COMPARATIVE ELECTROCARDIOGRAPHY 
Robert L. Hamlin and C. Roger Smith* 
Clinical and experimental observations indicate that 
the electrocardiogram has serious limitations in studying 
abnormalities of the heart in certain species (bovine, 
porcine, equine) in which inordinate degrees of cancel- 
lation occur during ventricular depolarization. In these 
species with a more complete penetration of Purkinje 
fibers throughout both ventricular free-walls, even 
massive ventricular hypertrophy or dilatation may pro- 
duce no or very minimal alterations in body surface 
potentials; while production of massive zones of 
ischemia of the left ventricular free-wall merely alters 
QRS vectors to appear like normal vectors in mammals 
which do not have such complete penetration of Purkinje 
fibers. 
If, in the pathogenesis of disease, alterations in ven- 
tricular activation are important, then animals with 
complete penetration of Purkinje fibers throughout both 
ventricular free-walls should not be used as models. For 
this reason, no matter how severe the heart lesion, ven- 
tricular activation may be changed only minimally. On 
the other hand, these species may serve as useful models 
to study the effects of lesions independent from changes 
in ventricular activation. 
Larger species (e.g., equidae) are of interest because 
of their propensity for developing atrial arrhythmias — 
in particular, atrial fibrillation — because of the absolute 
greatness of their atrial mass; while smaller species 
(e.g., small dogs) are of interest because they may have 
most severe atrial disease without developing atrial 
fibrillation. 
Another interesting aspect of comparative electro- 
cardiography lies with phenomena considered normal 
in certain mammals (e.g., second-degree A-V block and 
P-mitrale in equidae, pronounced sinus arrhythmia in 
dogs, wandering atrial pacemakers in dogs and horses) 
but considered to be abnormal in man. 
Differences exist in postures within the thorax of 
hearts of quadrupeds compared with hearts of primates ; 
emphasizing that, even though cardiac activation may 
be identical in certain quadrupeds and primates, body 
surface potentials of one may be grossly different from 
surface potentials of the other. 
INTRODUCTION 
To make optimal use of the electrocardiograph 
for monitoring electrophysiologic properties of 
* Department of Veterinary Physiology & Pharmacology, College 
of Veterinary Medicine, The Ohio State University, Columbus, Ohio 
43210. 
the heart, the investigator should be familiar 
with the normal electrocardiogram of the spe- 
cies used in the investigation. Too often an in- 
vestigator will interpret an electrocardiogram 
obtained from one species using criteria estab- 
lished for another. 
If, for example, one interprets the electro- 
cardiogram from a normal horse, dog, cow, or 
sea lion, using criteria established as either 
normal for man or as indicative of certain ab- 
normalities in man, one will claim the horse 
has left atrial enlargement because of the P- 
mitrale,^'2 the dog has left ventricular hyper- 
trophy because of the ST-T contour,^ the cow 
has either right bundle branch block or right 
ventricular hypertrophy because of orientation 
of mean and terminal vectors of QRS,*-^ and 
the sea lion has atrial injury because of the PQ 
segment deviation.^ 
DISCUSSION 
Variations in the form of the electrocardio- 
gram among the various species might arise 
for two reasons: either the cardiac activation 
process varies, or the position of the heart 
within the torso varies. 
We would like to present examples of these 
two determinants and to classify'^ many do- 
mestic mammals according to the form of QRS. 
Components of QRS are produced by uncan- 
celled boundaries between electrically active 
and resting ventricular myocardium. Thus, the 
time-course of ventricular depolarization deter- 
mines the form of QRS in any lead. That time 
course is determined by the distribution of 
Purkinje tissue within the ventricular mass. 
Thus, we proposed that differences in form of 
QRS reflect differences in the distribution of 
the Purkinje fibers in ventricles of the various 
individuals of each category of mammal. 
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