Effect of anesthetic agents and middle ear pressure application on distortion product otoacoustic emissions in the gerbil.

The functional status of the middle ear system has a crucial importance in the measurements of distortion product otoacoustic emissions (DPOAEs), because each emission signal has to be detected indirectly in the external canal. It was observed that DPOAEs were scarcely detectable in the gerbil anesthetized with pentobarbital. On the other hand, when ketamine was used as an anesthetic, the DPOAE levels were generally high. The differences in the effects of these anesthetic agents on the DPOAEs became less clear when the tympanic bulla was opened. This strongly suggests that the effects might be due to a modification of the middle ear pressure. This study was designed to elucidate the mechanisms of the effects of these anesthetics on the DPOAEs. Comparing the effects of pentobarbital and those of pressure application to the middle ear on the frequency characteristics of DPOAEs, the following conclusions emerged: (1) pentobarbital administration causes negative middle ear pressure in the gerbil; (2) the generated pressure strongly reduces DPOAE conduction through the middle ear; and thus (3) proper selection of anesthetic agents is very important in gerbil experiments that involve OAE measurements.