Gas composition and pressure in the middle ear: a model for the physiological steady state.

The gas contents of the physiological middle ear periodically cycle through two phases in steady state. During phase I, the eustachian tube is shut and the middle ear gas space is effectively closed. Gas is absorbed or produced at the mucosal surface, and the total pressure changes correspondingly. During phase II, which is of very short duration, the eustachian tube opens, a bolus of gas passes between the middle ear and the nasopharynx, and the total pressure in the middle ear rapidly adjusts to that in the nasopharynx. Since nasopharyngeal pressure fluctuates in time, so does the pressure in the middle ear. The effect of these pressure changes is to produce a level of ventilation in the middle ear, which depends on a combination of three parameters: the volume of the middle ear, multiplied by the mean amplitude of pressure variations in the nasopharynx, divided by the mean elapsed time between successive eustachian tube openings. Assuming steady-state conditions, the composition of middle ear gas can be computed and is predicted to range from PN2 = 621, PO2 = 46, PCO2 = 46, PH2O = 47 mm Hg in the case when nasopharynx fluctuations are small, to a match with nasopharyngeal gas composition, when the fluctuations are large.