Dr. Selcuk Guney- International publication

1. Ann Otol Rhinol Laryngol 108:1999
MASTOID BUFFERING PROPERTIES: I. GAS PARTIAL PRESSURES
EYAL RAVEH, MD JACOB SADE, MD
TEL-AVIV, ISRAEL TEL-AVIV, ISRAEL
HAYA MOVER-LEV, PHD SELCUK GUNEY, MD
TEL-AVIV, ISRAEL ISTANBUL, TURKEY
Differences in the gas partial pressures between the middle ear (ME) cavity and the blood are an important factor in ME gas
economy. Differences in gas partial pressures between various regions of the ME-mastoid air cell system (ME-MACS) could play a
role as well. To determine whether gas partial pressure differences do occur between various compartments in the ME, we measured
the rate of gas diffusion from one compartment to another in both an artificial model and in the ME-MACS of human temporal
bones. The rate of gas diffusion between various areas of the ME and the mastoid tip was found to be rapid, with a half-life on the
order of 2 minutes (range 0.8 to 5.3 minutes). We suggest that this high diffusion rate prevents the buildup of significant differences
in gas composition in the ME-MACS system, which can therefore be regarded as a homogeneous gas pocket.
KEY WORDS — chronic otitis media, diffusion of gases, mastoid, middle ear gases, middle ear pressure, partial pressure.
INTRODUCTION MATERIALS AND METHODS
Our current understanding of gas pressure dynam-
ics in the middle ear (ME)-mastoid air cell system
(MACS) includes an assumption that under normal
physiological conditions there is continuous absorp-
tion of gas, mainly nitrogen (N2) and oxygen (02),
from this cavity into the blood. The opening of the
eustachian tube that occurs during swallowing, yawn-
ing, etc, compensates for this loss by introducing gas
from the nasopharynx into the ME cavity.1-3
The impetus for gas exchange between the ME
and the blood is the difference in the partial pres-
sures of gases between these 2 locations.4-6
The to-
tal pressure is the sum of the gas partial pressures;
differences in gas partial pressures between the ME
and the MACS may influence gas exchange rates,
and consequently affect the total pressures of the sys-
tem. Since the mastoid is a labyrinth, its internal cel-
lular structure might be expected to attenuate the rate
of diffusion and redistribution of gases in the ME-
MACS after a compositional upset in one region. A
lag in gas diffusion and redistribution could cause
pressure differentials in the system that may possi-
bly be clinically significant. To determine whether
gas partial pressure differences do occur between var-
ious compartments in the ME-MACS, we measured
the rate of gas diffusion from one compartment to
another, in both an artificial model and in human tem-
poral bones.
From the Ear Research Laboratory, Department of Bioengineering, Faculty of Engineering and Sackler School of Medicine, Tel-Aviv Univer-
sity, Tel-Aviv (all authors), and the Department of Otolaryngology-Head and Neck Surgery, Rabin Medical Center, Beilinson Campus, Petah-Tikva
(Raveh), Israel.
Presented at the Third Extraordinary International Symposium on Recent Advances in Otitis Media, Copenhagen, Denmark, June 1-5, 1997.
CORRESPONDENCE — Jacob Sade, MD, Ear Research Laboratory, Bioengineering, Wolfson Building, Tel-Aviv University, Tel-Aviv 69978,
Israel.
Artificial Model. To examine gas diffusion rates
in a model, a 20-mL plastic syringe was divided into
2 cells by a partition (Fig 1). A4-mm opening in the
partition could be closed and opened by means of a
metallic cover operated by an external magnet. Two
50-mm lengths of polyethylene tubing, 1 mm in in-
ner diameter, permitted the introduction of a gas mix-
ture into cell I. Probes were connected to cell II for
measurement of pressure, temperature, and gas con-
centrations.
Initially, cell II contained room air, while cell I
was filled with a mixture resembling the steady-state
composition of gas in the ME: 81.39% N2, 9.37%
pressure transducer
thermocouple m
/~^ gas in
mass spectrometer
catheter cell Π | cell I
partition
gas out
Fig 1. Model for gas diffusion. Twenty-milliliter syringe
was divided into 2 cells by partition. Gas mixture was
introduced into cell I through polyethylene inlet and out-
let tubes. Cellular material was contained in cell II, which
was connected to mass spectrometer, thermocouple, and
pressure transducer. Partition had opening closed with me-
tallic cover that could be opened with external magnet.
