High-resolution measurements of middle ear gas volume changes in the rabbit enables estimation of its mucosal CO₂ conductance

Transmucosal CO₂ exchange in the middle ear (ME) of the New Zealand White rabbit (Oryctolagus cuniculus) was studied using an accurate novel detecting and recording system for measuring gas volume changes at constant pressure, based on a principle that was previously used by Kania et al. (Acta Otolaryngol 124:408-410, 2004). After the ME cavity was washed with ambient air, the initial diffusion rate of CO₂ (V̇_iCO₂ from the blood perfusing the ME mucosa was calculated from gas volume change measurements. In nine cases, V̇_iCO₂ calculated after normalization due to shifts in baseline was 314 ± 112 μL•h ^-1 (mean ± SD). In two cases where normalization was not needed, V̇_iCO₂ was 409 μL•h^-1 (276 and 543 μL•h^-1). Normalization of V̇_iCO ₂ data was also made in five additional cases where secretion of fluids from the lining of the ear canal was observed. In these cases V̇_iCO₂ was 245 ± 142 μL•h^-1. No differences were found between results obtained in the three groups. Thus, an overall mean value of V̇_iCO₂ 305 ± 131 μL•h^-1 (n = 16) was calculated. An effective coefficient of conductance of CO₂ (G₂) between the mucosal circulation and the ME gas cavity of the New Zealand White rabbit was estimated to be ≈0.05 μL (h•Pa)^-1 and compared to the G₂ estimated for humans in a different study.