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TECHNOLOGY, COMPUTING, AND SIMULATION

The Impact of Acoustic Stimulation on the AEP Monitor/2 Derived Composite Auditory Evoked Potential Index Under Awake and Anesthetized Conditions

Department of Anesthesiology, University Hospital Regensburg, Germany

Address correspondence and reprint requests to Frank Weber, MD, Department of Anesthesiology, University Hospital Regensburg, Franz-Josef-Strauß-Allee 11, D-93053 Regensburg, Germany. Address e-mail to frank.weber{at}klinik.uni-regensburg.de.

The 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^TM) 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.