204 
PULMONARY MODELS 
Figure 3. — Lucite window in place on support appara- 
tus. 
to surgical technique must be observed to pre- 
vent blood loss and replacement during prepara- 
tion of the model if it is to be valid. 
Trauma to the lung surface during insertion 
of the window must be carefully avoided as this 
caused interstitial edema and even hemorrhage 
which could be interpreted as due to the eifect 
of shock. For chronic animal preparation this is 
doubly important as trauma sets up a pleural 
reaction that prevents later observation. 
Positive pressure ventilation is another factor 
which affects pulmonary blood flow. We have 
observed that respiratory rate, tidal volume, 
end expiratory pressure and ventilatory cycle 
all alfect flow. Increased rates cause decreased 
blood flow. Excessive tidal volumes impede flow 
as well. The most normal flow on positive pres- 
sure ventilation is obtained with ventilatory 
volumes and rates close to the normals set forth 
in The Biological Handbook of Respiration and 
Circulation.^ 
If a phase adjustment is available on the ven- 
tilator it is best to set it so that the ventilatory 
cycle is Vs inspiratory and % expiratory as this 
gives the most normal blood flow. Finally the 
exhalation part of the ventilator should be 
placed under 5 cm of water to maintain a posi- 
tive end expiratory pressure. This is more effec- 
tive at preventing atalectasis and maintaining 
a normal function residual capacity than inter- 
mittant "sighs" of high pressure, and gives less 
change in the pulmonary circulation. Obviously 
the best preparation is a mildly sedated animal 
with a chronically implanted window and breath- 
ing on its own. Despite these considerations we 
wish to underline the suggestion that a careful 
survey of vascular changes always should be 
made in the control period. 
SUMMARY 
The use of the incident-light, dark-field sys- 
tem in conjunction with this newly modified 
thoracic window, permits microscopic study of 
the lung under truly physiologic conditions. We 
have minimized extraneous tissue motion, elim- 
inated the need for additional surgery required 
in transillumination techniques, and still pro- 
vided the light intensity for high magnification 
studies. We feel that this new system is a supe- 
rior method for studying the pulmonary micro- 
structure in the living animal, and particularly 
in the rapidly changing vascular states fol- 
lowing shock. The limitations and precautions 
of the system must, however, be observed if 
valid data are to be obtained. Finally we wish 
to point out that with the incident-light tech- 
nique it is possible to study changes in flow in 
many organs beside the lung. We have been able 
to photograph the microcirculation of the brain, 
liver, kidney, and gut with a satisfactory degree 
of resolution at magnifications to 450X. We 
therefore feel that this system can be recom- 
mended to investigators in a wide area of study 
concerning the response of laboratory animals 
to low-flow states. 
