Measurements of perilymphatic oxygen tension in guinea pigs exposed to loud sound

Using different types of custom-made oxygen-sensitive microelectrodes, the perilymphatic oxygen partial pressure (PO2) was determined in anesthetized guinea pigs. Cochlear temperature, heart rate, and arterial blood pressure and acid-base status were monitored. The PO2 in the basal scala tympani perilymph (200 microns below the round window membrane) was found to be 53 +/- 17 mmHg (mean +/- SD) in 33 normal animals. In 11 guinea pigs exposed to loud sound for 15 min (10 kHz pure tone, 125 dB SPL) there was on average a continuous decline in the perilymphatic PO2, which was significant only 30 min post-exposure. A considerable variation in response was found in individual animals. Mean arterial blood pressures showed a slightly increasing time course, while heart rates did not change significantly during the whole period of the experiment. Arterial acid-base status and PO2 values remained within normal limits and did not change significantly. Cochlear microphonics and compound action potentials were substantially decreased after acoustic overstimulation. The results are discussed with due consideration of sources of error.