ROBERT L. HAMLIN AND C. ROGER SMITH 
613 
species is activated with a burst of cancelling 
activity, it is electrocardiographically silent. 
Therefore, disease — including gross enlarge- 
ment — of either ventricle is obscured. One in- 
teresting feature of the these species,^^ hov^^ever, 
is that the left ventricular epicardium may be 
"dePurkinjenated" by producing ischemia of 
that region. Thus, the goat may be converted 
into a dog or man by merely removing the addi- 
tional Purkinje tissue with which he was en- 
dowed by his species superiority. We may speak 
of this as superiority because his complete pene- 
tration of Purkinje tissue expedites the activat- 
ing stimulus to all regions of the heart without 
need for much myocardial to myocardial acti- 
vation; plus the fact the goat cannot develop 
bundle branch block because of the previously 
mentioned anastomoses between the main 
bundles. 
As mentioned previously, differences in the 
form of body surface potentials between in- 
dividuals of the same species or different species 
may arise not only from differences in cardiac 
activation processes, but also from differences 
in the spatial orientation of the heart within 
the torso. Thus, the ventricular activation proc- 
ess of two animals may be identical, but QRS 
from identical leads in each animal may vary 
markedly. 
We may presume that the ventricular activa- 
tion process of man and dog are similar; yet 
QRS's from leads I, II and III in man are dif- 
ferent in configuration than from the same 
leads in the dog. Previously, we have heard of 
the dog heart as being similar to man's only 
electrically vertical ; since, if QRS's in leads I, 
II and III in man appeared as they do in a dog, 
normally, the man would be considered as hav- 
ing an electrically vertical heart. 
This is deceptive nomenclature, however, 
since it infers that when the human heart lies 
vertically within its torso, radii drawn to the 
epicardlal perimeter of the heart from lead aVp 
would subtend a portion of that heart which 
would be similar to that subtended in a like 
manner from lead aVp in the dog. As shown 
(Figure 8), this is untrue. Indeed, lead aVp of 
man "looks at" the left ventricular apex ; while 
the same lead, in the dog, "looks at" the left 
ventricular free-wall. What is termed the an- 
terior (or truly the ventral) surface of the 
human heart is, for the dog, the sinistrad sur- 
face. A frontal (or ventral) cross-section cut 
through the human heart parallels, more closely 
in the dog, a horizontal or coronal section. A 
frontal (ventral) cross-section cut through the 
dog heart parallels, more closely in man, a 
horizontal section. 
Another factor of importance in attempting 
to draw analogies between electrocardiograms 
for man and dog is the dependency, in the dog, 
on the position of the thoracic limbs with re- 
spect to the torso. Anatomical peculiarities to 
quadrupeds dictate that both thoracic limbs 
must be placed so that the humeri arise at a 
near right angle with the long axis of the torso. 
This is particularly important for the left tho- 
racic limb ; for it is pulled caudad, the form of 
QRS in avL may change from the normal Qr 
to an atypical qR. 
It should be considered, also, that lead I in 
man, which measures the potential difference 
between two thoracic limbs which attach to the 
torso at least near the geometrical X axis of 
the torso, has no simple analogue in the dog. 
For both of dog's thoracic limbs attach to the 
torso far craniad to the heart. Thus unipolar 
electrocardiograms from both thoracic limbs 
appear nearly identical in configuration because 
both limbs are more to the front of the heart 
than either is to the left or to the right of the 
heart. 
For this reason, when a dog develops even 
massive left ventricular enlargement, QRS in 
all leads may appear normal. In man, when the 
left ventricle enlarges it causes the electrical 
axis to shift leftward and craniad; while, for 
the dog with left ventricular enlargement, the 
electrical axis remains oriented caudad. The 
voltages may be augmented in leads monitoring 
the cranio-caudal axis of the torso (II, III, and 
aVp), but such wide variations in these voltages 
occur normally, one is hard pressed to differen- 
tiate normal from abnormal. 
With use of either additional thoracic leads ^'^ 
or a corrected orthogonal lead system such as 
the McFee axial,^^ the electrocardiogram may 
provide more useful information about left ven- 
tricular enlargement than can be obtained 
currently. 
