Ficune 1. Typical 0, and CO, blood-gas dissociation
curves for dogs of the local geographical area,

Ficune 2. Construction detail of the rigid-bubble capsule.

-3-—


Taste I
DOG AP P, “TISSUE”P, (Ps—“TISSUE”P,)
NO. mm Hg mm Hg mm Hg
1 48 731 683 P=<0.1%
+33 +0.9 +35
N-4 N-4 N-4
2 43.9 729.3 685.4 P=<0.1%
+09 +13 +2.0
N-13 N-13 N-13
3 41.6 729.5 687.9 P=<0.1%
+06 +0.9 +12
N-17 N-17 N-17

Aksnes and Rahn for mixed-venous blood, The
difference may be real and possibly accounted
for by the presence of a considerably larger
alveolar-arterial O, gradient in the anesthetized
versus unanesthetized dog.

DISCUSSION

The experimentally obtained mean AP values
(Table 1) lie within the range of values pre-
dicted for this parameter, Although the mean
P, values (Table 1) for the three dogs do not
differ significantly at the chosen statistical
probability level, the biological possibility does
exist that these small mean differences are, in
fact, real and are related to minor variations
in tissue metabolism at the capsule site. A
continuous measurement of P; might resolve
this possibility.

The potential uses of the capsule device
described in this study are manifold, A few
examples of these projected uses are as follows:

1. According to the work of Van Liew,
the equilibrium concentrations of any com-
bination of O., CO, and N, injected into a
subcutaneous gas bubble closely resemble the
concentrations of the same gases in the venous
blood of the tissue surrounding the bubble.
Since the pulmonary artery is the site of mixed-
venous blood from the whole organism, the
total gas pressure measured at this site by
means of a miniaturized version of the capsule
device should represent an approximation of
the mean total gas pressure of the whole
organism in conjunction with its existing state
of metabolism.

2. Since the total gas pressure in any tissue
is related to its concomitant metabolic gas
exchange, the presence of a miniaturized cap-
sule device in skeletal muscle could allow
the assessment of metabolic gas exchange dur-
ing rest versus various degrees of muscular
work.

The measurement of mean metabolic changes
occurring in the whole organism (e.g., at a
mixed-venous blood site) or of individual meta-
bolic changes at some specific site (e.g.,
skeletal muscle) in a broad variety of physio-
logical states should be of extreme value.

SUMMARY

An in vivo method for the quantitative esti-
mation of total gas pressure in mammalian
tissue has been established. This method utilizes
a rigid-walled capsule specially constructed to
be permeable to oxygen, carbon dioxide and
nitrogen (O,, CO,, and N,), but negligibly
permeable to water vapor. The results obtained
at equilibrium, after subcutaneous implantation
of this capsule, demonstrate directly that the
total gas pressure in the adjacent tissue, which
is represented by the intra-capsular total gas
pressure, is about 40 to 50 mm Hg less than
the concomitant atmospheric pressure, The dif-
ference beween these gas pressures is a result
of the metabolic gas exchange and may, there-
fore, be used to detect quantitative changes in
tissue gas metabolism. Work is in progress on
miniaturization and modification of the capsule

-~6—

de

te



The subcutaneous injection of a gas bubble
consisting of O,, CO, and N, is followed by the
gradual absorption of those gases by the sur-
rounding tissue and blood.® The absorption of
an injected mixture of these gases can be divided
arbitrarily into two phases. In the first phase,
each gas can diffuse into or out of the bubble
towards the site of its lowest partial pressure.
This trans-interface flux continues until the
intra-bubble concentrations of the three gases
become equal to those of the adjacent tissue
interface, This interface is composed of tissue
cells, tissue liquid and the capillary blood which
flows through the area, When constant gas-
composition in the bubble is attained, the
second phase of absorption commences. All
the gases now diffuse out of the bubble at an
overall rate depending on the physical prop-
erties of the slowest diffusing gas. Since the
loose, overlying skin does not significantly com-
press the bubble, the total gas pressure in the
bubble is essentially equal to that of the
atmosphere and remains so during the complete
absorption process, Since it has been empiri-
cally established that the bubble gases are
absorbed, it follows that the total gas pressure
in the tissue-liquid interface must be less than
that within the gas bubble and that this pres-
sure difference (the absorption gradient) is
the diffusion driving-force for the absorption
process. This gradient must also remain essen-
tially constant during the whole process of
absorption.

