Perfusion and diffusion limitations in middle ear gas exchange: The exchange of CO₂ as a test case

A long standing debate on perfusion/diffusion limitations in the context of middle ear (ME) gas exchange was revisited using data obtained from previous iso-pressure gas-exchange measurements in different mammals. We tried to determine whether the exchange of CO₂ in the ME is limited by perfusion or by diffusion by comparing the mass specific cardiac output (msQ̇) and the mass specific initial CO₂ flow rate into air-washed MEs (msVi̇CO2) of rabbits and rats. Based on previously published allometry at rest, the msQ̇ was 0.154mL/(ming) in rabbits (mean body weight: 2800g) and 0.259mL/(ming) in rats (mean body weight: 179.1g); msVi̇CO2 (Δt=0) was 0.109±0.047μL/(hg) in rabbits (n=16) and 0.170±0.094μL/(hg) in rats (n=9). Similar ratios were found when an allometric comparison was made between the ratio of msVi̇CO2 (Δt=0) (∼0.64), and the ratio of msQ̇s (∼0.59) in rabbits and rats.If the active mucosal surface areas of MEs of rabbits and rats are directly proportional to their masses as are the masses of their hearts and if their msQ̇s are proportional to the rates of blood flows in the ME mucosa, these results support the assumption that the exchange of CO₂ in the ME of mammals is mainly perfusion (and not diffusion) dependent.