750

2. Raveh et al, Mastoid Buffering Properties 751
2.5 ml syringe
gas in S
■ . iC e U I cell II
H IT"
gas Π
connected to
external meatus
mass
spectrometer
gas out
pressure transducer
thermocouple
B
Fig 2. Gas diffusion measurements in mastoid bone. A) Bone with 2.5-mL syringe connected to external
auditory canal. There were 2 openings in mastoid tip, 1 for mass spectrometer and other for pressure trans-
ducer and thermocouple. B) Syringe (2.5 mL) served as chamber for introduction of gas mixture into middle
ear. Like that in Fig 1, it was divided into 2 cells by partition; metallic cover over opening between cells could
be opened with external magnet. Cell II was hollow cylinder connected to external auditory canal. Cell I was
used for introduction of another gas mixture via polyethylene inlet and outlet tubes.
carbon dioxide (C02), 8.17% 02, and 1.07% argon
(Ar). After a 7-minute wash with this gas mixture,
all air in cell I was considered to have been replaced,
and both tube inlets were sealed. The partition sepa-
rating the 2 cells was then opened. The changes in
gas composition in the combined cells over time were
recorded until the mass spectrometer indicated that
a steady state had been reached. Diffusion time was
defined as the period elapsed between the opening
of the partition and the first reading of a gas compo-
sition that remained constant. These measurements
were repeated at volumes of 5, 10, and 15 mL.
To simulate the internal structure of the MACS,
the experiment was also done with cellular material
filling cell II. Organic and synthetic types of sponge
were used: the dried skeleton of a marine sponge,
and a flexible polyurethane spongy material, respec-
tively. Both of these had an interconnected open cel-
lular network, with cell sizes ranging from 0.5 to 3
mm. Gas partial pressure changes recorded with ei-
ther spongy material in the syringe were compared
with those recorded in their absence.
The model served as a control for the system re-
sponse time, and was used as a baseline against which
the temporal bone results could be evaluated.
Bone Selection andPreparation. In view ofthe dif-
ficulty of performing such a study in vivo, we used
8 human temporal bones in various stages of preser-
vation, categorized as dry, wet, or fresh, as described
below. The structure of the MACS (aerated, diploic,
or sclerotic) of each bone was analyzed with a Schul-
ler position radiograph.
The 6 dry bones had been preserved in formalde-
hyde solution. Their soft tissues were removed with
hypochloric sodium solution, and they were dried
by being washed with alcohol. The external surfaces
of these bones were then sealed with a coat of lac-
quer (collodion). Certain areas were also sealed with
epoxy resin. Sealing was repeated until no leakage
between the mastoid-ME cavity and the atmosphere
could be detected. Leakage was tested by raising the
pressure inside the bone and looking for a drift in
pressure with time.
One wet bone, still lined with its internal mucous
membrane, was used as a "pilot" for the fresh bone
measurements, also taken under wet conditions. Free
fluid was drained from a preserved bone, which was
then sealed with a coating of silicone RTV (room
temperature vulcanization).
Onefresh temporal bone was taken from a cadaver
3 hours after death. (Access to human bones for re-
search is very limited in Israel.) This bone was also
sealed externally with silicone RTV, with no other
treatment.
Bone Setup. A 2.5-mL syringe was connected to
the external auditory canal (Fig 2) for the introduc-
tion of gas mixtures into the ME. Like the model,
this syringe was also divided into 2 cells by a rigid
partition, which had a relatively wide (5 mm) open-
ing in it. Cell I ( 1 mL), distal to the bone, was set up
similarly to that in the artificial model. Cell II was
connected directly to the external auditory canal, with
a wide (8 to 9 mm) opening. Therefore, when the
partition was opened, cell I and the ME effectively
became a single cavity. Thus, we regarded the diffu-
sion rate between cell I and the tip of the mastoid as
the same as that expected to occur between the ME
and the mastoid.

3. 752 Raveh et al, Mastoid Buffering Properties
TABLE 1. GAS DIFFUSION RATE CONSTANTS
IN MODEL
Without Cellular With Cellular
Material Material
Volume k Tin k Tin
of Model (min'1
) (min) (min~l
) (min)
5mL 0.42 1.6 0.41 1.6
lOmL 0.45 1.5 0.44 1.6
15 mL 0.46 1.5 0.49 1.4
Data are averages of 3 readings.
k — diffusion rate constant; Tl/2 — diffusion half-life.
