M. N. LEVY 
663 
equivocal. We did, of course, use some pharma- 
cological interventions in the animals, but even 
there the results were not too good. In most of 
the experiments, if we cut the vagi and removed 
the stellates, we would block synchronization. 
In a little more than half of the experiments, we 
were not able to produce synchronization. But, 
in some experiments we were able to get syn- 
chronization even when the vagi and stellates 
were cut. I might emphasize that whenever we 
held blood pressure constant, we were never 
able to produce synchronization. In a little less 
than half of the cases, if we did an acute dener- 
vation of the heart, we could get some syn- 
chronization. It was very, very touchy. We had 
to adjust the pacemaker in the ventricle, or the 
frequency of the pacemaker, very, very care- 
fully to get it. 
You may, or may not, be familiar with the 
observation that was reported by both Hashi- 
moto and Chiba and by James, that if you 
change the pressure in the S-A node artery, the 
heart rate changes inversely as a function of 
the pressure in the S-A node artery. So, appar- 
ently there is a sort of localized barorecep- 
tor reflex within the S-A node itself. And this 
probably accounts for the synchronization even 
in the absence of the intact reflex pathway. We 
tried to use propanolol to block the sympathet- 
ics in some of our experiments, but that was un- 
satisfactory because the heart rate wouldn't 
stay constant enough. That is, we would get a 
gradual change in heart rate. The animal would 
then go into synchronization, but pretty soon 
because of this gradual shift in heart rate, it 
would go out of synchronization. So we found 
that surgical interruption of the reflex paths 
for our purposes was more satisfactory than 
using pharmcological agents. 
Walter Ehrlich, Johns Hopkins, Baltimore, 
Md. : Would you explain how you produced the 
heart block? Did you cut, or was the procedure 
acute? Was it performed on an opened chest 
dog? 
Dr. Levy: We pushed the wall of the right 
atrium over until it was right up against the in- 
ter-atrial septum, and then we advanced a small 
syringe needle into the interatrial septum and 
injected about 0.2 cc. of 95% alcohol until we 
produced a complete acute heart block. Yes, it 
was an open chest dog. 
