196 
PULMONARY MODELS 
600 
'e 
0 500 
* 400 
1 300 
■S" 200 
(r. 
> 
Q. 
100 
60 
I 
50 
E 
40 
E 
30 
< 
Q. 
20 
10 
• OS GROUP, OPEN 5,000' 
* GROUP, CLOSED 15.000' 
o~.. 
o- • 
ti — 
10 
30 60 
AGE IN DAYS 
90 
Figure 3. — Pulmonary arterial pressure and pulmon- 
ary vascular resistance of susceptible and resistant 
calves at 5,000 feet and after 2 hours at 15,000 feet 
altitude. 
the major reason for the greater hypertension 
in the susceptible animals (Figure 3) . 
When taken to the natural altitude of 10,000 
feet a similar but more dramatic difference in 
pulmonary vascular reactivity was observed be- 
tween the two groups (Figure 4). A progres- 
sive rise in mean pulmonary arterial pressure 
was observed in both groups but a much greater 
response was present in the susceptible animals. 
Clinical signs of right heart failure occurred be- 
tween 64-122 days at altitude in all but one of 
the susceptible animals and in none of the re- 
sistant animals. All of the affected animals were 
removed to 5,000 feet to recover with the first 
definitive signs of heart failure. The remaining 
susceptible calf developed signs of impending 
heart failure at 150 days at which time this ani- 
mal and the entire group of resistant animals 
were returned to 5,000 feet. In contrast to the 
susceptible group, 9 of the 11 resistant animals 
developed only moderate levels of pulmonary 
hypertension at 10,000 feet, whereas the re- 
maining two animals in this group showed sub- 
stantial pulmonary hypertension but no evi- 
dence of congestive failure. 
In an attempt to concentrate the genetic pool 
that might be responsible for resistance and 
susceptibility, parent females were bred to first 
generation males that were at the most extreme 
ends of the response curve for their respective 
groups; i.e., two males which under the chal- 
lenge of altitude developed the most severe pul- 
monary hypertension were bred to susceptible 
group females while the two males which 
showed the lowest pulmonary vascular resist- 
ance were bred to the resistant group females 
(Figure 2) . From these matings 12 susceptible 
and 10 resistant calves were produced. 
Cardiorespiratory studies were carried out 
when the animals were five months of age at 
5,000 feet altitude and in the hypobaric cham- 
ber after two hours at 15,000 feet.^^ Significant 
differences in pulmonary vascular reactivity 
were present between the two genetic groups 
with the susceptible calves showing greater 
/id HMD) 
(2fHMD) 
FIRST YEAR 
PROGENY 
•-S group(8) 
▲-R group(ll) 
4 16 30 64 78 106 124 172 
DAYS AT 10,000 FEET 
Figure 4. — Mean pulmonary arterial pressure in resist- 
ant and susceptible calves chronically exposed to 
10,000 feet natural altitude. 
