NORMAL GAS EXCHANGE IN THE HUMAN MIDDLE EAR A.
ELNER,
M.D.
LUND, SWEDEN
SUMMARY - Different kinds of biological gas pockets and the special character of the middle ear as a gas pocket are discussed. The pattern of gas absorption from the normal middle ear and from ears with blocked Eustachian tubes is presented. The amount of gas leaving the ear by diffusion and the resulting underpressure from this process are determined. The composition of air in the ear is compared to that in experimental gas pockets and the role of the partial pressures of CO 2 and O 2 in secretory otitis media is discussed.
There is a continuous absorption of gases from all air filled cavities in the body. The pattern of this absorption is somewhat different, depending on whether the cavity communicates with the outside and, if so, on the type of this communication. According to Hahn' there are only three kinds of biological gas pockets (Fig. 1). The first one is the normal lung. Rahn calls this an "open ventilated" gas pocket. The second one is "open nonventilated" or intermittently ventilated. This is the ear with a normally functioning Eustachian tube, and the normal sinus. Here we find another pattern of absorption. Carbon dioxide is in a state of equilibrium with the surrounding tissues and both oxygen and nitrogen are absorbed. This continuous process creates an underpressure which builds up slowly in the ear and is equilibrated by small amounts of air through the tube. The third one is "closed," a type which might be represented by the peritoneal cavity in case of pneumoperitoneum, a lung or a lung lobe with complete bronchial obstruction, and the middle ear with a blocked Eustachian tube. In this case there is absorption of all the components, which means that the carbon dioxide too is absorbed. A subcutaneous gas pocket devised in the rat by Rahn and Canfield" gradually contracted and eventually disappeared. The pressure of the gases inside the pocket was maintained by the elasticity of the skin and tissues. But in the ear the
situation is different (Fig. 2). The ear is a nearly rigid chamber which can decrease its volume only very little by the inward movement of the drum and by the increased volume of the mucous membrane lining the cavity. Consequently, when the Eustachian tube is blocked, there is a growing underpressure and when this reaches a certain limit (which is not known as far as I know) transudation occurs. The fluid reduces the volume of the airfilled cavity and a pressure difference between the cavity and the tissues is maintained by this mechanism. If the tube is blocked long enough, the whole cavity will ultimately be filled with transudate. This mechanism is well known in physiology and can be experimentally studied, for instance, in rigid chambers made of stainless steel netting and lodged subcutaneously in some experimental animal. An interesting point is the question of how large amounts of gas or air are absorbed from the middle ear and how fast an underpressure develops (Fig. 3). The sum of the partial pressures of the gases in the tissues is about 700 mm Hg and in the middle ear about 760 mm Hg. So, there is a diffusion gradient of about 60 mm Hg between the ear and the surrounding tissues. We have studied the absorption indirectly by measuring the volume passing through the Eustachian tube during a given period of time, compensating for the loss caused by absorption. The input must be the same as the output. This statement is valid if the Eu-
From the ENT Clinic, University Hospital, Lund, Sweden.
161
162
A.ELNER 0l'EN VENTLA18l
OPEN NON VENTILATED
NORMAL LUNG
NORMAL EAR
CLOSED
EAR WITH A ILOCKED 1\& LINl WITHIIIlONCHIAI. OIISTRUCTKlN
Fig. 1. Biological gas pockets according to Rahn.'
