Effects of prestimulatory carbogen inhalation on noise-induced temporary threshold shifts in humans and chinchilla.

Human subjects and chinchilla breathed either Carbogen or compressed air prior to high level sound stimulation. Following stimulation, threshold shifts and recovery to prestimulation thresholds were monitored. Behavioral audiometry was used for human subjects and electrocochleography was utilized to measure noise-induced threshold shifts in the animals. Results were similar for both human and animal subjects in that preinhalation of Carbogen (05 percent 02 and 5 percent CO2) was seen to decrease the amount of temporary threshold shift (TTS) and to increase recovery rate. Further, histologic study of a limited sample of the experimental animals suggested a notable reduction in the amount of sensory cell damage in subjects that breathed Carbogen before sound stimulation. To follow up on this encouraging indication, an additional experiment was conducted to investigate any differences between Carbogen-treated and non-Carbogen-treated animals with respect to histologically observable high level noise exposure damage. The follow-up experiment resulted in strong evidence that Carbogen inhalation prior to intense sound stimulation decreases the susceptibility to hair cell damage. Carbogen-treated chinchilla sustained cochlear sensory cell damage ranging between 1.1 percent and 1.5 percent, whereas non-Carbogen-treated, noise-exposed chinchilla were found to have between 1.6 percent and 4.6 percent sensory cell damage. The use of Carbogen in prophylactic treatment for unavoidable or accidental noise exposure is discussed.