A. F. ALEXANDER, C. S. CARD, R. S. JAENKE, J. L. HICKS, AND D. H. WILL 
197 
350 
300 
8 
I 2501- 
fa « 200 
Is 
8 Z 
> 8 150- 
I 100 
a. 
50 
I20 open 5,000 
lO^k closed 15.000' 
10 
20 
30 40 
50 60 
Mean Pulmonary Arterial Pressure 
mm Hg 
Figure 5. — Pulmonary vascular reactivity at 5,000 feet 
and after 2 hours at 15,000 feet; third year susceptible 
and resistant calves. 
mean pulmonary arterial pressures and vascu- 
lar resistance at both altitudes (Figure 5) . 
Five calves at the extreme ends of the re- 
sponse curve for both groups were selected for 
further study; i.e., the five calves of the suscep- 
tible group that displayed the greatest rise in 
pulmonary arterial pressure and vascular re- 
sistance, and the five calves of the resistant 
group which showed the least rise in these pa- 
rameters. At nine months of age these calves 
were again studied at 5,000 feet altitude and 
after two hours at a simulated altitude of 
15,000 feet in the hypobaric chamber to test 
whether the initial results were repeatable.^^ 
Again a striking difference in pulmonary vascu- 
lar reactivity to hypoxia between the two 
groups was observed (Figure 6). Although the 
reactivity of the susceptible calves had de- 
creased somewhat with age, the vascular resist- 
ance and the mean pulmonary arterial pressure 
was still significantly higher than in the resist- 
ant calves. 
The significant differences in apparent pul- 
monary vascular reactivity to both acute and 
chronic hypoxia in both generations of calves 
lends support to the hypothesis that susceptibil- 
ity to hypoxic induced pulmonary hypertension 
in cattle is at least partially heritable. Heritable 
factors probably exist in most all diseases and 
account for much of biologic variations ob- 
served. Inherited resistance and susceptibility 
to infectious diseases has been well demon- 
strated." Of particular interest, however, in 
the altitude susceptible and resistant calves 
is the fact that the apparent genetic in- 
fluence was well expressed in the initial 
offspring and did not require generations of ge- 
netic concentration. 
The reasons for the differences in response 
between the two groups of cattle remain un- 
clear. A stepwise regression analysis showed 
that pulmonary vascular resistance values ob- 
tained at 60 days of age at an altitude of 5,000 
feet served as the best single criterion (r = 
0.632) for predicting adaptive failure, i.e., high 
mountain disease in animals taken to high alti- 
tude. 
Although the exact mechanism of the in- 
creased pulmonary vascular resistance in cattle 
as well as other animals and man has not been 
defined, it is generally accepted that the effec- 
• 
u 
c 
o 
t « 
■i " 
8 " 
o 
E 
-350 
300 
250 
200 
150 
hioo 
50 
O • S open 5,000' 
A AR closed 15,000' 
(5 animals ea. group) 
10 20 30 40 50 60 
Mean Pulmonary Artery Pressure, 
mm Hg. 
70 
Figure 6. — Pulmonary vascular reactivity at 5,000 feet 
and after 2 hours at 15,000 feet; susceptible and re- 
sistant calves ; 5 and 9 months of age. 