stachian tube is the only way by which significant volumes of air can enter the middle ear and if absorption from the cavity to the capillaries in the mucous membrane is the only way by which air can leave the cavity. One of the problems involved is whether air can diffuse through the normal tympanic membrane (Fig. 4). Theoretically, there may be a passage of oxygen from the ear canal to the middle ear, of nitrogen from the middle ear to the ear canal, and of carbon dioxide from the ear to the ear canal. Carbon dioxide is in constant equilibrium and a "leakage" through the drum is of no importance. Oxygen and nitrogen can pass, though in very small amounts, as has been shown by us on fresh tympanic membrane preparations." Compared to the amount normally passing through the Eustachian tube in 2~ hours, it amounts to less than 0.1% an~ IS negligible. So, the air volume entering the middle ear through the tube must be the same as that leaving the cavity by absorption. As the middle ear is not a completely rigid chamber (Fig. 2), the change in the mass of the enclosed gas, L, V01> is counteracted by the volume displacement
of the drum when sucked inwards, L, V till, and also by the increased volume of the mucous membrane lining the cavity, L, Vm u c • Thus we get: L, Vd i fl = L, V t = L, Vm + L, V tm + L, VmucThe small change in the mass of the enclosed gas, L, Vm, can be calculated when we know the volume of the mastoid cells and the middle ear. With the method and equipment described by Elner et al,4,5 it has been possible to record the volume displacement of the drum L, V tm, when it recoils from a retracted position to its normal position, as small underpressures - created when the tube is kept closed for five to ten minutes _ are equilibrated by swallowing. From earlier investigation by Ingelstedt et al6 we know the magnitude of the volume change of the mucous membrane, L, Vmu c , per em H 2 0 underpressure in the ear. By adding the volumes above during the time covered by the experiment (Fig. 5) we can calculate the volume per hour or per 24 hours proSUM OF PARTIAL PRESSURES OF GASES IN THE TISSUES -700mm Hg
EAR CANAL EAR CANAL
EAR
TUBE
/
~~t
Fig. 2. Middle ear system with its variables. 6P t m - Pressure across the drum. 6 V, - Volume passing through the Eustachian tube.
t E.T.
Fig. 3. Diffusion gradient between the ear and the surrounding tissues.
GAS EXCHANGE
163
TABLE I GAS ABSORPTION FROM THE MIDDLE EAR IN 24 HOURS Riu et af
0.8 ml
Ingelstedt and Jonson'
1-2 ml
Elner et al' 0.7-1.1 ml [van Dishoeck' -5 • -8 em H,O/hr corresponding to -
vided the absorption conditions are identical. We have found the mean value to be 33 fLl/hour or 0.7-1.1 ml/24 hours under normal conditions. When the absorbed volume is known, we can also calculate the resulting underpressure. This is about 3-6.5 em H 2 0 per hour for a mastoid cell and middle ear system of 9-4 ml. If we have a larger volume, the resulting underpressure will be lower and in smaller systems somewhat higher. On the basis of recordings of the pressure drop in the ear in cases with perforated tympanic membranes (Table I) Riu et aF calculated the absorbed volumes and found approximately the same values. Ingelstedt and jonson" punctured the mastoid cells in normal ears and recorded the pressure drop behind an intact tympanic membrane and the flow in the ear canal caused by the tympanic membrane movement. Also van Dishoeck," using his pneumophone method, found the same values of the pressure drop. When the amount of gas that has been absorbed is known, the different values for oxygen and nitrogen can also be calculated, provided the gas composition is known. Melvill jones'? reported values that are in fair accordance with those found later by Riu et al,7 using a gas TYMPANIC
TABLE II GAS COMPOSITION IN THE MIDDLE EAR ° AND IN ELASTIC, SUBCUTANEOUS GAS POCKETS o O
N,
0,
CO,
Melvill jones" Riu et aio
84 % 9 % 7 %
Rahn and Canfield °°
88.3%
85 % 9.5% 5.7%
5.5% 6 %
In fact, Riu et aP found the same values in cases with transudate and a poorly functioning Eustachian tube as in normal cases, and in recordings made by Ingelstedt et al1 2 the oxygen and carbon dioxide tension in transudate aspirated from the ear was found to be 5.5% and 8% respectively. This might mean that there is no "ventilation" in the proper sense of the word of the middle ear via volume
/'"
50
PC02 EAR
_-\-~O.
EAR
chromatograph method (Table II). It is very interesting that these values are similar to those reported by van Liew'! and Rahn and Canfield" in subcutaneous elastic gas pockets devised experimentally in rats. It is thus possible that the composition of air in the ear is stable and not affected by the opening of the tube.
40 -
MEMBRANE
CANAL
0.5-1 ml]
2 02
MIDDLE EAR
"".
-:> PC0 ATM 2 EAR "'PH. ATM
POo
EAR
''''7
30
ATM
20 -
10 ~:':---!-:---L--.L..--L---l.--L---'_J--
10 20 30 40 50 GO 70 80 90
Fig. 4. Gas diffusion through the tympanic membrane.
time min"
Fig. 5. Recorded curve of gas absorption.
