Effect of Isoflurane Anesthesia on Cochlear Function in Domestic Pigs: A Pilot Study

Introduction: Domestic pigs have been considered robust animals for investigations in auditory sciences, due to their human-like endocochlear and genetic characteristics. However, isoflurane, one of the most commonly used anesthetics in animal experimentation, can affect cochlear mechanics and interfere with data interpretation. Therefore, in this pilot study, we aimed to analyze the initial effects of isoflurane on cochlear function in domestic pigs. Methods: Five large white pigs, approximately 20 kg, were anesthetized with intravenous propofol after receiving intramuscular ketamine and xylazine pre-anesthetics. Anesthesia was maintained with constant inhalation of 1.5 to 2% isoflurane. Cochlear function was measured using distortion product otoacoustic emission (DPOAE), where ƒ2 ranged from 1000 to 6000 Hz. Measurements were performed twice on the right ear. First measurement (Test 1) was performed approximately 40 minutes after the start of isoflurane, enough time for the other medications to leave the bloodstream. Second measurement (Test 2) was performed approximately 80 minutes later, ensuring a 40-minute difference between the two tests. The five ears included showed transient evoked otoacoustic emission amplitude greater than 0 dB SPL in at least four frequency bands at the beginning of Test 1, and all procedures were approved by the Institution`s Animal Use Ethics Committee (1989/2023). Findings: The mean amplitude and signal-to-noise ratio of DPOAE decreased at all frequencies in the Test 2 in comparison to Test 1; however, the difference was significant only at 1500, 2000, 3000 and 4000 Hz (t-Test p<0,05). Noise did not present a statistical difference between the two tests, regardless of the frequency tested. Discussion and Conclusion: Despite the small sample size, our findings consistently show a reduction in DPOAE responses under isoflurane anesthesia, suggesting a decrease in cochlear function. There are two main hypotheses to explain this: (1) isoflurane alters cochlear blood flow, directly affecting cochlear function, or (2) isoflurane alters middle ear mechanics, directly affecting the responses coming from the cochlea. We are currently investigating these hypotheses by conducting a research with a well-adjusted methodological design.