818 
ANATOMY AND PATHOLOGY 
materials or surfaces into extracorporeal shunts 
for purposes of evaluation. Promising mate- 
rials for cardiac valve surfaces have generally 
been assessed by intracardiac implantation at 
the specific valve site projected for their use^^" 
w^hile new^ly developed biologic surfaces for 
prosthetic devices have often been assessed 
v^hile serving as blood contacting parts and 
lining surfaces for circulatory assist devices. ^^'^^ 
Finally, resort has also been made to the use 
of ex vivo flow chambers and circuits for both 
the appraisal of synthetic surfaces and the 
study of early events in the process of throm- 
bosis.2^'2* 
It is noteworthy that in all of the systems 
cited above primary emphasis has been placed 
upon the amount of thrombotic deposit encoun- 
tered in situ upon the surface of the implant 
under study. Embolic phenomena generally 
have not been investigated in detail. Indeed 
most of the systems referenced above appear to 
be less than optimally suited for the detection 
of embolic episodes. This is due to the fact that 
in these preparations emboli of implant origin 
are not obligatorily channeled to receptor vas- 
cular beds which represent satisfactory biologic 
indicator systems for the consistent documenta- 
tion of embolic episodes. Minute emboli are 
particularly difficult to localize and identify 
within the peripheral vascular channels of most 
systemic organs. Lessor embolic episodes are 
also difficult to document within the pulmonary 
vascular bed, due first to the dual nature of the 
pulmonary circulation which protects the lung 
against embolic infarction,^^ and secondly to 
the action of the host fibrinolytic system which 
tends to bring about rapid dissolution of pul- 
monary emboli. 
PATHOPHYSIOLOGICAL BASIS FOR AN 
IN VIVO EMBOLUS TEST SYSTEM 
It has long been recognized that the tissues 
of various organs differ markedly with respect 
to their susceptibility to embolic infarction. 
Those tissues which possess extensive collateral 
circulation are relatively resistant to infarc- 
tion, while those tissues which possess an end 
organ circulatory pattern characterized by little 
or no collateral circulation have been known 
to be unusually susceptible to embolic infarc- i 
tion. This circumstance is due to the fact that 
tissue perfusion largely ceases in a given mass 
of tissue with little or no collateral blood sup- I 
ply, when an occlusive embolic particle lodges 
within its definitive artery of supply. Ischemic 
necrosis or infarction generally follows, and 
leaves a permanent anatomical marker of the 
embolic event. The pathophysiological relation- 
ships which exist in tissues possessing such an 
end organ circulatory pattern provide the basis 
for their use in animals as in vivo models for 
the detection of prosthesis-induced embolic 
events. Thus, if a given prosthetic surface is 
positioned in the path of blood which obligato- 
rily courses through an organ possessing an end 
organ circulatory pattern, emboli arising from 
the surface of the implant will be entrained 
within the blood stream, cause subsequent arte- 
rial occlusion, cessation of adequate perfusion 
and the production of tissue infarction. The 
latter will remain as a permanent marker of 
the embolic event. 
RENAL EMBOLUS TEST SYSTEM 
Design, Structure and Operation 
of the Model 
The relationships cited above have been uti- 
lized in the development of an in vivo model for 
the detection not only of thrombotic deposit ac- 
tually present upon the surface of a prosthetic 
implant, but also of embolic phenomena origi- 
nating at the implant site. The canine renal 
vascular bed and tissues were selected as an 
indicator system for the identification of em- 
bolic events, because of the known vulnerability 
of these organs to embolic infarction. They 
represent organs which readily display even 
minute infarcts and can be conveniently ex- 
amined in great detail by conventional patho- 
logical techniques. ^''^ 
The surface or material to be evaluated, pre- 
fabricated as a cylindrical ring implant with 
carefully beveled edges, is inserted into the 
abdominal aorta immediately above the origin 
of the renal arteries. The aorta is then sub- 
totally constricted (2-3 mm residual lumen) 
below the level of the renal arteries to divert 
