D. F. PATTERSON 
995 
Figure 2. — Ductus diverticulum. TOP: Lateral aortic 
angiogram of a 13-week-old male pup from a cross 
between a male Poodle with a ductus diverticulum 
and a normal female Coonhound. A funnel-shaped 
ductus diverticulum can be seen over the heart base. 
The distal extremity of the diverticulum extends well 
beyond the outer border of the aortic wall. The crista 
reuniens (CR) is formed by the adjacent walls of the 
ductus arteriosus and aorta. BOTTOM : Sagittal sec- 
tion of the aorta (A), ductus diverticulum, and pul- 
monary artery (P) of the same dog. The ductus di- 
verticulum extends beneath the crista reuniens (CR) 
to about one-half the length of the ductus arteriosus. 
Reprinted from reference #5 by permission of the 
American Heart Association, Inc. 
monary vascular resistance favors an increas- 
ing left to right shunt and volume overloading 
of the lungs and left heart, if the ductus arterio- 
sus remains patent. This view is consistent with 
the observations of Rudolph and co-workers, 
who have followed pulmonary arterial and right 
ventricular pressure changes in normal un- 
anesthetized pups from 1 to 36 days of age. Their 
studies show a rapid fall in peak right ventricu- 
lar pressure during the first week, followed by a 
more gradual decrease to adult levels over the 
next 4 weeks. Phillips et al.^^ found a similar 
fall in pulmonary arterial pressures and noted 
that lumen/wall ratios of the small pulmonary 
arteries in pups increased to adult values by 
four weeks of age. 
Pups which developed severe pulmonary hy- 
pertension with right to left or bidirectional 
shunts were distinguishable from other pups 
with PDA by the end of the second week. There 
was no evidence of substantial left to right 
shunting at any time : continuous murmurs ap- 
peared only briefly or not at all, and despite the 
presence of a patent ductus arteriosus which 
approached the aorta in size, signs of left heart 
failure did not ensue. In electrocardiographic 
studies, the "right ventricular hypertrophy" 
pattern seen normally at birth did not evolve to- 
ward the adult pattern of left ventricular domi- 
nance as it did in normal pups and other pups 
with PDA. Cyanosis of the caudal portion of the 
body was visible in some animals. Cardiac cath- 
eterization at 8 to 10 weeks of age revealed pul- 
monary arterial pressures which equaled or ex- 
ceeded aortic pressure throughout part or all of 
the cardiac cycle. The evolution of clinical signs 
suggests that in pups of this type the usual 
postnatal fall in pulmonary vascular resistance 
fails to occur. As has been discussed by Ru- 
dolph, failure of maturation of a low resistance 
pulmonary vasculature in such cases may be the 
result of constant exposure of the pulmonary 
arterial circulation to the high pressure of the 
systemic circuit.^''' Such a condition would be 
expected if the size of the communication be- 
tween the two circulations is sufiiciently large. 
Pathogenesis of Hereditary PDA 
Closure of the ductus arteriosus normally oc- 
curs in two stages. Active constriction of the 
muscular wall of the vessel takes place within 
hours after birth, resulting in 'physiologic clo- 
sure. Contraction of ductal smooth muscle ap- 
