644 
PHYSIOLOGY 
US in his "Isorhythmic A-V Dissociation," that 
the position of the P wave, or atrial contraction, 
is of major importance. If the atrium contracts 
prior to the ventricle, the BP increased to 
nearly double the value than if the contraction 
occurred after the QRS complex. Yet, in your 
carefully controlled study a BP comparison of 
sequential versus ventricular pacing showed no 
significant difference between both situations. 
Would you please comment on this ? 
Dr. Hawthorne: Well, I don't really know 
the answer to that. I think that one has to deal 
with the concept of what he's trying to do when 
constructing the ventricular function curve. All 
we really wanted to do is to change the end 
diastolic size. It doesn't matter what happens, 
so long as you do not get significant mitral re- 
gurgitation so that you really can't estimate the 
stroke volume. In all of these situations where 
there was a bizarre relationship between the 
atrial contraction and the ventricular contrac- 
tion, the alteration was such that the atrium 
was contracting against the closed mitral valves 
so that it did not affect our data. I think that I 
would, as a physiologist, have to go with se- 
quential pacing because that's nearer the nor- 
mal in a man. 
John Kennedy, Baylor College of Medicine, 
Houston, Texas: I rise to compliment the es- 
sayist on this really very nicely worked out ex- 
perimental series and ask if he would hazard a 
clinical extrapolation of these data? Occasion- 
ally, patients with symptomatic bradycardia in 
whom a trial of pacing is used prior to implan- 
tation of a permanent pacemaker exhibit car- 
diac output falls. I wonder if you can comment 
on that in light of your own experimental ob- 
servations ? 
Dr. Hawthorne: I would guess that if you 
are doing ventricular pacing, you do not get the 
assist from the increase in myocardial contrac- 
tility, and that might create a problem in terms 
of cardiac output. I've not thought about that. 
One says that the cardiac output does not 
change very much over this wide range, and 
we've found in these data that it really doesn't. 
The cardiac output stays pretty level over this 
range of pacing. I think we were more con- 
cerned with whether or not you could use the 
curve at all. I really don't know how to answer 
your question. 
K. L. Gould, University of Washington, 
Seattle: Your last thoughts included measure 
of wall force. What did you actually measure — 
tension or wall stress? And, if you measured 
stress, you would need wall thickness. How did 
you get that in your very nice preparations? 
Dr. Hawthorne: Fantastic that you'd ask 
me that question. I measured wall force, if you 
agree with me that wall force would be Pi times 
the internal radius, times the pressure, times a 
conversion factor for pressure, which is 1.36. 
Wall force I think is defined that way. We were 
very careful not to indicate that we were meas- 
uring stress. We had no measurements of thick- 
ness. We believed that one would have to deal 
with midwall tensile stress which would require 
that. It would also require you to make other as- 
sumptions about geometry. We don't have to 
make any assumptions if we measure wall 
force. As far as I know, the way that hoop 
stress, midwall stresses of various kinds, and 
wall force all vary together, it is very unlikely 
that one would see different data if you had 
measured it any way. 
K. Sagawa, Johns Hopkins, Baltimore, 
Maryland: Did you do something about reflex- 
ing? If not, the direct receptor reflex does not 
seem to do much, if your ingestation is right. 
Dr. Hawthorne : You're absolutely right. It 
doesn't seem to do much. I think that if I re- 
member Dean Franklin's paper at the American 
Heart Meeting, he would agree that it doesn't 
do much in the conscious dog. His statement is 
that the carotid mechanoreceptors have their 
primary effect on peripheral resistance. But I 
don't think that is all of the story. Dr. Sagawa. 
I think that here we're dealing with something 
which is very curious and has a lot more to it. 
Theoretically, when you get a pacing rate up to 
3.5 Hertz, one should at least see the phenome- 
non. Curiously, pacing the ventricle does not 
show that. I'm not prepared at this moment to 
deal with that, but we are trying to work on it. 
Chairman Hamlin: Did you change the 
pacing rate suddenly or gradually, and did you 
see any effect when you changed it gradually? 
Dr. Hawthorne: We monitored these dogs 
over anywhere from ten seconds to five minute 