Two openings 2 mm in diameter were drilled into
the cortical bone adjacent to the mastoid tip (Fig 2A),
through whichthemeasuring probes of the mass spec-
trometer, the thermocouple, and the pressure trans-
ducer were inserted. Before each run, we checked
the system for leakage by looking for pressure drift.
Diffusion Rate Measurement. The experiment was
started by filling the mastoid, the ME, and cell II
with gas, either N2 or Ar. The partition between the
cells in the syringe was then closed. Cell I was then
flushed with 02 (if cell II held N2) or N2 (if cell II
containedAr), following which the tubes were closed.
At this stage, we had a closed system with 2 sepa-
rate cavities: cell I with one gas; and the mastoid,
ME, and cell II with another gas (02-N2 or N2-Ar).
The next step was to use a magnet to open the parti-
tion between the cells, and record the rate and pat-
tern of the gas composition changes with time. The
diffusion process was followed from the opening of
the partition until the gas concentrations reached a
steady state, as measured at the tip of the mastoid.
The 2 gas combinations (02-N2 and N2-Ar) were
chosen so that at the end of the diffusion process,
the gas components would resemble those in the ME
under normal conditions, and would be similar to
the calibration gas concentrations — to increase the
accuracy of readings of gas partial pressures.
In the 02-N2 combination, partial pressures of both
gases were recorded. Therefore, data presented for
02 are of its diffusion rate into N2 and vice versa. In
the N2-ÀT experiment, for technical reasons the mass
spectrometer was calibrated only with Ar; therefore,
only Ar concentrations were recorded.
Two other processes were used as controls. One
was the response time of our measuring system. The
probe of the spectrometer was inserted into a vessel
of known gas composition (calibration gas), and par-
tial pressure changes from room air levels were re-
corded until a plateau of calibrating gas composition
had been reached. The second reference process, as
mentioned, was the diffusion rate of gas in the artifi-
cial model.
Measuring Techniques. An Amis 2000 mass spec-
trometer (Innovision, Odense, Denmark) was used
for gas composition measurements, at a sampling rate
of 2 x 10~6
mL/s. The response time reported by the
manufacturer is 20 to 40 seconds. The mass spec-
trometer was calibrated before and after each experi-
ment.
Pressure and temperature readings were taken con-
tinuously. Pressure measurements were made with a
Validyne DP-103 pressure transducer (accuracy
0.02%) and confirmed with a U-tube water manome-
ter. Temperature was monitored with a copper-con-
stantan thermocouple (Ellab, Copenhagen, Denmark;
accurate to ±0.1 °C).
Data Evaluation. The change of gas concentra-
tion with time had an exponential pattern, best de-
scribed by a monoexponential equation: C = C°°(l -
ekt
) + CO where C is the concentration of the gas at a
given time (t, in minutes), e is the natural logarithm,
and k is the rate constant (in minutes-1
) in which we
are interested. The half-life of this diffusion process,
Tl/2, can be calculated as ln2/k.
The correlation r2
between the measured data and
the exponential fitted line was calculated with
SIMFIT software (courtesy of Dr Bardeley, Universi-
ty of Manchester, England). Correlations of the rate
constants of the different gases and the different types
of bones were calculated by the Kruskal-Wallis non-
parametric technique, using SAS software (SAS In-
stitute, Inc, Cary, NC). Ap value of <.05 was consid-
ered significant.
RESULTS
Gas Diffusion Rates in Model. The average gas
diffusion rate constant (k) between the 2 cells in the
model was 0.45 min-1
for all gases (Table 1). The
average Tl/2 was 1.5 minutes. Neither varying the
volume of the model nor the presence or absence of
spongy material had any significant effect on the rate
of equalization of gas composition.
Gas Diffusion Rates in Temporal Bones. Figure 3
shows the typical exponential shape of a recording
of gas diffusion between the mastoid and the ME.
The correlation constant, r2
, between measured data
and those calculated from the exponential fitted line
was .96 to .99 in the O2-N2 experiments and .92 to
.99 in the N2-Ar measurements (Table 2).
