Gas flow into and within the middle ear

Both a normal and a narrowed eustachian tube (ET) are capable of equilibrating pressures between the middle ear (ME) and the atmosphere almost instantaneously. The objective of this study was to assess experimentally the effect of narrowing a simulated ET isthmus on air passage into the ME. A Perspex ME model (0.5 mL) was constructed in which a 1.5-mm long ET of 0.07- to 1.0-mm diameter and a "mastoid" of 0- to 10-mL volume were changeable. The ET could be opened and closed with a valve. A -5 mm H2O pressure difference between the system and the atmosphere was created by withdrawing gas from the system. The time required to equalize these pressures after opening the valve to the atmosphere were measured with a pressure transducer. A pressure difference of -5 mm H2O was created in the system when 1.3 to 6.5 microL of ME gas was removed. On ET valve opening, the pressure was equalized within 0.1 and 0.15 to 0.3 seconds for ET diameters of 1.00 and 0.07 mm, respectively, depending also on the "mastoid" volume. Similar results were obtained when the pressure was measured through the "tympanic isthmus" and "aditus ad antrum." Our model shows that under ordinary physiological conditions, the amount of gas that can pass through the ET during swallowing time (0.4 sec) is potentially higher than required to equalize a negative pressure. This is also the case when the ET is very narrow and open for a very short time. It is unlikely that any narrowing of the tube will, by itself, hamper gas transfer into or within the ME, as long as the ET is not totally obstructed.