DIRECT DEMONSTRATION OF GAS-DIFFUSION INTO THE MIDDLE-EAR

The gas composition of the middle ear differs from that of air, and re sembles the gas composition of mixed venous blood. This observation su ggests the existence of a bi-directional route for gas diffusion betwe en the middle ear and blood. In an attempt to demonstrate this route i n a direct way, we tracheotomized guinea pigs in such a way that they breathed freon-22 directly into the distal part of their tracheostomy. The proximal part of the trachea was sealed so that air by-passed the oropharynx and nasopharynx, thus preventing freon-22 from making cont act with the eustachian tube orifice. At the same time middle ear gase s were monitored with a mass spectrometer, through a measuring probe w hich was inserted into a hole in the bulla. The appearance of freon-22 in all middle ears-after 8 min on the average-demonstrated direct gas diffusion from the blood into the middle ear, since freon-22 could re ach the middle ear only from the blood stream, i.e., by diffusion. Dif ferences in gas diffusion rates into and from the middle ear may there fore play a role in regulating middle ear gas economy, and therefore i n middle ear pressure.