Role of nitrogen in transmucosal gas exchange rate in the rat middle ear

This study investigates the role of nitrogen (N-2) in transmucosal gas exchange of the middle ear (ME). We used an experimental rat model to measure gas volume variations in the ME cavity at constant pressure. We disturbed the steady-state gas composition with either air or N-2 to measure resulting changes in volume at ambient pressure. Changes in gas volume over time could be characterized by three phases: a primary transient increase with time (phase 1), followed by a linear decrease (phase II), and then a gradual decrease (phase III). The mean slope of phase II was -0.128 mu l/min (SD 0.023) in the air group (n = 10) and -0.105 mu l/min (SD 0.032) in the N-2 group (n = 10), but the difference was not significant (P = 0.13), which suggests that the rate of gas loss can be attributed mainly to the same steady-state partial pressure gradient of N2 reached in this phase. Furthermore, a mathematical model was developed analyzing the transmucosal N-2 exchange in phase II. The model takes gas diffusion into account, predicting that, in the absence of change in mucosal blood flow rate, gas volume in the ME should show a linear decrease with time after steady-state conditions and gas composition are established. In accordance with the experimental results, the mathematical model also suggested that transmucosal gas absorption of the rat ME during steady-state conditions is governed mainly by diffusive N-2 exchange between the ME gas and its mucosal blood circulation.