Anesthesia effects on the electrically evoked middle latency response in guinea pigs

Recent data indicate that the electrically evoked middle latency response (EMLR) is useful for patient selection for cochlear implantation and may provide a test for determining safe levels of electrical stimulation in cochlear implant recipients. Some anesthetic agents have been reported to alter the auditory evoked middle latency response. The aim of our study was to examine the effects of ketamine and xylazine anesthesia on the EMLR in guinea pigs. A consistent, reproducible, and significant depression in the EMLR was observed after anesthesia. Response latencies were increased and the suprathreshold amplitudes were depressed initially, but later increased above preanesthetic values. Changes followed a predictable time course of depression and overshoot, which allows the investigator to compensate for these effects of anesthesia. No change in threshold was observed. The lack of threshold change and the predictable course of suprathreshold depression indicates that the EMLR may be useful to evaluate responsiveness of the auditory system to electrical stimulation in the anesthetized animal.