1002 
ANIMAL DISEASES 
the nature of the genetic influence is ill-defined. 
Although not ideal, the dog is eminently better 
suited for genetic investigation than man and it 
can be anticipated that the further study of 
strains of dogs having a high incidence of spe- 
cific cardiovascular defects will contribute sub- 
stantially to our understanding of the nature 
and mode of action of the genetic factors under- 
lying congenital heart disease. 
The second important implication is that dogs 
with predictable cardiovascular defects can now 
be produced for a variety of research purposes 
by selective breeding. They will be particularly 
useful in studies of the pathogenesis of cardio- 
vascular malformations, since embryos with 
known defects can be obtained at any desired 
stage of development and used to investigate 
the tissue, cellular, and biochemical disturb- 
ances which lead to abnormalities of form. Pre- 
viously, embryologic studies have been largely 
confined to normal embryos, or embryos in 
which defects were induced by exposure to var- 
ious environmental insults. Procedures of this 
type usually lack the specificity necessary for 
the study of a particular type of developmental 
defect. 
As was shown in congenital incomplete right 
bundle branch block and the Wolff'-Parkinson- 
White syndrome, canine models may also be 
helpful in investigating the physiologic de- 
rangements which accompany congenital car- 
diovascular defects, particularly where they 
cannot be studied in human patients. Further 
examples include the study of cardiac activation 
in dogs with right ventricular hypertrophy ac- 
companying congenital heart disease, and 
the investigation of coronary blood flow altera- 
tions in dogs with naturally occurring subaortic 
stenosis. In the latter case, dogs with heredi- 
tary subaortic stenosis were studied over a pe- 
riod of weeks after implantation of blood flow 
transducers, providing the only evidence yet 
available regarding the chronic effects of aortic 
stenosis on phasic coronary blood flow patterns. 
Although they have not yet been utilized for 
this purpose, dogs with congenital cardiovascu- 
lar defects could be valuable in the development 
and testing of surgical procedures aimed at the 
treatment of cardiovascular malformations. 
They would provide a far more realistic me- 
dium than normal animals or animals in which 
anatomic defects have been surgically created. 
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