J. N. ROSS, JR., J. M. FUQUA, JR., B. S. RUARK AND J. H. KENNEDY 
423 
ticularly fat, thick and inelastic chest wall and 
this, coupled with the relative smallness of the 
thoracic cavity of the pig in relation to the over- 
all size of the animal, makes thoracic exposure 
in this species difficult.^* The pony's chest wall is 
relatively thick and may present some difficulty 
with rib retraction. Thoracic exposure presents 
no particular difficulties in the primate, dog or 
ruminant; however, among the latter, the au- 
thors have found the calf superior to both the 
goat and sheep. 
The obvious advantage of the larger primates 
is their anatomic similarity to man. Blood 
groups in Papio species and clotting factors 
in non-human primates in general^^ are re- 
ported to be similar to man. The pig has been 
frequently suggested as a laboratory animal 
for cardiovascular research largely because its 
coronary blood supply is very similar to the 
human's in anatomy and distribution.^^-^^-®^ 
Whereas all of these species are relatively sim- 
ilar hemodynamically, differences exist in terms 
of the distribution of Purkinje fibers and ven- 
tricular activation process,^^-^*'! and in the anat- 
omy of the great vessels — notably the short 
aortic arch giving rise to a single brachio- 
cephalic trunk, and the persistent vena hemi- 
azygous in ruminants. ^^^^ 
The pig is not well suited for chronic studies 
necessitating cardiopulmonary bypass. Our ex- 
perience with the pig has been similar to others 
in that total body perfusion has been consist- 
ently fatal within 8 hours after CPBP opera- 
tions in this species.*^'^* Their poor tolerance to 
CPBP is characterized by rapid and progressive 
right heart distension associated with post- 
perfusion pulmonary insufficiency similar to 
that described in man-^"^ The pig may be a 
promising model for the study of this syndrome. 
The primate and calf^s.sT.ss tolerate CPBP 
well, and are preferred over the goat, sheep or 
dog for this purpose. The authors have no 
experience with CPBP in the pony or goat and 
no literature was found regarding total body 
perfusion in these species. 
The friability of the cardiovascular tissues in 
the pig,39.44 sheep,<'4.io4 ^nd dog,i9.32,33 makes 
these species less suitable than the calf and pony 
in experimental cardiovascular surgical pro- 
cedures. 
Postoperative management of primates can 
be particularly difficult because of their mobility 
and manual dexterity, and this may be a de- 
terrent to their use in studies necessitating 
chronic hard wire instrumentation and/or 
mechanical circulatory assistance (MCA). Sim- 
ilarly, dogs are basically undesirable for experi- 
ments requiring chronic transcutaneous con- 
duits due to their tendency to bite, chew, 
scratch and in general disrupt external leads 
or assist devices. Considerable difficulty may 
be encountered in managing the pony in the im- 
mediate postoperative period of anesthetic re- 
covery. All species present postoperative prob- 
lems. In our experience, the calf presents fewer 
difficulties than any of the other species. 
The calf and pony are relatively easy to man- 
age and seem to tolerate chronic instrumenta- 
tion well.^'*'^^ The authors find the pig, primate, 
and to some extent the dog, to be relatively un- 
manageable in studies involving prolonged me- 
chanical circulatory assistance and instrumen- 
tation. 
The equine species has historically contrib- 
uted much to our knowledge of the cardiovas- 
cular system and circulation,"^-^*'^ but it has been 
only recently that the pony has been adapted 
for routine use in a laboratory setting.^^-^^-^'^-^^ 
In general, the pony's potential as an experi- 
mental animal for cardiovascular surgery is 
appealing, but further studies are needed to 
define its tolerance to CPBP and prolonged me- 
chanical circulatory assistance or replacement. 
The calf has been frequently used and has sur- 
vived longer than any of the other species dur- 
ing prolonged mechanical circulatory assistance 
and replacement.8'S3.94,io9,iio 
Unquestionably, dogs have been used more 
than the other experimental animals, and an 
abundance of well documented surgical and 
physiological data has accumulated to serve as 
a firm basis for comparative investigation. Be- 
cause of the difficulties encountered with cardio- 
pulmonary bypass, transcutaneous instrumen- 
tation and restraint, availability in large size, 
tolerance to chronically applied circulatory as- 
sist methods or devices and spontaneous dis- 
eases such as heartworm infestation and homol- 
ogous blood reactions,^^'^* the use of other 
species seems appropriate. To date, the calf 
