In vivo measurement of O(2) and CO(2) gas exchange across the human tympanic membrane.

CONCLUSION: The hypothesis that the human tympanic membrane (TM) is permeable to CO(2) and O(2) at physiologic pressure gradients is supported but additional experiments need to be done to validate this methodology.
OBJECTIVE: Gas exchange between the middle ear and adjacent compartments determines the trajectory of middle ear pressure change. Little information is available regarding the permeability of the TM to physiological gases. This study aimed to determine in vivo if the human TM is permeable to O(2) and CO(2) at physiologic transTM pressure gradients. SUBJECTS AND METHODS: An ear canal (EC) probe (ECP) constructed from a custom-fitted acrylic body, a glass capillary tube enclosing an oil meniscus to maintain ambient ECP + EC pressure and a silica glass microtube linked to a mass spectrometer (MS) for measuring gas composition was hermetically sealed within one EC in each of 15 adults. ECP + EC volume was measured and gas samples were taken at 10 min intervals for 1 h. Epinephrine (1:100 000) was applied topically to the ipsilateral TM to decrease blood flow and the experiment was repeated. The ECP + EC pressures of O(2) (32 AMU) and CO(2) (44 AMU) were regressed on time and the slope divided by the predicted transTM partial-pressure gradients to yield estimates of transTM O(2) and CO(2) conductance.
RESULTS: Consistent with expectations for transTM gas exchange, ECP + EC O(2) decreased and CO(2) increased during the experiments. CO(2) increase was faster after application of epinephrine to the TM. The ratio of O(2)/CO(2) conductances was not consistent with the gas exchange through a primarily water or lipid diffusion barrier.