Effects of carbon dioxide in the middle ear cavity upon the cochlear potentials and cochlear pH

To elucidate the effects of CO2 in the middle ear upon the cochlea, measurements were made of the cochlear potentials (compound action potential and endocochlear potential) and of the pH of the inner ear fluids and the organ of Corti. Gas containing CO2 did not affect the AP threshold, except for a slight decrease in AP threshold elicited by an 8 kHz tone burst with 10% CO2 flow. The EP did not vary with CO2 gas. The CO2 gas mixture reduced the pH in perilymph significantly, by 0.11 +/- 0.05 with 5% CO2 and by 0.17 +/- 0.04 with 10% CO2, in comparison with 100% N2. The CO2 gas slightly but significantly decreased the endolymph pH, by 0.05 +/- 0.04 with 5% CO2 and by 0.09 +/- 0.06 with 10% CO2. The removal of perilymph led to a greater acidification of endolymph with CO2 gas. Acidification of the organ of Corti was also noted with the CO2 gas flush. These findings indicate that CO2 in the middle ear influences the acid-base regulation of inner ear fluids and the cochlear function.