W. W. WAGNER, L. P. LATHAM, AND D. R. HALSEY 
257 
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torn. If this were the case, then the level of re- 
cruitment should correlate with the magnitude 
of the rise in Ppa. Such is not the case (see Fig- 
ure 4), since recruitment is independent of the 
level of change in Ppa. This conclusion does de- 
pend on the accuracy of the grid counts and, 
therefore, should be viewed with caution. How- 
ever, because there is some recruitment at the 
bottom of the lung with hypoxia, precapillary 
shifting of the blood from the bottom to the top 
is an unlikely explanation for recruitment. 
Pulmonary venoconstriction is left as the 
most probable explanation. There is substantial 
evidence that hypoxia causes constriction of the 
pulmonary veins in the dog; such varied tech- 
niques as dimension transducers implanted on 
lobar veins, ^- pulmonary venography,^^ and 
various hemodynamic measurements^^-^^ have 
been used to demonstrate this phenomenon. 
We conclude, therefore, that pulmonary veno- 
constriction is probably an important factor 
in causing capillary recruitment with hy- 
350 
Figure 2. — Capillary grid count data. Each bar repre- 
sents counts made on the same field during control 
and hypoxia. 
account for the capillary recruitment we have 
observed with hypoxia : ( 1 ) changes in cardiac 
output, (2) the rise in Ppa, and (3) pulmonary 
venoconstriction. 
Recruitment could be explained by an in- 
crease in cardiac output. This possibility has 
been examined by plotting the increase in capil- 
lary counts with hypoxia against the change 
cardiac output caused by hypoxia. The results 
are shown for the top of the lung in Figure 3. 
There is no correlation since recruitment oc- 
curred in all cases regardless of the direction of 
change in cardiac output with hypoxia. This con- 
clusion is probably valid because it depends 
more on the trend of the capillary counts rather 
than on the accuracy of grid counts per se. 
A second explanation is that the rise in Ppa 
could partially overcome the hydrostatic perfu- 
sion gradient and shift blood from the bottom 
to the top of the lung thereby causing recruit- 
ment at the top and derecruitment at the bot- 
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% CHANGE IN CARDIAC OUTPUT 
Figure 3. — Hypoxia produces changes in both capil- 
lary counts and cardiac output. In 5 cases cardiac 
output fell during hypoxia but recruitment still oc- 
curred indicating that increased capillary perfusion 
was not caused by cardiac output alterations. Data 
shown in this figure are from top of lung. 
