DIRECTIONAL ASYMMETRY IN THE MEASURED N2O TIME-CONSTANT FOR MIDDLE EAR TRANSMUCOSAL GAS EXCHANGE

摘要

Simple, two-compartment models of transmucosal gas exchange for the middle ear (ME) are useful for describing ME pressure behavior under a variety of conditions. The most well developed exchange models require input of an experimentally determined, lumped-parameter, exchange-constant for each represented gas species. Evidences from past studies suggest that the value of the exchange-constant for inert gases is specific to the direction of the extant pressure-gradient, an unexpected result. In this study, the N2O transmucosal exchange-constant (time-constant) for 16 ears of 8 monkeys was measured for positive and negative ME-blood N2O gradients using previously published methods. The results showed that the two time-constant measures were highly correlated, but that the ME-blood time-constant was approximately 13 times greater than the blood-ME time-constant. This directional effect depends on the value of a gradient-ratio (arterial-venous/arterial-ME) included as a parameter in the time-constant for perfusion-limited gas exchange. All values of that ratio not equal to 1 introduce directional asymmetry into the measured time-constant. The significance of this phenomenon to current models and as a lead to the development of more complex, distributed models of transmucosal gas exchange is discussed.