• Anesthesia and analgesia · Aug 2005

    The impact of acoustic stimulation on the AEP monitor/2 derived composite auditory evoked potential index under awake and anesthetized conditions.

    • Frank Weber, Markus Zimmermann, and Thomas Bein.
    • Department of Anesthesiology, University Hospital Regensburg, Germany. frank.weber@klinik.uni-regensburg.de
    • Anesth. Analg. 2005 Aug 1;101(2):435-9, table of contents.

    UnlabelledThe AEP Monitor/2 features an auditory evoked potential (AEP) and electroencephalogram (EEG)-derived hybrid index of the patient's hypnotic state. The composite AEP index (AAI) is preferably calculated from the AEP, but in case of low signal quality it is based entirely on the spontaneous EEG. We investigated the impact of auditory input on the AAI in 16 patients with correctly positioned headphones for acoustic stimulation and headphones disconnected from the patient's ears under awake and anesthetized conditions. The AAI and the Narcotrend Index (NI), another EEG-based measure of hypnotic depth, were recorded simultaneously. AAI values under awake and anesthetized conditions were higher with correctly positioned headphones than with headphones disconnected from the patient's ears (P < 0.05) but remained within the range indicating the patient's actual hypnotic state as given by the manufacturer of the monitor. Under awake conditions with correctly positioned headphones we observed frequent fluctuations between AEP-derived and EEG-derived AAI, whereas with headphones disconnected from the patient's ears the AAI calculation was completely EEG based. Acoustic stimulation had no impact on the Narcotrend Index. Although relevant misinterpretations of the patient's hypnotic state as a consequence of a turnover from AEP-derived to EEG-derived AAI values should not occur, an improved harmonization of the two methods of indexing would be desirable.ImplicationsThe AEP Monitor/2 generates an Index (AAITM) indicating the patient's hypnotic state by analyzing either auditory evoked potentials (AEP) or spontaneous electroencephalographic (EEG) activity. We demonstrate that, though significantly different under AEP-derived or EEG-derived conditions, AAI values remain within the range indicating the patient's actual hypnotic state as given by the manufacturer of the device.

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