Human Auditory Steady-State Responses during Repeated Exposure to Hypobaric Hypoxia

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Abstract
This study was aimed at evaluating the time course of auditory steady-state response (SSR) variations during two consecutive exposures to hypobaric hypoxia. Six normal subjects were examined in a hypobaric chamber at ground level. Then, they climbed to a simulated altitude of 17000 ft (5182 m), where SSRs were recorded after 6, 12, 18, 24 and 30 min. Thereafter, they breathed 100% O2 and SSRs were recorded after 2 min of reoxygenation. A second exposure to hypoxia followed, with SSR recordings after 6 and 12 min. Finally, the subjects returned to ground level for recovery recording. A phase shift of SSR sinus wave was observed at the beginning of both exposures to hypoxia, although in the first recording (i.e. at 6 min) during the first exposure, the result was not statistically significant. A slight SSR phase shift was still detectable on return to ground level. The central acoustic pathway involved in SSR genesis was probably the area which was found to be most sensitive to hypoxia, compared to other parts of the auditory apparatus (e.g. the cochlea). This data could suggest an impairment of compensation mechanisms when consecutive exposures to hypoxia are performed.