Transmucosal O2 and CO2 exchange rates for the human middle ear

Estimate the transmucosal CO(2) and O(2) rate-constants for adult middle ears (MEs).

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% CO(2), Balance N(2) to create transmucosal CO(2) 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 CO(2) and O(2) pressures were divided by their established transmucosal gradients to yield CO(2) and O(2) rate-constants.

The average (±STD) transmucosal CO(2) 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)mmHg/min/mmHg, respectively. The average half-life for the CO(2) and O(2) gradient was 11.1 and 61.6 min. The average CO(2):O(2) rate-constant ratio was 8.1 ± 4.0 (N=8, range: 3.6-14.6).

For adult human MEs, transmucosal CO(2) exchange is rapid and much faster than transmucosal O(2) exchange. The estimated CO(2)/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.