In vivo measurement of O₂ and CO₂ gas exchange across the human tympanic membrane

Abstract

Conclusion. The hypothesis that the human tympanic membrane (TM) is permeable to CO₂ and O₂ 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 O₂ and CO₂ 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 O₂ (32 AMU) and CO₂ (44 AMU) were regressed on time and the slope divided by the predicted transTM partial-pressure gradients to yield estimates of transTM O₂ and CO₂ conductance. Results. Consistent with expectations for transTM gas exchange, ECP + EC O₂ decreased and CO₂ increased during the experiments. CO₂ increase was faster after application of epinephrine to the TM. The ratio of O₂/CO₂ conductances was not consistent with the gas exchange through a primarily water or lipid diffusion barrier.

• Middle ear
• tympanic membrane
• gas exchange