Abstract
The middle ear is a semi-rigid biological gas pocket which is closed most of the time. It is believed that pressure in the middle ear (ME) is regulated by a complex combined action of the Eustachian tube muscles, eardrum deformation and gas exchange with the blood circulating in the mucosal lining. Sudden pressure variations of thousands of kPa occur in everyday situations and can be slow as well as fast. Both external pressure changes and ME gas exchange processes generate differences between the ME and ambient pressure. To understand the mechanisms and effects involved with quasistatic high amplitude pressure variations, we need to know how middle ear pressure varies in normal circumstances, investigate both fast and slow regulation mechanisms, and determine the effect of such pressures on middle ear mechanics. To measure the normal variations in middle ear pressure, we developed a monitoring system which is used in ambulant patients. We will show some results from direct measurements in intact ears. To investigate a possible fast regulation process, we performed measurements of eardrum deformation in gerbils. We found that volume displacement of the pars flaccida is small compared to middle ear volume, so that its fast pressure regulating function is limited to a very small pressure range. Finally, we will discuss some recent results, obtained in rabbits, of the effect of static pressure variations on middle ear ossicle motions. With heterodyne interferometry, we measured motions of umbo and stapes at pressure change rates between 200 Pa/s and 1.5 kPa/s. We will show that hysteresis in these motions increases as pressure change rate decreases, quite in opposite to the common notion that such hysteresis is mainly caused by visco-elasticity. We conclude that static and dynamic friction are important aspects of ossicle motion at slow pressure variations, new aspects which should be taken into account when trying to model quasi-static ossicle mechanics.
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