164
A. ELNER
the tube and that the small amounts of air entering the ear do not affect the composition of the enclosed gas. The Eustachian tube would then function more or less as a pressure regulator only. This may also mean that the composition of
air in the middle ear in cases of secretory otitis media is the same as in normal cases and that the carbon dioxide and oxygen tensions do not act as "noxae" on the mucous membrane in secretory otitis media.
REFERENCES 7. Riu R, Flottes L, Bouche J, et al: La 1. Rahn H: The role of N, gas in various biological processes with particular reference Physiologie de la Trompe d'Eustache. Paris, Librairie Arnette, 1966 to the lung, in The Harvey Lectures, Series 55. SUNY, Buffalo, New York, 1961, pp 1738. Ingelstedt S, Jonson B: Mechanism of 199 the gas exchange in the normal human mid2. Rahn H, Canfield RE: Volume changes dle ear. Acta Otolaryngol [Suppl.] (Stockh) and steady state behaviour of gas pockets 224:452, 1967 within body cavities. WADC TR 357, 1955 9. van Dishoeck HAE: Negative pressure 3. Elner A: Gas diffusion through the tymand loss of hearing in tubal catarrh. Acta Otopanic membrane. Acta Otolaryngol (Stockh) laryngol (Stockh) 29:303-312, 1941 69: 185, 1970 10. Me1vill Jones G: Pressure changes in the 4. Elner A, Ingelstedt S, Ivarsson A: A middle ear after altering the composition of method for studies of the middle ear mechan- contained gas. Acta Otolaryngol (Stockh) ics. Acta Otolaryngol (Stockh) 72: 191, 1971 53:1, 1961 5. Elner A: Indirect determination of gas 11. van Liew HD: Tissue PO, and 0, esabsorption from the middle ear. Acta Oto- timation with rat subcutaneous gas pockets. laryngol (Stockh) 74:191, 1972 J Appl Physiol 17:851, 1962 6. Ingelstedt, S, Ivarsson A, Jonson B: 12. Inge1stedt S, Jonson B, Rundcrantz H: Mechanics of the human middle ear. Acta Oto- Gas tension and pH in middle ear effusion. laryngol [Suppl.] (Stockh) 228, 1967 Ann Otol Rhinol Laryngol 84:198-207, 1975 REPRINTS - A. Elner, M.D., ENT Clinic, University Hospital, Lund, Sweden.
ELNER,
M.D.
LUND, SWEDEN
SUMMARY - Different kinds of biological gas pockets and the special character of the middle ear as a gas pocket are discussed. The pattern of gas absorption from the normal middle ear and from ears with blocked Eustachian tubes is presented. The amount of gas leaving the ear by diffusion and the resulting underpressure from this process are determined. The composition of air in the ear is compared to that in experimental gas pockets and the role of the partial pressures of CO 2 and O 2 in secretory otitis media is discussed.
There is a continuous absorption of gases from all air filled cavities in the body. The pattern of this absorption is somewhat different, depending on whether the cavity communicates with the outside and, if so, on the type of this communication. According to Hahn' there are only three kinds of biological gas pockets (Fig. 1). The first one is the normal lung. Rahn calls this an "open ventilated" gas pocket. The second one is "open nonventilated" or intermittently ventilated. This is the ear with a normally functioning Eustachian tube, and the normal sinus. Here we find another pattern of absorption. Carbon dioxide is in a state of equilibrium with the surrounding tissues and both oxygen and nitrogen are absorbed. This continuous process creates an underpressure which builds up slowly in the ear and is equilibrated by small amounts of air through the tube. The third one is "closed," a type which might be represented by the peritoneal cavity in case of pneumoperitoneum, a lung or a lung lobe with complete bronchial obstruction, and the middle ear with a blocked Eustachian tube. In this case there is absorption of all the components, which means that the carbon dioxide too is absorbed. A subcutaneous gas pocket devised in the rat by Rahn and Canfield" gradually contracted and eventually disappeared. The pressure of the gases inside the pocket was maintained by the elasticity of the skin and tissues. But in the ear the
situation is different (Fig. 2). The ear is a nearly rigid chamber which can decrease its volume only very little by the inward movement of the drum and by the increased volume of the mucous membrane lining the cavity. Consequently, when the Eustachian tube is blocked, there is a growing underpressure and when this reaches a certain limit (which is not known as far as I know) transudation occurs. The fluid reduces the volume of the airfilled cavity and a pressure difference between the cavity and the tissues is maintained by this mechanism. If the tube is blocked long enough, the whole cavity will ultimately be filled with transudate. This mechanism is well known in physiology and can be experimentally studied, for instance, in rigid chambers made of stainless steel netting and lodged subcutaneously in some experimental animal. An interesting point is the question of how large amounts of gas or air are absorbed from the middle ear and how fast an underpressure develops (Fig. 3). The sum of the partial pressures of the gases in the tissues is about 700 mm Hg and in the middle ear about 760 mm Hg. So, there is a diffusion gradient of about 60 mm Hg between the ear and the surrounding tissues. We have studied the absorption indirectly by measuring the volume passing through the Eustachian tube during a given period of time, compensating for the loss caused by absorption. The input must be the same as the output. This statement is valid if the Eu-
From the ENT Clinic, University Hospital, Lund, Sweden.