The nature of this gradient may be visualized
in Figure 1, which presents typical CO, and O,
blood-gas dissociation curves for dogs of the
surrounding geographical region, It has been
assumed that these curves and the arterio-
venous O; and CO, differences shown in Fig-
ure 1 represent the essential conditions for the
gas exchange between the capillary blood and
the tissue-liquid interface of the subcutaneous
bubble. The addition of 4 volumes % CO, by
the tissue cells to the traversing capillary blood
results in an increase of 6 mm Hg in the total
gas pressure of the blood. Concomitantly, the
unloading of 5 volumes % O, into the tissue
cells by the blood results in a decrease of
47 mm Hg in the total gas pressure of the
blood. The net effect of these two processes
is a 41 mm Hg decrease in the total gas pres-
sure of the blood, An additional decrement

may occur as the combined result of the
alveolar-capillary-blood barrier and the dilution
effect of intra-pulmonic arterio-venous shunts.
In “normal,” unanesthetized, air-breathing dogs,
this additional decrement could approximate
10 mm Hg. Since these were the conditions
under which the data in this study were ob-
tained, one would predict that the total decre-
ment or equilibruim absorption gradient should
approximate 40 to 50 mm Hg, The subtraction
of this gradient from the concomitant baro-
metric pressure should yield a quantitative
estimate of the total gas pressure of the tissue-
liquid interface,

METHODS

The equilibrium absorption gradient may be
measured by using a specially-constructed, con-
stant volume, rigid-walled bubble which is
permeable to O., CO,, and N, but with rel-
atively negligible permeability to water vapor.
The total gas pressure in such a subcutaneously
implanted bubble will come into equilibrium
with the total gas pressure of the tissue-liquid
interface; the absorption gradient manifesting
itself at equilibrium as a constant, negative,
hydrostatic pressure within the bubble.

The construction detail of the rigid-bubble
capsule is shown in Figure 2, The volume cham-
ber and two membrane-retention rings are
made of acrylic plastic. The volume chamber
communicates freely with the lumen of the
externalizing mylar-plastic tube and metal
stopper-housing. A rigid, wire mesh screen
is flush-mounted on each side of the volume
chamber. A 5-mil thick, siliconated rubber
membrane is placed on each noncollapsible
wire screen and locked in, water-tight, by the
plastic retention rings, The CO, diffusibility
constant of a one mil thick portion of this
membrane as measured at 25 degrees centi-
grade is 0.56 ml/min/cm’ surface area at a
gradient of one atmosphere (Dow Corning).
This diffusibility is listed as being 3053 times
that of water vapor. The permeability of the
mylar tubing to O., CO,, N: and water vapor is
virtually zero. The internal gas volumes of
the three capsules used in this study were
1.0, 0.75, and 0.70 ml respectively. Figure 3
shows the general appearance of an assembled
capsule.

—2-



Ficure 3. General appearance of an assembled rigid-bubble
capsule.

This type of capsule was surgically implanted
subcutaneously in each of three dogs under
Nembutal anesthesia. The subcutaneous posi-
tion of the capsule in each dog is shown in
Figure 4. Following one week allowed for
surgical healing, the metal housing was inserted
into the external orifice of the mylar tube and
occluded with a rubber stopper. Measure-
ments of the intracapsular negative-pressure
were obtained for pressure-buildup durations
of 1 to 6 days, The intracapsular pressure was
reduced to zero (atmospheric pressure) at the
start of each pressure-buildup period. The
dogs were unanesthetized during all the meas-
urement and pressure-buildup periods. A small-
bore water manometer was used as the pressure
measuring device. It was connected to the
intracapsular volume via a hypodermic-needle
forced into the self-sealing rubber stopper.

RESULTS

The change in the intracapsular negative-
pressure with respect to time is shown in

Figure 5. The mean pressures for the 2nd, 3rd,
and 4th days are not significantly different at
the 5% level of probability. Therefore, all the
pressure determinations representing pressure-
buildup periods of 2 or more days duration
were considered representative of the equilib-
rium state. It is this pressure which represents
the absorption gradient. The subtraction of
this value from the concomitant barometric
pressure yields a quantitative estimate of the
total gas pressure of the tissue-liquid interface.

The summarized equilibrium data are shown
in Table 1. The means and standard errors are
shown for the absorption gradient (AP), the
concomitant barometric pressure (Ps) and the
calculated “tissue” total gas pressure (P;). The
three mean values for P; are not significantly
different at the 5% level of probability, Each
mean P, is, however, significantly different
from its related mean Ps in all three cases.
Two of the three mean values of AP are
significantly less than the corresponding con-
comitant value of 54 mm Hg obtained by

~4-



device so that it may be used at intravascular
as well as tissue sites in a broad variety of
physiological states.

