Influence of General Anesthesia on Transiently Evoked Otoacoustic Emissions in Humans

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Abstract
The influence of general anesthesia (GA) on transiently evoked otoacoustic emissions (TEOAEs) was studied in 19 normally hearing women undergoing surgery. Emissions were measured on the day before the operation, after premedication but before the beginning of the operation, and during and after the operation. There were no significant differences in TEOAE amplitude or in reproducibility between results obtained the day before the operation and after premedication. Ten patients received nitrous oxide (N2O) during GA (N2O group), and 9 patients did not (non-N2O group). The amplitude of TEOAEs was reduced during GA in 9 of 10 patients in the N2O group and in 7 of 9 patients in the non-N2O group. However, the average decrease of amplitude after the first 10 minutes was greater in the N2O group (4 ± 3.4 dB) than in the non-N2O group (0.18 ± 1.4 dB). The corresponding mean reproducibility of the response decreased in 9 of 10 patients of the N2O group (29% ± 24%) and was nearly unchanged in the non-N2O group (2.3% ± 7.2%). The time course of the amplitude reduction was similar in both groups. The smallest amplitudes were reached on an average by 19.3 ± 11.4 minutes in the N2O group and by 17 ± 13.6 minutes in the non-N2O group. Preoperative and postoperative TEOAEs were comparable in level and reproducibility. Differential frequency effects imply a middle ear effect for the greater reduction of TEO AE amplitudes in the N2O group due to gas diffusion into the middle ear. The decreases in the amplitudes of TEOAEs in patients not receiving N2O indicate the presence of additional factors affecting the response during GA.