Middle ear gas exchange in isobaric counterdiffusion.

Nitrous oxide entry into the middle ear gas space was studied in cats in relation to anesthesia and the vestibular dysfunction caused by isobaric inert gas counter-diffusion in diving. A catheter implanted in the auditory bulla was used for direct gas sampling and pressure measurements. Experiments were designed to evaluate the participation of the eustachian tube, mucosal blood vessels, and tympanic membrane in middle ear gas exchange. The eustachian tube did not contribute to N2O entry and the mucosal blood supply only contributed about one-third of the total N2O accumulation. Diffusion across the tympanic membrane accounted for most of the N2O entering the middle ear from ambient and respiratory environments containing N2O.