 2900
From:  (Robert Cardwell)
Date: Sat, 16 Jul 94 14:29:38 +1000
Subject: CRYONICS Carotid Perfusion

In Message #2884 Mike Darwin writes:

>Robert writes that the (I paraphrase here) "carotids do not provide adequate
>flow in most patients and that is why Alcor cannulates the aorta..."
>
>Actually the reverse is true.  In most people the carotids provide plenty of
>flow, in fact you could probably perfuse a person's entire brain adequately
>through *one* carotid -- this is because the two carotids and two vertebrals

>join a circular structure (almost exactly like a European Traffic Circle) 
called
>(appropriately enough) the Circle of Willis.
...
[much omitted]
...
>Another problem with perfusing through the carotids is
>that some of your flow will "escape" through the Circle of Willis and down the

>vertebrals (and thus down the drain).  This is annoying (and costly) when doing
>closed circuit perfusion.  However, it is not normally a limitation when doing
>open circuit perfusion.  As long as you keep your perfusion pressure at 40-60
>mmHg you will perfuse the brain (as well of course, as loose some flow
>retrograde down the vertebrals and to the drain).

When I made my earlier remark about carotid perfusion I had in mind Steve
Harris's messages #738 and #1296. I quote below the parts of his messages
that relate to this matter.

In message #738 Steve wrote, in the course of a discussion with Edgar
Swank:
...
>For
>your information, the brain is perfused by not only two carotids,
>but also two vertebral arteries (which join at the brain stem).
>If the person happens to have a fully intact and widely patent
>Circle of Willis (this occurs in about 25% of people) you run the
>risk that blood forced up the carotids will mostly escape down
>the vertebrals into the body without perfusing the brain (after
>all, why should blood go through a 5 or 10 micron wide capillary
>when it can go retrograde down a major artery instead?).  Since
>TransTime doesn't do skull bur-holes to monitor brain perfusion,
>how are you going to know how you are doing?  And for those
>people without intact or fully patent Circle of Willis connec-
>tions, the hind-brain is not going to perfuse at all through the
>carotids (Alcor has seen this in dogs perfused though the
>carotids).
...

And in message #1296 Steve wrote, referring to the Susan White
suspension:
...
>   Four vessels feed the brain, and these four vessels are con-
>nected perfectly to each other at the base of the brain (in a
>fully-formed "circle of Willis") in only 25% of people.  The
>other 75% of people are probably best perfused through all four
>vessels.  That means open chest surgery to isolate the ascending
>aorta.  Mr. White & Co. discovered that one carotid is not enough
>(duh-- surprise) but the lesson is in fact more general.  Even
>both carotids will not be enough for most people, judging by both
>Alcor's dog work and the known facts of human neurovascular
>anatomy.

It appears to me that Mike and Steve are contradicting each other on a
couple of important points. Leaving aside the possibility of
atherosclerotic blockage of the neck vessels, the preceding quotations
from Mike and Steve appear to differ as follows:

(1) Does carotid perfusion have any path to reach the entire brain? Steve
says that the Circle of Willis is intact in only 25% of people and that
"for those people without intact or fully patent Circle of Willis
connections, the hind-brain is not going to perfuse at all through the
carotids." On the other hand, Mike says "In most people the carotids
provide plenty of flow, in fact you could probably perfuse a person's
entire brain adequately through *one* carotid."

(2) Even if a patient has a complete Circle of Willis then some proportion
of the perfusate will not reach the brain because it will be shunted away
via the vertebrals. Is this important? Mike says that this loss "is not
normally a limitation" while Steve says "you run the risk that blood
forced up the carotids will mostly escape down the vertebrals into the
body without perfusing the brain (after all, why should blood go through a
5 or 10 micron wide capillary when it can go retrograde down a major
artery instead?)."

I would cheerfully concede that my reasoning on this subject was incomplete
- thanks for describing the other reasons why perfusion is done via the
chest, Mike. However, in light of the apparent contradictions in the expert
opinions expressed above I am still left with my (conservative) conclusion
that perfusion via the carotids alone should be considered only as a
last resort for situations where washout and prompt shipment to a perfusion
facility is impossible. I should add that you are the only expert I have
heard suggest carotid perfusion as a first choice, whether for remote cases
or any others.

I should also add that these choices are not mine to make, of course. The
suspension of an Alcor member here will be under the direction of Alcor's
Suspension Services Manager, who may actually be present when legal death
occurs or soon thereafter. I am simply the local person who has been
designated as a member of the Alcor transport team, to notify Alcor and
follow their instructions.


Long life,
Robert

Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=2900