Effects of the depth of anesthesia on distortion product otoacoustic emissions

Generation of distortion product otoacoustic emissions in infants with a combined air and bone conduction stimulus

OBJECTIVES

Evaluation of Distortion Product Otoacoustic Emissions (DPOAEs) by combining Air Conduction (AC) and Bone Conduction (BC) stimuli in infants.

METHODS

Measurements were performed in 19 normal hearing infants, and in 23 adults serving as a control group. The stimulus consisted either of two AC tones, or of combined AC/BC tones. DPOAEs were measured for f2 at 0.7, 1, 2, 4 kHz, and a constant ratio of f2/f1 = 1.22. Sound pressure level of the primary stimulus L1 was held constant at 70 dB SPL, while the level of L2 was decreased in 10 dB steps from 70 to 40 dB SPL. A response was included for further analysis when DPOAEs reached a Signal to Noise Ratio (SNR) of ≥6 dB. Additional DPOAE responses of <6 dB SNR were included when visual inspection of the measurements indicated clear DPOAEs.

RESULTS

DPOAEs could be elicited in infants at 2 and 4 kHz for the AC/BC stimulus. DPOAE amplitudes evoked by the AC/AC stimulus were larger than those by the AC/BC stimulus, with the exception of 1 kHz. The highest amplitudes of DPOAEs were registered for a stimulation level of L1 = L2 = 70 dB, with the exception of AC/AC at 1 kHz, where the highest amplitudes were with L1-L2 = 10 dB.

CONCLUSIONS

We demonstrated that DPOAEs can be generated in infants by a combined AC/BC stimulus at 2 and 4 kHz. The high noise floor needs to be further reduced to achieve more valid measurements in frequencies <2 kHz.

Gas Anesthesia Impairs Peripheral Auditory Sensitivity in Barn Owls (Tyto alba)

Auditory nerve single-unit recordings were obtained from two groups of young barn owls (age, between posthatching days 11 and 86) in terminal experiments under two different anesthetic regimes: ketamine (6-11 mg/kg) plus xylazine (∼2 mg/kg); or isoflurane (1-1.5%) in oxygen, delivered via artificial respiration. In a second series of minimally invasive experiments, auditory brainstem responses (ABRs) were recorded in the same four adult barn owls (Tyto alba; age, between 5 and 32 months) under three different anesthetic protocols: ketamine (10 mg/kg) plus xylazine (3 mg/kg), isoflurane (1-1.5%), and sevoflurane (2-3%) in carbogen. Finally, the ABR measurements on adult owls were repeated in terminal experiments including more invasive procedures such as artificial respiration and higher isoflurane dosage. The main finding was a significant deterioration of auditory sensitivity in barn owls under gas anesthesia, at the level of the auditory nerve (i.e., a very peripheral level of the auditory system). The effect was drastic in the young animals that experienced threshold elevations in auditory nerve single-unit responses of ≥20 dB. ABR thresholds assessed repeatedly in experiments on adult owls were also significantly higher under isoflurane and sevoflurane, on average by 7 and 15 dB, compared with ketamine/xylazine. This difference already occurred with minimal dosages and was reversibly enlarged with increased isoflurane concentration. Finally, there was evidence for confounding detrimental effects associated with artificial respiration over many hours, which suggested oxygen toxicity.

Comparison of anesthetic agents on otoacoustic emissions in children: propofol vs ketamine

BACKGROUND

Otoacoustic emission (OAE) tests are important evaluation tools for diagnosis of peripheral auditory pathology. Sedation or general anesthesia may be required for the performance of the OAE tests. The aim of this retrospective study was to compare the effects of anesthetic agents, propofol and ketamine, on OAEs in children.

METHODS

Fifty healthy children who underwent tonsillectomy and/or adenoidectomy under general anesthesia were included in this study. Three anesthesia induction protocols were defined for this study and the anesthesiologist applied his or her own choice. Transient evoked otoacoustic emissions (TEOAEs) and distortion-product otoacoustic emissions (DPOAEs) were automatically recorded in both ears of each patient prior to anesthetic (predrug) and following the loss of consciousness 5 min later (postdrug) by an audiologist blinded to the method of anesthesia. Acceptable TEOAEs were defined as signal noise ratio (S/N) of above 3 dB SPL (decibel sound pressure level) and DPOAEs of 6 dB SPL or above. Between-group and within-group comparisons and correlations were performed for statistical analysis.

RESULTS

Retrospective review of the anesthesia charts from 44 cases that completed the study showed that propofol, ketamine, and sevoflurane induction protocols were used in 21, 18, and 5 cases, respectively. Measurements of 36 ears in the propofol group and 34 ears in the ketamine group were included in the final analysis. Postdrug TEOAE and DPOAE amplitudes were significantly lower than predrug amplitudes except at 8 kHz in the ketamine group. There was no significant statistical difference in postdrug DPOAE measurements between propofol and ketamine groups but a significant difference was observed at 2 and 3 kHz of postdrug TEOAE measurements. TEOAE measurements were below 3 dB in 8 of 34 ears after ketamine and in 1 of 36 ears after propofol administration. There was a significant difference between the groups with respect to the incidence of successful measurements of TEOAEs. The DPOAE measurements were affected less by these drugs.

CONCLUSION

DPOAE measurements were reduced similarly by propofol and ketamine anesthesia. Lower false outcome ratio in TEOAE measurements made propofol a better option than ketamine.