Variation of distortion product otoacoustic emissions with furosemide injection

Cochlear function was monitored in adult gerbils using distortion product otoacoustic emissions (DPOAE) during intraperitoneal injection of furosemide. All stimulus parameters were varied independently over a wide range, the stimulus frequencies f1 and f2 from 1 to 16 kHz, and the stimulus levels L1 and L2 from 20 to 80 dB SPL. The observed emissions at 2f1-f2 and 3f1-2f2 could be considered to be made up of two distinct components: (1) an 'active' source which depended in a complex way on the stimulus frequencies and levels, which was dominant at low and moderate stimulus levels, and which, by definition, was eliminated by sufficient furosemide intoxication; and (2) a 'passive' source which was essentially the same at all frequencies, with a level dependence given approximately by a simple power law distribution. The change from the active to the passive source was usually accompanied by an abrupt shift in emission phase angle. A simple summation model was shown to account for the observed form of this transition. The amount of the decrease in 2f1-f2 emission amplitude after furosemide injection was approximately independent of frequency and consistent for the middle frequency ratios and intensity levels (f2/f1 approximately equal to 1.3, L1 x L2 approximately equal to 55 x 50 dB SPL). It was concluded that the combination of DPOAE with furosemide injection can usefully be employed as a probe of active cochlear mechanics.