Gas composition of the normal and the ventilated middle ear cavity.

Epidemiologic and controlled studies indicate that late minimal hearing impairment is a sequelae after the use of a ventilation tube in early childhood. The patho-physiology is unknown, but abnormal middle ear gas composition might be important. Therefore it is mandatory to measure middle ear gas composition in order to understand the gas exchange in the normal middle ear, as well as the change in gas composition associated with ventilation tubes. Accordingly, the aim of this study was to measure middle ear gas composition both in the physiologic state and in artificial ventilation by a transtympanic tube. Employing puncture of the typanic membrane, through a liquid seal, we aspirated 300 microliters of middle ear gas. The procedure was carried out under the otomicroscope on adults without any anesthesia. A total of 58 normals participated, along with 10 persons with unilateral ventilation tubes and 1 with a patent Eustachian tube. The mean values of physiologic state were: Partial pressure of oxygen in the middle ear cavity = 39 mm Hg, and partial pressure of carbondioxide in the middle ear cavity = 48 mm Hg. The mean values of artificially ventilated ears were: Partial pressure of oxygen in the middle ear cavity = 138 mm Hg, and partial pressure of carbondioxid in the middle ear cavity = 15 mm Hg. The total imprecision was 4.2/4.4 mm Hg and the accuracy seems fair,--especially because we found a quasi equilibrium to the "most probable value", the venous blood gases. It is concluded that artificial ventilation of the middle ear cavity, with a ventilation tube increases the oxygen content of the middle ear cavity with a factor 3.2. This constitutes a relative hyperoxic atmosphere with a subsequent possibility for a toxic tissue damage.