Abstract SNACC-32

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The Influence of Artefacts and EEG Pattern on Calculated Index Values of Four Different Commercially Available DOA Monitors.

1Karatchiviev V, 1Schneider G, 2Kreuzer M, 2Kochs E, 1Pilge S
1Witten/Herdecke University, Wuppertal, Wuppertal, , Germany; 2Technische Universität München, München, , Germany

During anesthesia, the EEG signal may contain some artefacts. These artefacts may influence calculated indices of the level of anesthesia. This study evaluates the influence of artefacts on calculated index values for different commercially available DOA monitors.

For the present analysis, recorded EEG data from a previous study were used. In it, patients were requested to squeeze the investigators hand in a 30s interval during slow induction and emergence phases of general anesthesia, thus allowing to assess their responsiveness every half a minute. The recorded EEG signal was later replayed to 4 different DOA monitor devices (Aspect A2000; BIS®; EntropyTM Module; CSI monitor; Narcotrend® monitor). We investigated the index values that the monitors displayed at the 30s intervals before and after the first change of the patient’s response and the 60s EEG sequences that immediately preceded them. A visual analysis of the EEG sequences was performed by three independent investigators, who evaluated them for the occurrence of artefacts/typical EEG pattern. Calculated index values at the 30s intervals before and after the first change of the patient’s response were defined as ”erroneous” if the value was higher than 80 for unresponsive patients, or lower than 60 or for responsive patients.

The predominant artefacts/EEG pattern found were high frequency signals (HF) and eye blinks (EB). Out of the 284 sequences analysed, 198 (70%) contained HF, 135(48%) contained EB and 78(27%) were free of these. The number of erroneous and missing index values is summarized in Graph 1. Table 1 shows the error rate, estimated as the fraction of erroneous indices in relation to the valid ones, for the groups of EEG sequences containing HF, EB and no artefacts. All monitors had a greater error rate in the groups of EEG sequences containing HF or EB than those containing no artefacts.

The greater prevalence of erroneous indices in the group of unconscious patients may reflect the monitors’ tendency to declare an unconscious patient conscious, thus making them more sensitive to a possible awareness episode. All but the NCT monitor seem to follow this approach, while the NCT has a greater number of missing values during induction/emergence, i.e. no index value was calculated from these dynamic periods.
The present data show a higher error rate for both high frequency signals and eye blink artefacts during unconsciousness, in contrast to only minor differences during consciousness. This may be due to the influence of high frequency signals and eye blinks, shifting EEG analysis towards "conscious" index values.

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