Rate of gas exchange in the middle ear of guinea pig

Gas exchange between blood in the middle ear (ME) mucosa and ambient ME gas may be limited by diffusion through tissue or blood perfusion. In order to study the limiting factors in ME gas exchange, a hole was drilled in the bulla of 14 anesthetized guinea pigs through which a mass spectrometer probe was inserted and sealed in place. The rate at which oxygen (O2), carbon dioxide (CO2), nitrogen, and argon concentrations changed toward their steady state values was recorded. From the exponential fitted curves, gas rate constants (Kg) were calculated. The ratio KCO2/KO2 was 4:1, which is lower than expected from a diffusion-limited process in an aqueous compartment. The different rate ratios of CO2 and O2 indicate a diffusion-limited process. However, the deviation of the KCO2/KO2 ratio from that expected in aqueous solutions may indicate the involvement of a lipid compartment in gas exchange or other physiological mechanisms such as local acidity.