124 
CARDIAC MODELS 
Figure 6. — Photomicrograph showing the calibre of the intramyocardial septal artery in dog No. 11 (a) and 
branching arterioles (b) in counterpulsated dog No. 8. 
ing lactate but was extracting it from arterial 
blood. 
Comments— Study I 
The chronic coronary insufficiency prepara- 
tion of Litvak and Vineberg ^ served as an ex- 
cellent reproducible ischemic preparation for 
evaluating the effects of counterpulsation on 
coronary collateral circulation. As in acute liga- 
tion experiments, consistent effects on coronary 
blood flow depend upon proper placement of 
constrictors.^'^ The constrictor must be placed 
about the anterior descending artery immedi- 
ately distal to the takeoff of the septal artery 
and at the origin of the circumflex artery. Coat- 
ing the formalized casein constrictor with par- 
affin slows the rate of fluid absorption so that 
when 2.77 mm. I.D. constrictors are used in 20 
kilogram dogs, total occlusion of the coronary 
arteries does not occur before one month.^ After 
ten to fourteen days, a relatively stable period 
of coronary insuflficiency is produced despite 
progressive proximal coronary artery constric- 
tion. This basic ischemic heart model can then 
be readily adapted according to experimental 
design to hemodynamic or metabolic studies. 
In Experiment I, 80% of control animals 
succumbed within one month, whereas 80% 
of the treated dogs survived and 60% lived long- 
term despite almost complete proximal occlu- 
sion of the anterior descending and circumflex | 
arteries. The sudden death of control animals 
suggests a terminal fatal arrhythmic episode, 
probably ventricular fibrillation. The explana- 
tion for prolonged survival of treated animals 
was apparent in the postmortem angiograms. 
One must assume, therefore, that potential col- 
lateral channels exist within the heart. Burton 
has shown that vascular resistance in non- 
functional closed vessels is infinitesimally great 
and requires a critical opening pressure to over- 
come the surface and wall tension.^ Increased 