161
162
A.ELNER 0l'EN VENTLA18l
OPEN NON VENTILATED
NORMAL LUNG
NORMAL EAR
CLOSED
EAR WITH A ILOCKED 1\& LINl WITHIIIlONCHIAI. OIISTRUCTKlN
Fig. 1. Biological gas pockets according to Rahn.'
stachian tube is the only way by which significant volumes of air can enter the middle ear and if absorption from the cavity to the capillaries in the mucous membrane is the only way by which air can leave the cavity. One of the problems involved is whether air can diffuse through the normal tympanic membrane (Fig. 4). Theoretically, there may be a passage of oxygen from the ear canal to the middle ear, of nitrogen from the middle ear to the ear canal, and of carbon dioxide from the ear to the ear canal. Carbon dioxide is in constant equilibrium and a "leakage" through the drum is of no importance. Oxygen and nitrogen can pass, though in very small amounts, as has been shown by us on fresh tympanic membrane preparations." Compared to the amount normally passing through the Eustachian tube in 2~ hours, it amounts to less than 0.1% an~ IS negligible. So, the air volume entering the middle ear through the tube must be the same as that leaving the cavity by absorption. As the middle ear is not a completely rigid chamber (Fig. 2), the change in the mass of the enclosed gas, L, V01> is counteracted by the volume displacement
of the drum when sucked inwards, L, V till, and also by the increased volume of the mucous membrane lining the cavity, L, Vm u c • Thus we get: L, Vd i fl = L, V t = L, Vm + L, V tm + L, VmucThe small change in the mass of the enclosed gas, L, Vm, can be calculated when we know the volume of the mastoid cells and the middle ear. With the method and equipment described by Elner et al,4,5 it has been possible to record the volume displacement of the drum L, V tm, when it recoils from a retracted position to its normal position, as small underpressures - created when the tube is kept closed for five to ten minutes _ are equilibrated by swallowing. From earlier investigation by Ingelstedt et al6 we know the magnitude of the volume change of the mucous membrane, L, Vmu c , per em H 2 0 underpressure in the ear. By adding the volumes above during the time covered by the experiment (Fig. 5) we can calculate the volume per hour or per 24 hours proSUM OF PARTIAL PRESSURES OF GASES IN THE TISSUES -700mm Hg
EAR CANAL EAR CANAL
EAR
TUBE
/
~~t
Fig. 2. Middle ear system with its variables. 6P t m - Pressure across the drum. 6 V, - Volume passing through the Eustachian tube.
t E.T.
Fig. 3. Diffusion gradient between the ear and the surrounding tissues.