In the O2-N2 experiments, the gas diffusion rate
constant for oxygen (k02) in the 6 dry bones varied
from 0.31 to 0.60 min-1
, with an average (±1 SD) of
0.40 ±0.1 min-1
(Table 2). This gives an average
Tl/2 of 1.80 ± 0.4 minutes, with a range of 1.1 to 2.2
minutes. The diffusion rate of the wet bone (Tl/2,

4. Raveh et al, Mastoid Buffering Properties 753
9.0
6.5
4.0
1.5
0.0
Calculated value from
the exponential fitted line
O Measured datum
0.0 2.5 5.0 7.5
Time (min)
10.0
Fig 3. Diffusion rate. Typical recording, with exponen-
tial shape, of gas diffusion (oxygen, in this case) between
middle ear and mastoid.
5.3 minutes), though somewhat slower, was not sig-
nificantly different from that of the fresh or dry bones
(p > .05).
With the exception of that for the fresh bone, the
diffusion rate constants calculated for nitrogen were
similar to those for 02 in all 02-N2 experiments (p
> .05). The slower N2 diffusion rate constants of the
wet (Tl/2,4.1 minutes) and fresh (Tl/2,2.8 minutes)
bones did not differ significantly from the rates of
the dry bones (average Tl/2, 1.95 ± 0.4 minutes; p >
.05).
In the experiment of argon diffusing into N2, kAr
values were similar to those of 02 into N2. In the
fresh bone, however, Ar, like N2, had a slower diffu-
sion rate, with a Tl/2 of 3.1 (2.8 for N2), compared
to 1.4 minutes for 02.
Two of the dry bones had a sclerotic type of mas-
toid, whereas all 6 of the other bones had a well-
developed MACS. This was demonstrated by imag-
ing and verified when we drilled the openings in the
bones for the measuring probes. One of the 2 sclerotic
bones (No. 1) exhibited the fastest diffusion rates
(Table 2).
No significant statistical difference was found be-
tween the k values of the dry aerated, dry sclerotic,
wet, or fresh bones (p > .05), nor between k values
of the various gases (02, N2, and Ar).
Correction of the results for variations in tempera-
ture resulted in only minor changes, on the order of
0.1% or less.
ControlRates. The Tl/2 ofthe calibration measure-
ments was 0.5 to 1.2 minutes, and averaged 1.5 min-
utes in the model.
DISCUSSION
The role of the MACS is not well understood. Pos-
sible functions include physical defense for the in-
ner ear and the brain7
and a resonance box for specif-
ic frequencies.8
It is accepted that an association ex-
ists between a small MACS and the chronic otitis
media syndrome,912
and that there is a linkage be-
tween these various disease entities and negative
pressure development in the ME-MACS.1
·5
·10
Dif-
fusion of gas between the ME cavity and the blood,
like the introduction of gas through the eustachian
tube, is a major factor in the pressure dynamics of
the ME.4
"6
·13
We studied partial pressure distribution within the
ME-MACS by measuring the rate at which gases
diffuse between these 2 compartments. The ques-
tion posed was, how will a change of partial pres-
sure in one location affect that in a nearby location?
For example, would a change in the diffusion proper-
TABLE 2. GAS DIFFUSION RATE CONSTANTS IN MIDDLE EAR-MASTOID AIR CELL SYSTEM
Dry bones
1*
2
3*
4
5
6
Average
SD
Wet bone
Fresh bone
Data are averages
k
(mirr1
)
0.60
0.32
0.36
0.36
0.42
0.31
0.40
0.1
0.13
0.56
of 2 readings.
k — diffusion rate constant; Tl/2 —
Oxygen
Tin
(min)
1.1
2.1
1.9
1.9
1.6
2.2
1.80
0.4
5.3
1.4
diffusion half-life
r2
.98
.97
.98
.98
.98
.99
.99
.99
r2
—
k
(min'1
)
0.49
0.32
0.34
0.41
0.37
0.26
0.37
0.1
0.17
0.25
Nitrogen
Tl/2
(min)
1.4
2.1
2.0
1.7
1.9
2.6
1.95
0.4
4.1
2.8
correlation coefficient.
r2
.96
.96
.97
.97
.98
.97
.99
.98
k
(min~l
)
0.80
0.45
0.30
0.32
0.36
0.24
0.41
0.2
0.17
0.22
Argon
Tl/2
(min)
0.8
1.5
2.3
2.1
1.9
2.8
1.90
0.6
4.1
3.1
r2
.92
.99
.96
.94
.98
.99
.99
.99
*Bones 1 and 3 were sclerotic; all others had aerated structure.

