PERILYMPH PRESSURE DURING HYPOBARIC CONDITIONS - COCHLEAR AQUEDUCT OBSTRUCTED

Nine cats with the cochlear aqueduct surgically blocked were anaesthet ised, put in a pressure chamber, and stepwise exposed to hypobaric pre ssure, each of the two steps lasting 5 min. The chamber pressure was r educed to -5 and -7 kPa. Concluding the experiment the cats were obser ved for 5 min after restoration of atmospheric pressure. The perilymph , tympanic cavity, cerebrospinal fluid, central venous, arterial and s urrounding chamber pressures were continuously recorded. Reduced chamb er pressure caused relative overpressure within the tympanic cavity, t hereby inducing pressure gradients between the pressure chamber, the t ympanic cavity, the labyrinth and the cerebrospinal fluid and vascular pressures respectively. Our results demonstrate that hypobaric effect s on the labyrinth are mediated via pressure changes in the tympanic c avity and not by systemic, vascular or cerebrospinal fluid influence. Substantial perilymph overpressure lasted throughout the hypobaric con ditions. The magnitude and the duration of this overpressure were infl uenced by the patency of the Eustachian tube whereas the rate of the p ressure change in the chamber was of little importance. As atmospheric pressure was restored in the chamber the overpressure in the labyrint h was reversed to underpressure - relative to pre-experimental conditi ons.