Trans-mucosal inert gas exchange constants for the monkey middle ear

Objective: The purpose of the study was to estimate the exchange constants for Argon and N2 across the middle ear mucosa of monkeys for use in mathematical models of middle ear pressure regulation. Methods: In five anesthetized monkeys, the tympanic membrane was perforated and, with the animal breathing room air, a 13-ml probe attached to an Argon gas source and to a pressure transducer was introduced into the ear canal and sealed. The probe and middle ear volume was washed with Argon and then closed to the gas source. Gas samples were withdrawn from the probe at 15-min intervals for composition analysis using an online mass spectrometer. Probe partial pressures of Argon and N2 were regressed on time and the slopes of those functions were divided by the respective average partial pressure gradient for the interval to estimate the exchange constants. Results: The average trans-mucosal exchange constants for Argon and N2 were 0.0007±0.0001 and 0.0003±0.0001/min, respectively. The average ratio of the two constants was 2.60±0.36 which is not different from the value of 2.3 predicted for perfusion limited gas exchange. Conclusion: These results confirm a very slow, perfusion limited trans-mucosal exchange of N2 and other inert gases across the middle ear mucosa. They infer that the required frequency of Eustachian tube openings to prevent the development of middle ear effusion is low and on the order of one/day.