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.


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.
Department of Pediatric Otolaryngology, Children's Hospital of Pittsburgh, Pennsylvania 15213, USA.
Ulrik N
UN
Felding
Juliane M
JM
Banks
William J
WJ
Doyle
D000818:Animals; D001128:Argon; D005064:Eustachian Tube / Q000502:physiology*; D006371:Helium; D008252:Macaca fascicularis; D008416:Mastoid / Q000502:physiology*; D008876:Middle Ear Ventilation; D010313:Partial Pressure; D013997:Time Factors; D014432:Tympanic Membrane / Q000502:physiology
D006371:Helium; D001128:Argon