Under normal circumstances the Eustachian tube is closed, so that the middle ear forms a closed gas pocket. Over longer time scales the amount of gas in the pocket may vary due to different mechanisms such as Eustachian tube action or gas exchange, but on the short time scale the amount of gas is a fixed quantity, as is the temperature of the gas. Changes in ambient pressure lead to pressure differences between the pressure in the ear canal and the pressure in the middle ear. These pressure differences will cause the eardrum to deform, and change the volume in which the middle ear gas is enclosed. From the universal gas law, we know that under conditions of fixed temperature and gas quantity, the volume of a gas is inversely proportional to its pressure. When the eardrum deforms it will change the pressure in the middle ear, reducing the pressure difference between the middle ear and the ear canal. A perfectly flaccid membrane would perfectly buffer any pressure change, but the eardrum has non-linear elastic characteristics. As a consequence it will buffer small pressure variations well, but at larger pressure differences it will not be able to deform more. In our basic research we have measured eardrum deformation in human temporal bones and in animal model preparations. Our measurements on pars flaccida in gerbil have shown that the pressure regulating role is limited to a pressure range of a mere 400 Pa. Measurements of human eardrum deformation shows that the pressure buffering role is better in ears with small gas content than in ears with large gas content. These findings are relevant in the clinical context: in an ear with de-regulated pressure it may be beneficial to reduce the middle ear gas content by partial obliteration of the mastoid so that the pressure regulative function of the eardrum will increase.Keywords: pressure, gas exchange, eardrum.
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