J. ROSENWEIG AND S. CHATTERJEE 
125 
Figure 7. — Photomicrograph (low power) of intramyocardial arteries and arterioles in the left ventricular myo- 
cardium in dog No. 5 (a) and in dog No. 8 (b). 
intraluminal pressure alters the transmural 
pressure and increases the vessel diameter. 
There is a dynamic relationship between flow, 
pressure, volume and caliber in distensible 
blood vessels. Since resistance is related to the 
inverse of the radius to the fourth power 
(Poiseuille's equation), resistance to flow can 
be rapidly diminished by increments above 
critical opening pressure. Beyond a certain 
point, the resistance in small vessels becomes 
constant and the flow is then solely related to 
the driving pressure. 
In the ischemic heart, it is likely that coun- 
terpulsation particularly affects the caliber of 
small arteries and arterioles which communicate 
with distal branches of proximally occluded cor- 
onary arteries. Because of their lack of external 
support, epicardial vessels are especially vul- 
nerable. The diameter of small vessels is in- 
creased and potential non-functioning channels 
are opened. Once collateral flow is established, 
it persists, even after cessation of pumping. 
Flow is then maintained by the pressure grad- 
ient across the ischemic bed and the resultant 
diminished vascular resistance is enhanced by 
the local metabolic milieu favoring vasodilation. 
The angiogram and histologic studies of long- 
term survivors suggest that with passage of 
time, the pattern of collateral circulation be- 
comes prominently established as the new chan- 
nels accommodate to the increased flow. A two 
phased process of transformation of a small 
myocardial arteriole into a functionally efficient 
intercoronary anastomotic artery in the dog 
has been previously described by Schaper and 
Vandesteene.^ 
The collateral circulation which developed 
naturally following ameroid constriction in un- 
treated control animals was insufficient for ade- 
quate protection of the myocardium. The stabi- 
lized, although diminished, distal flow beyond 
the fourteenth day was interpreted as reflect- 
ing maximal natural development of collateral 
circulation in the ischemic heart. However, in 
