Effects of hypobaric hypoxia on the middle-latency and steady-state auditory evoked potentials.

The aim of the present paper was the investigation of middle-latency responses (MLRs) and steady-state responses (SSRs) during and after a 30-min exposure to hypobaric hypoxia (5182 m above sea level). The test was performed in a hypobaric chamber on 8 male audiologically normal volunteers. The auditory stimulus (500-Hz tone burst), delivered at rates of 10 and 40/s for MLRs and SSRs, respectively, was recalibrated in the hypobaric condition because of the reduced air density. Absolute latencies of waves Na and Pa and their interpeak amplitude were the MLR parameters investigated; for the SSRs, the first positive wave (P1) absolute latency and the interpeak amplitude between P1 and the first negative wave (N1) were considered. The results showed an absence of statistically significant modification of the MLRs. On the contrary, the SSRs showed a significant (p < 0.025) latency increase during hypoxia, with an immediate recovery upon return to ground level. No significant changes of SSR amplitudes were observed. One possible data interpretation is related to the higher stimulation rate adopted for the SSRs; a second possibility could be a different electrogenesis between MLRs and SSRs.