168 
CARDIAC MODELS 
over to copper wire, and have pretreated with 
heparin, which seems to overcome this critical 
period. Some metals, such as iron, copper and 
similar metals will cause clotting within five to 
ten minutes. However, using a small-diameter 
wire and pretreating again with 4,000 units of 
heparin, we found that we can obtain an occlu- 
sion in two hours. This seems to be a very nice 
model and a fine material, to be used if you are 
particularly interested in fibrinolytic agents of 
one type or another. Magnesium is hard to find. 
In reply to your second question : you merely se- 
lect dogs that have coronary arteries of about 
this size. In dogs weighing 15 to 20 kilograms, 
the wire fits around a 19-gauge needle. It works 
very well. 
