Middle ear pressure after changes in steady state

The combined effect of changes in middle ear (ME) gas composition and changes in systemic arterial blood oxygenation on total ME pressure was studied in three anesthetized juvenile Rhesus monkeys (Macaca mulatta). The gas composition of the ME was altered by ME inflation (politzerization) using either pure nitrogen, oxygen, or carbon dioxide while the animal was ventilated with either room air or 100% oxygen at constant carbon dioxide blood gas tension. Total ME pressure was measured indirectly by tympanometry (acoustic impedance) for a 5-hour duration. The changes in ME pressure were consistent and reproducible, exhibiting different pressure-time patterns unique for each gas. Carbon dioxide resulted in the most rapid decrease in ME pressure, followed by oxygen. The slowest decrease was observed in experiments with nitrogen. The systemic hyperoxygenation had little or no effect on the results. The findings were explained by the differences in relative permeabilities of these gases influencing ME gas diffusion, but the lack of systemic hyperoxygenation effect remained unexplained.