Middle ear gas exchange in the air phase.

Objective: The rate of gas exchange between the tympanum and mastoid is important for middle ear pressure regulation. The best-studied model of middle ear pressure regulation is the cynomolgus monkey. The aim of this study was to determine the time required for the equilibration of tympanum and mastoid partial pressures for two inert gases, argon and helium, in cadaveric cynomolgus monkey middle ears.

Material And Methods: In separate experiments on 5 air-dried temporal bones from monkeys, helium or argon was introduced into the tympanum to bring the total pressure to 300 mmH2O above ambient and the partial pressures of all relevant gases in the tympanum and mastoid were measured using mass spectrometry at 0, 1, 3, 5, 10 or 15 min.

Results: The average ratios of tympanum:mastoid partial pressure at 0 min were 4.7 +/- 1.9 and 3.9 +/- 2.0 for helium and argon, respectively. By 5 min those ratios had decreased to 1.2 +/- 0.2 and 1.1 +/- 0.1, respectively, suggesting approximate equilibrium.

Conclusions: Air-phase gas exchange between the tympanum and mastoid is rapid. Partial-pressure gradients across the middle ear will be dissipated quickly and are not an important contributor to middle ear pressure regulation in the studied model.