Gas Exchange Across the Middle Ear Mucosa in Monkeys: Estimation of Exchange Rate

Objective: To estimate the rate of exchange of selected gases across the middle ear (ME) mucosa and define the exchange limitations. Design: At separate sessions, the ME was inflated via the eustachian tube with a bolus of pure nitrogen, carbon dioxide, oxygen, or nitrous oxide, and ME pressures were recorded by tympanometry at selected intervals for up to 4 hours. The slope of the function relating pressure change to pressure was calculated by least squares regression and used as an estimate of the rate constant for exchange of that gas (experiment 1). Because of the slow rate of nitrogen exchange, a second experiment was performed in which the tensor veli palatini muscle was unilaterally paralyzed. The ME was inflated with nitrogen, and the slope of the rate-pressure function for measurements at 24-hour intervals was used to estimate the rate constant. Subjects: Ten juvenile cynomolgus monkeys, six for experiment 1 and four for experiment 2. Results: The relative, average rate constants for carbon dioxide, nitrous oxide, oxygen, and nitrogen were 1, 10.7, 18.6, and greater than 700, respectively. Comparisons of these rates with those predicted by theory show that oxygen and carbon dioxide exchange is diffusion limited, and nitrous oxide and nitrogen is perfusion limited. Conclusions: The perfusion limitation for nitrogen suggests that its exchange rate is notably increased by inflammation from increased mucosal blood flow. Targeting inflammation for therapy of persistent ME effusions may decrease the rate of nitrogen exchange and reestablish normal ME pressure regulation. (Arch Otolaryngol Head Neck Surg. 1995;121:887-892)