This study investigates the role of nitrogen (N2) and blood flow in the trans-mucosal gas exchange function of the middle ear (ME). We used an experimental rat model for measuring gas volume variations of the ME cavity at constant pressure. By initially disturbing the gas composition of the ME either with air or N2, the ME volume changes in air- and N2- flushed ME were compared.
Twenty rats were maintained under general anesthesia, which keeps the Eustachian tubes (ET) closed. The tympanic membranes were punctured and a transparent glass capillary was connected to the external ear canal, kept horizontal and sealed. The ME was then flushed with either room air (n=10) or nitrogen (n=10). A droplet of liquid was placed inside the lumen of each capillary allowing the gas volume changes to be monitored at constant pressure and temperature.
Changes in ME gas volume with time followed 3 phases. A primary transient volume increase with time (phase I) was observed, followed by a linear volume decrease (phase II). This was followed by phase III where the volume declined exponentially with time. Phase I was significantly more pronounced in ME flushed with N2 only, compared with air-flushed ME. The mean slope of phase II was -0.128μL/min ± 0.023SD (n=10) in the air group, and it was -0.105μL/min ± 0.032SD (n=10) in the N2 group. There was no significant difference in the slope values between the groups (p=0.13), suggesting that the rate of gas loss in phase II can be attributed mainly to the same steady-state nitrogen partial pressure gradient reached, whether the initial washout was with air or nitrogen and depends on the steady-state blood flow perfusing the ME. The time course of phase III suggests a possible role of mucosa accumulation in the trans-mucosal diffusive gas path.
Based on these results, a mathematical model was proposed to include the trans-mucosal N2 exchange in phase II. It takes gas diffusion, blood gas partial pressures and flow rate into account, predicting that ME gas volume should show a linear decrease with time after a steady-state ME blood flow rates and gas composition are established. The model predicts that the calculated effective blood flow in the ME is: ∼310 μL/min. Using the rates of the measured gas loss and the estimated blood flow, the ratio of ME effective blood perfusion to its Eustachian tube ventilation in an awake animal is about 2650 to 1.
Both the experimental results and the mathematical gas exchange model showed that the trans-mucosal gas absorption of the rat ME during steady-state conditions are controled mainly by diffusive N2 exchange between the ME gas and the mucosal blood circulation.