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Dynamic measurement of gas composition in the middle ear. II: Steady state values.

On-line measurement of ME gas composition in normal middle ears of 5 anesthetized guinea pigs at an established steady state was performed by mass spectrometry. The mean values for the gas composition of the middle ear were found to be: PN2: 82.4%, PO2: 7.6% and PCO2: 10.0%. This composition is very different from that of atmospheric air, and very similar to the gas composition of mixed venous blood. Our conclusions are that the gas composition is basically controlled by interchange with gases present in the blood and not by introduction of air through the eustachian tube. It is therefore proposed that middle ear gas deficiency is secondary not to eustachian tube input failure but to excess loss of middle ear gas due to enhanced diffusion into the blood. This situation exists especially under inflammatory conditions when there is an enlarged number of blood vessels promoting increased gas diffusion into them. Under ordinary conditions this middle ear gas deficiency will probably cause no significant underpressure because the mastoid pneumatization will act as a pressure buffer. When mastoid pneumatization is lacking, as happens in the otitis media syndrome, a pathological negative pressure will ensue.