GAS EXCHANGE
163
TABLE I GAS ABSORPTION FROM THE MIDDLE EAR IN 24 HOURS Riu et af
0.8 ml
Ingelstedt and Jonson'
1-2 ml
Elner et al' 0.7-1.1 ml [van Dishoeck' -5 • -8 em H,O/hr corresponding to -
vided the absorption conditions are identical. We have found the mean value to be 33 fLl/hour or 0.7-1.1 ml/24 hours under normal conditions. When the absorbed volume is known, we can also calculate the resulting underpressure. This is about 3-6.5 em H 2 0 per hour for a mastoid cell and middle ear system of 9-4 ml. If we have a larger volume, the resulting underpressure will be lower and in smaller systems somewhat higher. On the basis of recordings of the pressure drop in the ear in cases with perforated tympanic membranes (Table I) Riu et aF calculated the absorbed volumes and found approximately the same values. Ingelstedt and jonson" punctured the mastoid cells in normal ears and recorded the pressure drop behind an intact tympanic membrane and the flow in the ear canal caused by the tympanic membrane movement. Also van Dishoeck," using his pneumophone method, found the same values of the pressure drop. When the amount of gas that has been absorbed is known, the different values for oxygen and nitrogen can also be calculated, provided the gas composition is known. Melvill jones'? reported values that are in fair accordance with those found later by Riu et al,7 using a gas TYMPANIC
TABLE II GAS COMPOSITION IN THE MIDDLE EAR ° AND IN ELASTIC, SUBCUTANEOUS GAS POCKETS o O
N,
0,
CO,
Melvill jones" Riu et aio
84 % 9 % 7 %
Rahn and Canfield °°
88.3%
85 % 9.5% 5.7%
5.5% 6 %
In fact, Riu et aP found the same values in cases with transudate and a poorly functioning Eustachian tube as in normal cases, and in recordings made by Ingelstedt et al1 2 the oxygen and carbon dioxide tension in transudate aspirated from the ear was found to be 5.5% and 8% respectively. This might mean that there is no "ventilation" in the proper sense of the word of the middle ear via volume
/'"
50
PC02 EAR
_-\-~O.
EAR
chromatograph method (Table II). It is very interesting that these values are similar to those reported by van Liew'! and Rahn and Canfield" in subcutaneous elastic gas pockets devised experimentally in rats. It is thus possible that the composition of air in the ear is stable and not affected by the opening of the tube.
40 -
MEMBRANE
CANAL
0.5-1 ml]
2 02
MIDDLE EAR
"".
-:> PC0 ATM 2 EAR "'PH. ATM
POo
EAR
''''7
30
ATM
20 -
10 ~:':---!-:---L--.L..--L---l.--L---'_J--
10 20 30 40 50 GO 70 80 90
Fig. 4. Gas diffusion through the tympanic membrane.
time min"
Fig. 5. Recorded curve of gas absorption.
164
A. ELNER
the tube and that the small amounts of air entering the ear do not affect the composition of the enclosed gas. The Eustachian tube would then function more or less as a pressure regulator only. This may also mean that the composition of
air in the middle ear in cases of secretory otitis media is the same as in normal cases and that the carbon dioxide and oxygen tensions do not act as "noxae" on the mucous membrane in secretory otitis media.
REFERENCES 7. Riu R, Flottes L, Bouche J, et al: La 1. Rahn H: The role of N, gas in various biological processes with particular reference Physiologie de la Trompe d'Eustache. Paris, Librairie Arnette, 1966 to the lung, in The Harvey Lectures, Series 55. SUNY, Buffalo, New York, 1961, pp 1738. Ingelstedt S, Jonson B: Mechanism of 199 the gas exchange in the normal human mid2. Rahn H, Canfield RE: Volume changes dle ear. Acta Otolaryngol [Suppl.] (Stockh) and steady state behaviour of gas pockets 224:452, 1967 within body cavities. WADC TR 357, 1955 9. van Dishoeck HAE: Negative pressure 3. Elner A: Gas diffusion through the tymand loss of hearing in tubal catarrh. Acta Otopanic membrane. Acta Otolaryngol (Stockh) laryngol (Stockh) 29:303-312, 1941 69: 185, 1970 10. Me1vill Jones G: Pressure changes in the 4. Elner A, Ingelstedt S, Ivarsson A: A middle ear after altering the composition of method for studies of the middle ear mechan- contained gas. Acta Otolaryngol (Stockh) ics. Acta Otolaryngol (Stockh) 72: 191, 1971 53:1, 1961 5. Elner A: Indirect determination of gas 11. van Liew HD: Tissue PO, and 0, esabsorption from the middle ear. Acta Oto- timation with rat subcutaneous gas pockets. laryngol (Stockh) 74:191, 1972 J Appl Physiol 17:851, 1962 6. Ingelstedt, S, Ivarsson A, Jonson B: 12. Inge1stedt S, Jonson B, Rundcrantz H: Mechanics of the human middle ear. Acta Oto- Gas tension and pH in middle ear effusion. laryngol [Suppl.] (Stockh) 228, 1967 Ann Otol Rhinol Laryngol 84:198-207, 1975 REPRINTS - A. Elner, M.D., ENT Clinic, University Hospital, Lund, Sweden.
