Transmucosal O2 and CO2 exchange rates for the human middle ear

Abstract Objective Estimate the transmucosal CO2 and O2 rate-constants for adult middle ears (MEs). Methods Ten adults with healthy MEs had a unilateral myringotomy. A custom-fitted acrylic mold with a valved line to a mass spectrometer (MS) and central tube coupled to a 3-way valve and connected to a pressure transducer (the probe) was sealed with adhesive glue within the ipsilateral ear-canal. A second 3-way valve was attached to the probe valve, a flow-regulated tank gas source and paired syringes. Volumes of the ME and probe were measured. On sequential days, the probe + ME was washed for 15-min with 6% O2 , Balance N2 and 25% O2 , 6% CO2 , Balance N2 to create transmucosal CO2 and O2 gradients, respectively. After washing, the probe + ME was isolated from the gas source, and baseline and 10-min gas samples were obtained for MS analysis of gas partial-pressures. The rates of change in ME CO2 and O2 pressures were divided by their established transmucosal gradients to yield CO2 and O2 rate-constants. Results The average (±STD) transmucosal CO2 and O2 rate-constants were 0.062 ± 0.034 ( N = 10, range: 0.032–0.119) and 0.011 ± 0.009 ( N = 8, range: 0.002–0.032) mm Hg/min/mm Hg, respectively. The average half-life for the CO2 and O2 gradient was 11.1 and 61.6 min. The average CO2 :O2 rate-constant ratio was 8.1 ± 4.0 ( N = 8, range: 3.6–14.6). Conclusions For adult human MEs, transmucosal CO2 exchange is rapid and much faster than transmucosal O2 exchange. The estimated CO2 /O2 rate-constant ratio for the human ME is not consistent with that predicted for diffusion-limited gas exchange across a water-based barrier.