The author gratefully acknowledges the

technical assistance of Norma M. Graham, 3.

REFERENCES

. Krough, A. Skandinav, Arch. F. Physiol. 20:259, 1908.
. Aksnes, E.G. and H. Rahn. Jour. Appl. Physiol.

10:173, 1957.
Van Liew, H. D. Jour. Appl. Physiol. 17:851, 1962.

4% U, § GOVERNMENT PRINTING OFFICE : 1963 0 - 712-218



“IN VIVO” MEASUREMENT OF TOTAL
GAS PRESSURE IN MAMMALIAN TISSUE

MICHAEL T. LATEGOLA, Ph. D.
Chief, Respiration Section
Biodynamics Branch

63-11

FEDERAL AVIATION AGENCY
AVIATION MEDICAL SERVICE
AEROMEDICAL RESEARCH DIVISION
CIVIL AEROMEDICAL RESEARCH INSTITUTE
OKLAHOMA CITY, OKLAHOMA
JULY 1963



roune 4. Subcutaneous location of the capsule system in
the

1g.

swe Te
-40- /N=8 Ne3
/
/
nal / (DOG 3)
INTRA-
CAPSULAR |" /
P /
-20b
(mmHg) L Nea
-l0F
L ! Ll _
%% 1 2 3 4
TIME (DAYS)

Ficure 5. A graphical plot of the negative intracapsular
pressure versus time indicating the minimum amount of
time necessary to attain the equilibrium value of the

absorption pressure-gradient.

5H





“IN VIVO” MEASUREMENT OF TOTAL GAS PRESSURE
IN MAMMALIAN TISSUE

Michael T. Lategola, Ph. D.

ABSTRACT

An in vivo method for the quantitative estimation of total gas pressure in mammalian tissue
has been established. This method utilizes a rigid-walled capsule specially constructed to
be permeable to oxygen, carbon dioxide and nitrogen (O,,CO,, and N,), but negligibly per-
meable to water vapor. The results obtained at equilibrium, after subcutaneous implantation
of this capsule, demonstrate directly that the total gas pressure in the adjacent tissue, which
is represented by the intracapsular total gas pressure, is about 40 to 50 mm Hg less than
the concomitant atmospheric pressure. The difference between these gas pressures is a
result of the metabolic gas exchange and may, therefore, be used to detect quantitative
changes in tissue gas metabolism. Work is in progress on miniaturization and modification
of the capsule device so that it may be used at intravascular as well as tissue sites in a

broad variety of physiological states.

The metabolizing tissues of mammals combust
carbon (food stuff) in oxygen with the conse-
quent production of carbon dioxide. Under
“steady state” conditions, the metabolic amounts
of O, used and CO, produced in any particular
tissue manifest a fair degree of constancy, thus
establishing fairly constant partial pressures of
these gases in the tissue. Any sudden metabolic
increase in such tissue produces an increased
O, utilization and CO, production whereby the
partial pressure of oxygen (Po:) decreases and
that of carbon dioxide (Pco.) increases, Be-
cause of the marked difference in slope of the
O, versus the CO, dissociation curves of the
blood, the decrease in tissue Po, will be much
larger than the increase in tissue Pco,. There-
fore, the total gas pressure in the tissue (P;)
should also decrease, since the partial pressure
of the metabolically inert nitrogen (N:) re-
mains essentially constant. Thus, in any tissue,
a change in Py reflects metabolic changes af-
fecting the Po, and Pco, which, as yet, cannot
be measured readily. It is for this reason that

the availability of direct P, measurements
would be extremely useful for the detection
of these metabolic effects in any tissue.

One of the earliest measurements of total gas
pressure in mammalian tissue was obtained in
1908 by August Krogh’, Using an acute
preparation and an “in vivo” volumetric method,
Krogh ascertained that the total gas pressure in
the arterial blood of the anesthetized rabbit was
less than that of the atmosphere. Aksnes and
RahnTM in 1956 repeated and extended Krogh’s
measurements of total gas pressure. Using an
“in vitro” method, these investigators were able
to obtain estimates of mixed-venous as well as
arterial total gas pressures.in the anesthetized
dog. The measurements were obtained using
blood samples withdrawn during “steady state”
conditions. They found the mixed-venous total
as pressure to be 54 mm Hg less than the con-
comitant atmospheric pressure (average of 12
data). This technique as that of Krogh, does
not permit the chronic measurement of the total
as pressure in the blood or other tissue of the
unanesthetized animal.

-1-