Effects of changes in arterial oxygen and carbon dioxide upon cochlear microphonics.

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Abstract
In some cats, reduction of arterial O2 from an avg. max. possible value of 14.1 vol. % to 6 to 9 vol. % was accompanied by definite though reversible depressions in microphonics. Variability among cats was considerable with regard to effectiveness of the less severe degrees of hypoxia in reducing microphonics. In this connection, the possible significance of circulatory adjustments and of incidental changes in arterial CO2 was mentioned. The percentage fall in microphonics during hypoxia was unrelated to the initial amt. of cochlear response at the moderate levels elicited in this study. Complete recovery of microphonic output followed arterial O2 concns. as low as 2 to 3 vol. % maintained for as long as approx. 30 min. No instance of complete recovery followed hypoxia of sufficient severity to lower arterial O2 significantly below 2 vol. %. When arterial O2 was below about 3 vol. %, the lower the level of arterial O2, the longer was the time required to reach max. recovery. When the decline in microphonics during hypoxia exceeded 80%, subsequent recovery upon termination of hypoxia was reduced and its temporal course was greatly prolonged. No more than 8 to 9 sec. need elapse between the inhalation of O2 in room air and a significant increase in microphonics already depressed by hypoxia. Cochlear microphonics were reversibly reduced by inspired CO2 in O2 (or air) in concns. between 5.2% and 25%.

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