Transmucosal O2 and CO2 exchange rates for the human middle ear

Objective: Estimate the transmucosal CO2 and O-2 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% O-2, Balance N-2 and 25% O-2, 6% CO2, Balance N-2 to create transmucosal CO2 and O-2 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 O-2 pressures were divided by their established transmucosal gradients to yield CO2 and O-2 rate-constants. Results: The average (+/- STD) transmucosal CO2 and O-2 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 O-2 gradient was 11.1 and 61.6 min. The average CO2:O-2 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 O-2 exchange. The estimated CO2/O-2 rate-constant ratio for the human ME is not consistent with that predicted for diffusion-limited gas exchange across a water-based barrier. (C) 2011 Elsevier Ireland Ltd. All rights reserved.