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HEMODYNAMICS 
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DISCUSSION 
Kenneth L. Gould, University of Washing- 
ton, Seattle, Washington : When you put in your 
internal diameter measuring transducers, you 
have to lead the wires out through the ventricu- 
lar wall. Now, depending on how one does this, 
a scar that forms around the wire may inter- 
fere -ttith the normal thickening of the wall dur- 
ing systole. How do you circumvent that prob- 
lem? 
Dr. Cothran : Yes. We were not able to cir- 
cumvent it. We did use transducers where the 
cross-sectional area of the disc was considerably 
larger than the cross-sectional area of the wire. 
You do get a core in the center of connective tis- 
sue when making these measurements. 
Dr. Gould : Do you think that where the wire 
went through the wall restricted the wall's abil- 
ity to thicken in the internal diameter where 
you measured it? 
Dr. Cothran : In these studies, it may be re- 
stricted in measurements of internal diameter 
to some extent. However, in most of these ani- 
mals, we monitored left ventricular wall thick- 
ness either directly where the gauges on the 
intemal surface and external surface were cou- 
pled by means of an elastic band. We simply 
took precautions to make certain that there was 
no dimpling of the ventricle which would indi- 
cate that they were too tightly coupled. 
Dr. Gould : The second question regards 
thick versus thin wall theories. In dogs, the re- 
sults are very impressive. In humans, where the 
heart is larger, people have shown that if you 
calculate stress as you did, the error is not very 
large, 5 or 15 '^c using the thick versus thin wall. 
In your experience, is this calculation appropri- 
ate for humans since the heart is larger, or is 
this difference primarily a function of size of 
the tissues that we're dealing with ? 
Dr. Cothran: Well, first of all, the error 
which you get I think depends upon who made 
the calculation. I think Dr. Sandler has made 
calculations in humans and has indicated that 
you get something like a 30 to 60% error. Sec- 
ondly, I think it would be dependent upon the 
ratio of wall thickness to internal dimensions. 
Having made no studies in humans, I don't 
know what these values are. But if it exceeds .1, 
then the thick wall theory should be used. 
Dr. Gould: In humans, differences between 
thick and thin walls really aren't very large. It's 
also very interesting that in some 70% of our 
patients we get exactly the same stress curves 
as you do, and the relationship between my- 
ocardial stress and epicardial stress is very sim- 
ilar to yours and, in this sense, very compar- 
able, but this again is using the thin wall theory. 
Would one of you comment comparing dogs and 
humans using the thick versus thin wall theory. 
Dr. Cothran: I think I'll defer to answer 
this. 
Dr. Hawthorne: Hal, do you want to take 
that one ? 
Harold Sandler, NASA-Ames Research 
Center, Moffett Field, California: Your state- 
ment about measurement is indeed true, as long 
as you're dealing with absolutely one species, 
and as long as the people with coronary artery 
disease do not have left ventricular hypertro- 
phy. Because calculations in the hypertrophied 
heart by the thin wall theory are going to be 
wrong, you will get the wrong answers. So it is 
diflicult to say which is the right way. You must 
know the limits under which you're ordinarily 
operating. Some of your questions about the 
techniques that Dr. Cothran was using have 
been covered in papers published at a sympos- 
ium of Federation pro-proceedings a year and a 
half ago. In that sjmiposium two much more ele- 
gant techniques for studying dynamic geometry 
in cardiac systems were presented; one by 
Mitchell, where wet beads were inserted 
through the wall and radiographs were taken, 
