EFFECT OF HYPOXEMIA AND ETHACRYNIC-ACID ON ABR AND DISTORTION-PRODUCTEMISSION THRESHOLDS

Various studies have shown that induction of hypoxemia in animals such that arterial blood oxygen tensions reach 20-30 mm Hg is accompanied by reversible threshold elevations of the auditory nerve-brain-stem ev oked response (ABR). In this state, the endocochlear potential (EP) is depressed, causing a smaller potential difference across the hair cel ls and/or reduced activity of the cochlear amplifier of the outer hair cells. In order to test these possibilities, ABR threshold (an expres sion of the overall sensitivity of the cochlea) and changes in thresho ld of the cubic (2f(1)-f(2)) distortion product emissions (DPE) (an ex pression of activity of the cochlear amplifier) were measured in the s ame cats while the EP was depressed by hypoxemia or by ethacrynic acid . During the episodes of hypoxemia, DPE thresholds were elevated by 10 dB while ABR thresholds were elevated by 22.8 dB. Therefore, it seems that a normal EP is necessary both for normal cochlear transduction ( inner hair cells) and for normal cochlear amplification (outer hair ce lls). The human fetus in utero is relatively hypoxic and there is evid ence that its auditory threshold is also similarly elevated. Therefore the threshold elevation in the fetus in utero, estimated to be about 20 dB, is a consequence of both reduced transduction current through t he inner hair cells (about 10 dB) and an additional 10 dB reduction in the activity of the cochlear amplifier of the outer hair cells.