Study of middle ear pressure in relation to Eustachian tube patency

The aviation environment exposes the aircrew to rapidly changing ambient pressures, Such pressure changes, which occur, more so in military flying affect the air containing cavities of the body, which include the semi-closed middle ear cavity. The aircrew must be able to equilibrate the pressure on both sides of the tympanic membrance and this is possible only with a normal tubal function. Failure to equalize pressure across the tympanic membrance may result in otitic barotraumas [1-7]. Difficulty with clearing the ears during flight is a common cause of temporary or even permanent grounding of aircrew.

Ascent to altitude usually offers no trouble because the gas in the middle ear cavity expands and escapes along the Eustachian tube into the nasopharynx, so that the pressure remains equal on both sides of the tympanic membrane. [1,8,9]. During descent, equilibration is brought about by the passage of air into the middle ear through the Eustachian tube. During descent, gas from the nasopharynx must enter the middle ear in order to maintain equilibrium between the atmospheric pressure outside and the gas pressure in the middle ear. The one-way valve mechanism of the Eustachian tube prevents the back flow of gas back into the middle ear cavity. The resultant relative increase of pressure on the outside of the tympanic membrance pushes the membrane into the middle ear cavity. As descent continues the membrane is pushed further into the middle ear cavity unless gas enters it though the Eustachian tube to equalize the pressure across the tympanic membrane preventing the development of otitic barotraumas.

It is usually necessary to perform some active manoeuvre to open the Eustachian tubes. Although there are several simple manoeuvres such as swallowing, yawning and jaw movements (methods of free choice), which may open the tube, these are often not very effective [1]. These individuals have to raise the pressure in the nasopharynx in order to force gas into the middle ear cavities. The rise in pressure is usually achieved by performing either a Valsalva or Frenzel's manoeuvre. Valsalva's manoeuvre is generally not recommended because of problems of performing it in the critical phase of flight or when wearing oxygen mask. It may also cause alternobaric vertigo or can cause convulsive syncope in pilots with low 'G' tolerance [1, 8, 9, 10]. The Frenzel's manoeuvre has the advantage of opening the Eustachian tube at lower nasopharyngeal pressures and of being performed during any phase of respiration but is also not recommended because of problems of performing in the critical phase of flight. Normal Eustachian tube function is, thus, an essential prerequisite for acceptance for aircrew duties.

Tympanometry is a noninvasive and objective method, which facilitates a complete evaluation of patency of the Eustachian tube, measurement of MEP and compliance of ear drum [2, 11-23]. When the tympanic membrane is intact, tubal function can be assessed during tympanometry by noting the MEP at maximum compliance. If MEP lies within the normal range, then the normal Eustachian tube function can be assumed.

The middle ear pressure is expressed in mm of H₂O pressure or deka Pascal (daPa) units (1 mm of H₂O pressure = 1.02 deka Pascal unit). If there is no air space in the middle ear cavity (e.g. when the tympanic membrane has undergone adhesive changes or the middle ear cavity is filled up totally with fluid) there is no peak (or maximum compliant point) and the tympanogram is flat. Similarly, if the Eustachian tube is blocked there will be a negative pressure because of continuous absorption of gases from the middle ear space [24]. High negative middle ear resting pressure is indicative of obstruction of the Eustachian tube. The finding of normal resting middle ear pressure does not necessarily indicate normal Eustachian tube function, but the finding of the negative middle ear pressure is presumptive evidence of Eustachian tube dysfunction [25].

The normal range for the middle ear pressure is somewhat controversial, as different researchers have suggested slightly different ranges. Jerger [26] postulates that negative MEP in excess of 100 mm of H₂O pressure only should be accepted as pathological. Negative middle ear pressure is attributed to faulty Eustachian tube function [15]. Presently internationally accepted range of normal middle ear pressure is + 100 mm of H₂O pressure [6, 11, 13, 15, 20, 25, 26].

A prerequisite for studying the pressure equilibrating function of the Eustachian tube is a pressure gradient between the middle ear and the ambient pressure, In experiments it is possible to induce such pressure gradients directly by changing ambient pressure by means of a hypobaric or decompression chamber. By these means it is possible to induce both positive and negative middle ear pressure in subjects with intact eardrums, independent of their active equilibrating capacity [1, 2, 27].

The present study was undertaken because the current IAF practice of assessment of the Eustachian tube function by checking the middle ear pressure by tympanometry was in variance with the International standard. The study aimed to review the present Indian standard and to define or identify the range of middle ear pressure that could be accepted fit for flying. Objective reevaluation of the range of middle ear pressure as safe for military flying in IAF, would help in reducing the grounding time of asymptomatic aircrew, having middle ear pressure outside the present accepted range of + 25 mm of H₂O.