Perilymph pressure during hypobaric conditions--cochlear aqueduct obstructed.

Nine cats with the cochlear aqueduct surgically blocked were anaesthetised, put in a pressure chamber, and stepwise exposed to hypobaric pressure, each of the two steps lasting 5 min. The chamber pressure was reduced to -5 and -7 kPa. Concluding the experiment the cats were observed for 5 min after restoration of atmospheric pressure. The perilymph, tympanic cavity, cerebrospinal fluid, central venous, arterial and surrounding chamber pressures were continuously recorded. Reduced chamber pressure caused relative overpressure within the tympanic cavity, thereby inducing pressure gradients between the chamber, the tympanic cavity, the labyrinth and the cerebrospinal fluid and vascular pressures respectively. Our results demonstrate that hypobaric effects on the labyrinth are mediated via pressure changes in the tympanic cavity and not by systemic, vascular or cerebrospinal fluid influence. Substantial perilymph overpressure lasted throughout the hypobaric conditions. The magnitude and the duration of this overpressure were influenced by the patency of the Eustachian tube whereas the rate of the pressure change in the chamber was of little importance. As atmospheric pressure was restored in the chamber the overpressure in the labyrinth was reversed to underpressure--relative to pre-experimental conditions.