5. 754 Raveh et al, Mastoid Buffering Properties
ties of the ME mucosal linings during infection af-
fect the partial pressure in the mastoid? Our work-
ing hypothesis was that if a rapid diffusion rate were
to be found, partial pressures throughout the ME-
MACS would equalize rapidly after an upset in one
part, resulting in the rapid reestablishment of a uni-
form gas composition throughout the system.
In the 6 dry bones, the Tl/2 for the diffusion of 02
between the ME and the MACS ranged from 1.1 to
2.2 minutes. The Tl/2 for N2 and Ar were similar
(N2, 1.4 to 2.6 minutes; Ar, 0.8 to 2.8 minutes). The
wet bone had a slower diffusion rate (Tl/2, 5.3 min-
utes for 02, 4.1 minutes for both N2 and Ar). The
fresh bone had a Tl/2 of 1.4,2.8, and 3.1 minutes for
02, N2, and Ar, respectively. The slower rate in the
wet bone can be explained by the solubility of these
gases in water, which would slow the gas mixing
process in the cavity (a chromatography-like effect).
Even though the diffusion process in the wet bones
was slower than in the dry bones, this difference was
not statistically significant, and the overall picture
is of a diffusion process with a Tl/2 on the order of
only 2 to 3 minutes. Because of the small number of
bones available to us, no conclusions could be drawn
about the extent to which diffusion rates are affected
by the volume or morphology of the bone.
Do the diffusion rates we have found hold any
physiological or clinical significance? Of our 2 con-
trol measurements taken for comparison with the
temporal bone results, the Tl/2 values ranged from
0.5 to 1.2 minutes (the response time of our measur-
ing system) or averaged 1.5 minutes (the diffusion
rate in the model). The averages of the Tl/2 values
measured within the human ME-MACS were of a
similar order.
On the basis of these results, it can be concluded
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6. Hergils L, Magnuson B. Morning pressure in the middle
that the diffusion process between the ME and the
MACS is quick and within the range of the response
time of our measuring system. Thus, no significant
gas pressure differences are likely to build up be-
tween the ME and the MACS, and this system can
therefore be regarded as 1 chamber with a homogene-
ous gas concentration.
Several conditions could produce a differential in
gas partial pressures in various regions of the ME-
MACS complex. These include inflammatory condi-
tions, which may influence the rate of gas exchange
between the ME cavity and the blood or partially
obstruct the aditus, isolating the mastoid. A perfora-
tion in the tympanic membrane, either in cases of
chronic otitis media or in the presence of a ventilat-
ing tube,14
is another possibility. However, we could
find no data in the literature indicating that differenc-
es in partial pressures of the gases in the ME-MACS
occur. In view of the fast diffusion rates between the
ME and the MACS that we found, we propose that
if compositional differences in the gases within these
cavities do occur, they can be expected to be tran-
sient and therefore have little or no clinical signifi-
cance.
In our study, 6 of the 8 bones had no mucosal lin-
ing. Under physiological conditions, in the presence
of a mucosal lining, the connections between the
mastoid cells might be narrower and the diffusion
rate slower. As the diffusion rates of fresh bones with
intact mucosal membranes were not significantly
slower than those of the dry bones, we suggest that
the ME-MACS complex under in vivo conditions
also has a uniform profile of gas partial pressures.
However, because of the very limited number of fresh
bones available to us, this point warrants further ex-
amination.
ear. Arch Otolaryngol 1985; 111:86-9.
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8. Tumarkin A. On the nature and vicissitudes of accessory
air spaces of the middle ear. J Laryngol Otol 1957;71:65-99.
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middle ear disease to mastoid hypocellularity. Acta Otolaryngol
[Suppl] (Stockh) 1963(suppl 182): 1-4.
10. Flisberg K, Zsigmond M. The size of the mastoid air cell
system. Acta Otolaryngol (Stockh) 1964;60:23-9.
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12. Sederberg-Olsen JF, Sederberg-Olsen AM, Jensen AM.
The prognostic significance of the air volume in the middle ear
ACKNOWLEDGMENT — This paper was prepared with the assistance of Editorial Services, The Hospital for Sick Children, Toronto, Canada.
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6. Raveh et al, Mastoid Buffering Properties 755
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