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PULMONARY MODELS 
The most obvious approach for precise micro- 
vascular study is direct microscopic observation 
of the surface of the lung in vivo. Indeed this 
technique has seemed sufficiently promising to 
have attracted investigators since the seven- 
teenth century. Unfortunately the method is 
associated v^^ith so many problems that few 
workers have been successful. The problems can 
be divided into three categories: (1) respira- 
tory and cardiac movement of the surface of the 
lung, (2) maintaining the whole animal and the 
pulmonary observation field in a normal phys- 
iological condition during the experiment, and 
(3) extracting data once microscopic observa- 
tions are feasible. All these problems have to be 
solved satisfactorily before meaningful studies 
can be made. 
The purpose of this paper is to outline a tech- 
nique for in vivo microscopy of the pulmonary 
circulation and to report some preliminary ob- 
servations on the response of the microcircula- 
tion to airway hypoxia. 
MATERIALS AND METHODS 
Young, adult mongrel dogs have been used in 
these studies, because they have a satisfactory 
pulmonary vasopressor response to hypoxia, tol- 
erate the surgery well, and have a thin visceral 
pleura. Anesthesia is induced by intravenous 
administration of sodium thiopental (15 
mg/kg) and is maintained by intravenous 
injection of a-chloralose (60 mg/kg) dissolved 
in polyethelene glycol 200. Following a complete 
forequarter amputation, a thoracic window is 
implanted in the chest wall in place of a re- 
sected portion of the second rib.*' The lung 
under this rib has minimal respiratory move- 
ment. The window is designed to form an air- 
tight seal with the chest wall and permit spon- 
taneous respiration once the pneumothorax is 
aspirated. The air is removed from the chest by 
two concentric suction manifolds that are de- 
signed to arrest all movement of the surface of 
the lung under the window pane.'' 
Catheters are passed via the jugular vein and 
femoral artery into the main pulmonary artery 
and descending aorta, respectively. Blood pres- 
sures are measured by Statham P 23 Db trans- 
ducers energized and monitored by an Electron- 
ics for Medicine DR-8 recorder ; mean pressures 
are obtained electrically. Cardiac outputs are 
determined by indocyanine green dye dilution 
curves via a Waters XP-302 densitometer. Ar- 
terial blood samples are collected anaerobically 
for pH, PCO2, and POo measurements made 
with appropriate electrodes (Radiometer model 
PHM 72). Arterial oxygen saturations are cal- 
culated from the oxyhemoglobin dissociation 
curve corrected for pH and temperature. 
During the control period, the animal sponta- 
neously breathes a mixture of 40 per cent Oo in 
nitrogen. Airway hypoxia is produced by ad- 
ministration of a mixture containing 12 per 
cent Oo, 5 per cent CO.2, and 83 per cent N2. An 
average of 10 min is allowed for the animal to 
equilibrate on each gas mixture before any 
measurements are made. 
The animal is placed in one of two positions 
for each experiment. With the microscope lo- 
cated above the animal, which is lying on its 
side, the most superior (highest) portion of the 
lung is viewed. This is defined as the top of the 
lung; the region is in zone 2 about 15 cm above 
the heart.^ In the second position, the animal 
lies on top of the microscope. Viewing is done 
from the floor upward through a hole in the 
table. This we define as the bottom of the lung ; 
it is in zone 3 being about 4 cm below the 
heart. ^ 
Observations and cinemicrographs are made 
with a Leitz Ultropak surface illuminating mi- 
croscope. An IIX objective (N.A. 0.25), a tube 
factor -of IX, and a Leitz 4X projection eyepiece 
are used in combination with a Kodak Special 
Reflex Model 1 motion picture camera equipped 
with an Angenieux 100 mm f/2 lens to produce 
a film magnification of 17X. The light source is 
a 500 watt tungsten projector bulb controlled 
by a variac. The light is filtered by a pair of di- 
chroic infrared heat reflecting filters and a 
Kodak Wratten #64 filter. The green illuminat- 
ing beam has the characteristics of being cool 
and increasing the contrast of the blood to the 
tissue." When the animal has reached steady 
state on the control or hypoxic gas mixture, cin- 
emicrographs are taken at 24 frames per sec 
for 30 sec on Ektachrome EF film. 
After the film is developed, the individual se- 
quences are cut from the roll and viewed using 
