Middle ear pressure change during controlled breathing with gas mixtures containing nitrous oxide

The change in middle ear pressure while breathing gas mixtures containing N(2)O was studied in four monkeys. At each of three experimental sessions, monkeys were anesthetized, acclimated for 60 min, breathed with room air for 60 min, and then breathed with 5, 10, or 20% N(2)O for 60 min. Middle ear pressure, rectal temperature, and vital signs were recorded throughout. The time constant for blood-middle ear N(2)O exchange was calculated from these data. Middle ear pressure decreased during acclimation, was stable during air breathing, and increased during N(2)O breathing. The rate of pressure change was similar for both ears of each animal and was directly related to N(2)O percent. The calculated time constant ranged from 0.003 to 0.008 min(-1) across animals but was not different for a given ear across sessions. These results show that breathing gas mixtures containing N(2)O causes predictable and quantifiable increases in middle ear pressure.