Directional Asymmetry in the Measured Nitrous Oxide Time Constant for Middle Ear Transmucosal Gas Exchange

Abstract

Objectives:

Simple, 2-compartment mathematical models of middle ear (ME) transmucosal gas exchange reproduce observed ME pressure behavior. These models require input of an experimentally determined, lumped-parameter exchange constant for each represented gas species. Previous model applications assumed directional asymmetry for those parameters, which has not been experimentally validated.

Methods:

As a surrogate for the inert gas nitrogen (N₂), for which exchange is too slow to be measurable, the nitrous oxide (N₂O) transmucosal exchange constant for 16 ears of 8 monkeys was measured for positive and negative ME blood N₂O gradients.

Results:

The paired exchange constants for each ear were highly correlated, but the ME-blood/blood-ME exchange constant ratio was approximately 13. Modeling shows this asymmetry to depend on the value of the arterial-venous/arterial-ME ratio, a variable in the exchange constant for perfusion-limited gases.

Conclusions:

These results support an asymmetric rate of transmucosal N₂O and, by extension, N₂ exchange for the ME. Because the primary controlling parameter for ME pressure behavior in the absence of eustachian tube opening is the rate of transmucosal N₂ exchange, this effect needs to be incorporated into the simple 2-compartment exchange models for predictive accuracy. The gradient ratio dependence suggests that parameter-free modeling may require treating the ME mucosa as having a distributed gradient for certain gas species.