Background and Goal of Study: The dose dependent suppression of mid-latency auditory evoked potentials by general anesthetics has been proposed to measure depth of anesthesia.1 In this study, perioperatively recorded mid-latency auditory evoked potentials were analysed in a time-frequency space to identify significant changes induced by general anesthesia.
Materials and Methods: Perioperatively recorded AEPs (binaural stimuli, 1000 sweeps) of 19 patients were submitted to a multi scale wavelet analysis. We calculated energy contents of the signal in several frequency bands. Multiple comparison test with Bonferroni correction was performed on the data. We plotted the energy density of signal fragments on a 10 ms grid for the frequency bands (0-57.1 Hz, 57.1-114.3Hz, 114.3-228.6Hz and 228.6-457.1 Hz).
Results and Discussions: Statistical evaluation showed a highly significant decrease of the wavelet energies for the frequency bands 57.1-114.3 Hz (p = 1.49 × 10−8) and 114.3-228.6 Hz (p = 1.35 × 10−8) for the measuring points representing deep general anesthesia, accompanied by a significant decrease of the wavelet energy of the frequency band 228.6-457.1 Hz (p = 0.0006) and a decrease in the wavelet energy of the frequency band 0-57.1 Hz of no statistical significance (p = 0.03), most prominent in the post stimulus interval between 10 ms and 30 ms.
Conclusion(s): These high frequency components may therefore not only reflect the response of the target organ of anesthesia, but a surrogate parameter, i.e. muscle activity.
1 Drummond JC: Monitoring depth of anesthesia: with emphasis on the application of the bispectral index and the middle latency auditory evoked response to the prevention of recall. Anesthesiology 2000; 93:876-882.
2 O'Beirne GA, Patuzzi RB: Basic properties of the sound-evoked post-auricular muscle response (PAMR). Hear Res 1999; 138:115-132.