D. F. PATTERSON 
997 
non contractile wall structure of the aorta to an 
increasing segment of the ductus arteriosus, 
progressively impairing its capacity to undergo 
physiologic closure. The developmental mecha- 
nisms involved are as yet unexplored. Abnormal 
partitioning or differential growth of the aortic 
arch system may be involved. 
CONGENITAL DEFECTS IN CARDIAC 
ACTIVATION 
In man, a number of abnormalities in cardiac 
activation may occur in the absence of any ob- 
vious gross anatomic abnormalities of the 
heart. They include incomplete and complete 
right bundle branch block, various degrees of 
atrioventricular block, and the Wolff-Parkinson- 
White syndrome. These abnormalities have also 
been observed in dogs, providing an opportunity 
to investigate their etiology, pathogenesis, and 
the relationship between the alteration in car- 
diac activation and the body surface electrocar- 
diogram. 
Incomplete Right Bundle Branch Block (IRBBB) 
This electrocardiographic pattern occurs in 
association with a variety of congenital and ac- 
quired cardiac lesions and occasionally is ob- 
served in patients who have no other evidence 
of cardiac abnormality. As the name IRBBB 
implies, the underlying disturbance has gener- 
ally been assumed to be delayed activation of 
the right ventricle owing to slowed conduction 
along the right bundle branch or periphfiral 
Purkinje conduction system. Recently, IRBBB 
(Figure 4) was discovered in a family of dogs 
resulting from an experimental mating between 
a Beagle with valvular pulmonic stenosis and a 
Beagle with a small subaortic interventricular 
septal defect (Figure 5). All seven members of 
the Fi generation had varying degrees of the 
IRBBB pattern but no other clinical, hemody- 
namic or postmortem abnormalities. When 
mated inter se, these animals produced an F^ 
generation in which ventricular septal defect and 
valvular pulmonic stenosis occurred separately 
and together, as well as dogs with IRBBB but 
no obvious gross anatomic lesions (Figure 5) . 
Electrophysiologic studies in six members of 
the Fi generation as well as two unrelated dogs 
with the IRBBB pattern showed that the elec- 
trocardiographic abnormality is associated with 
delayed activation of the surface of the right 
ventricle.** Epicardial "break-through" of right 
ventricular activation in the outflow tract re- 
gion was delayed beyond any points on the sur- 
face of the left ventricle, and occurred at the 
time of the abnormal rightwardly directed ter- 
minal QRS forces. Conduction velocity down the 
right bundle branch was normal. When intra- 
mural activation of the right ventricular wall 
was studied using plunge electrodes, it was 
found that the wave of activation was not de- 
layed in reaching the endocardium of the right 
ventricle, but that spread through the right 
ventricular free wall in the region of the out- 
flow tract was prolonged due to a localized in- 
crease in wall thickness. 
The IRBBB pattern in this case was shown to 
result not from any abnormality in conduction 
through the right bundle branch, its subdivi- 
sions, or the peripheral Purkinje network, but 
from what appears to be a genetically deter- 
mined focal variation in right ventricular wall 
thickness. It is of interest that no accompany- 
ing hemodynamic or structural abnormalities 
could be detected which would account for the 
hypertrophy. The localized nature of the thick- 
ened area is significant, for it points out that 
prolongation of the intramural spread of acti- 
vation in only a small region of the right ventri- 
cle can result in changes in phase relationships 
of a sufficient degree to markedly alter the body 
surface electrocardiogram. 
The Wolff-Parkinson-White Syndrome 
The Wolff-Parkinson- White syndrome is an 
electrocardiographic abnormality characterized 
by an abnormally short P-R interval and a wid- 
ened QRS complex having a slurred initial por- 
tion (delta wave). Affected individuals are 
prone to attacks of supraventricular tachycar- 
dia. It has long been known that the anomalous 
electrocardiographic pattern in this syndrome 
is the result of an accessory atrioventricular 
(AV) conduction pathway which allows the 
wave of atrial depolarization to enter the ven- 
tricle at an abnormal site, prior to arrival of the 
