Offline comparison of processed electroencephalogram monitors for anaesthetic-induced electroencephalogram changes in older adults

Abstract

Background: Several devices record and interpret patient brain activity via electroencephalogram (EEG) to aid physician assessment of anaesthetic effect. Few studies have compared EEG monitors on data from the same patient. Here, we describe a set-up to simultaneously compare the performance of three processed EEG monitors using pre-recorded EEG signals from older surgical patients.

Methods: A playback system was designed to replay EEG signals into three different commercially available EEG monitors. We could then simultaneously calculate indices from the SedLine® Root (Masimo Inc., Irvine, CA, USA; patient state index [PSI]), bilateral BIS VISTA™ (Medtronic Inc., Minneapolis, MN, USA; bispectral index [BIS]), and Datex Ohmeda S/5 monitor with the Entropy™ Module (GE Healthcare, Chicago, IL, USA; E-entropy index [Entropy]). We tested the ability of each system to distinguish activity before anaesthesia administration (pre-med) and before/after loss of responsiveness (LOR), and to detect suppression incidences in EEG recorded from older surgical patients receiving beta-adrenergic blockers. We show examples of processed EEG monitor output tested on 29 EEG recordings from older surgical patients.

Results: All monitors showed significantly different indices and high effect sizes between comparisons pre-med to after LOR and before/after LOR. Both PSI and BIS showed the highest percentage of deeply anaesthetised indices during periods with suppression ratios (SRs) > 25%. We observed significant negative correlations between percentage of suppression and indices for all monitors (at SR >5%).

Conclusions: All monitors distinguished EEG changes occurring before anaesthesia administration and during LOR. The PSI and BIS best detected suppressed periods. Our results suggest that the PSI and BIS monitors might be preferable for older patients with risk factors for intraoperative awareness or increased sensitivity to anaesthesia.

Keywords: EEG; bispectral index (BIS); brain monitoring; entropy; patient state index (PSI); processed EEG.

Copyright © 2021 British Journal of Anaesthesia. Published by Elsevier Ltd. All rights reserved.

Figures

Fig 1

The anaesthesia playback system equipment set-up. (a) Pre-recorded EEG files are converted and replayed on the National Instruments (NI) PXIe-8821 to the NI PXI-6733 digital-to-analogue converter, which then directs EEG signals to the NI CA-1000. The NI CA-1000 is directly connected to and outputs EEG to individual monitors simultaneously. Outputs from the three monitors are collected on a laptop with RugloopII and ADC software programs installed (these programs collect and store the monitor indices). Arrows indicate the direction of data flow through the playback system. (b) Processed EEG monitors approximate electrode locations for the SedLine Root (red), bilateral BIS VISTA (blue), and the Datex Ohmeda S/5 monitor with the Entropy Module (orange). The Root and Entropy electrode locations closely follow those of the standard 10–20 electrode locations, as indicated. However, the BIS electrode positions (labeled LT and RT) are located on the zygomatic arch as it extends laterally approximately an inch anterior and a quarter of an inch superior from the external acoustic meatus.

Fig 2

Example of brain activity and monitor trajectories for a full anaesthesia case. Spectrograms of the EEG activity throughout an entire anaesthesia case (top F7; middle F8). Displayed in the bottom-line plot is the patient state index (PSI), bispectral index from the left (BIS-L) and right (BIS-R) electrodes, response entropy (Entropy-R), and state entropy (Entropy-S). Note that the entropy measures are only calculated unilaterally. The moment when the patient no longer responded to verbal commands (loss of response [LOR]) and recovery of response (ROR) to verbal commands are indicated with dashed vertical lines at the beginning and end of the case, respectively. The patient lightening at ∼20 min (indicated by increase in high-frequency power and indices) is when surgical stimulation began and nitrous oxide was added for anaesthetic maintenance.

Fig 3

Representative patient around loss of response (LOR) to verbal stimuli. Density spectral arrays showing EEG spectral activity around LOR for (a) F7 and (b) F8. (c) The trajectories of the indices outputted by the monitors in the same time period are shown. All monitors show appropriate decreases in indices around LOR as anaesthetic depth increases, although some have more abrupt transitions than others. (d) The index ranges outputted by the processed EEG monitors from 27 patients during 20 s segments before anaesthesia are given (pre-med) and before (Pre-LOR) and after (Post-LOR) LOR. All indices show decreases across patients. The central mark on the box plot indicates the median; the bottom and top edges are the 25th and 75th percentiles, respectively; and the whiskers extend to the most extreme data points not considered outliers. The ‘+’ indicates individual outlier data points. Wilcoxon signed-rank significance values (P) reported are uncorrected.

Fig 4

Example of brain activity and monitor trajectories from awake to EEG suppression during patient induction. (a) A 6 min clip of raw EEG activity (blue trace) from F7 is shown, where period of suppressed EEG has been identified in red. Suppressed periods were identified manually. (b) Clear decreases in the EEG spectral activity can be seen for periods of suppression in the corresponding density spectral array. (c) The percentage of suppression (per 4 s epoch) closely tracks EEG suppression onset and offset. (d) The trajectories of the outputted indices synchronised with this EEG activity.

Fig 5

Example of brain activity and monitor trajectories in a patient exhibiting several incidences of intraoperative EEG suppression. The synchronised density spectral array from (a) F7 and (b) F8 is shown with (c) corresponding percentages of EEG suppression in 4 s epochs (F7, blue traces; F8, grey traces). Two insets above the percentage of EEG suppression represent 4 s clips of EEG activity from corresponding time points indicated by the arrows and red vertical bars. These represent F7 clips with 31% suppression (left trace) and 77% suppression (right trace) with identified suppressed periods in red (c, insets). Left and right red bars correspond to the left and right traces, respectively. (d) The trajectories of the synchronised monitor indices are shown with labels indicating relevant clinical events. BIS-L, bispectral index from the left; BIS-R, bispectral index from the right; Entropy-R, response entropy; Entropy-S, state entropy; PSI, patient state index.

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