CO₂ gas exchange across the human tympanic membrane is not appreciably affected by pathology

Abstract

Past in vivo studies in humans showed that the tympanic membrane (TM) is permeable to physiological gases. Animal studies show that transTM CO₂ conductance is increased by TM pathology. The objective of the study was to determine if transTM CO₂ exchange in humans is affected by atrophic and sclerotic pathologies. The study used 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 that was hermetically sealed within the ear canal of the test ear. ECP + EC volume was measured and gas samples taken at 10 min intervals for 1 h. The fractional CO₂ pressure measured in the ECP + EC for each sample was regressed on time and the slope of the function multiplied by the ECP + EC volume and divided by the estimated transTM CO₂ gradient at the start of the experiment to yield transTM CO₂ conductance (µL/min/Pa). Data were complete for 15 normal, 13 sclerotic and 9 atrophic TMs. The average (+std) transTM CO₂ conductances were 1.76 × 10⁻⁴ + 7.27 × 10⁻⁵, 2.26 × 10⁻⁴ + 1.5 × 10⁻⁴ and 2.36 × 10⁻⁴ + 1.14 × 10⁻⁴ µL/min/Pa/TM for the normal, sclerotic and atrophic TMs, respectively. A pairwise comparison of data for the normal and atrophic TMs under the directional hypothesis of a greater CO₂ exchange rate for thinner TMs approached statistical significance (P = 0.07). A similar pairwise comparison for the sclerotic and normal TMs did not approach statistical significance (P = 0.28). The effect of TM pathologies on CO₂ conductance was limited.