Response of the GE Entropy™ monitor to neuromuscular block in awake volunteers.

BACKGROUND: The GE Entropy™ monitor analyses the frontal electroencephalogram (EEG) and generates two indices intended to represent the degree of anaesthetic drug effect on the brain. It is frequently used in the context of neuromuscular block. We have shown that a similar device, the Bispectral Index monitor (BIS), does not generate correct values in awake volunteers when neuromuscular blocking drugs are administered. METHODS: We replayed the EEGs recorded during awake paralysis from the original study to an Entropy monitor via a calibrated electronic playback system. Each EEG was replayed 30 times to evaluate the consistency of the Entropy output. RESULTS: Both State Entropy and Response Entropy decreased during periods of neuromuscular block to values consistent with anaesthesia, despite there being no change in conscious state (State Entropy <60 in eight of nine rocuronium trials and nine of 10 suxamethonium trials). Entropy values did not return to pre-test levels until after the return of movement. Entropy did not generate exactly the same results when the same EEG was replayed multiple times, which is primarily because of a cyclical state within the Entropy system itself. CONCLUSIONS: The GE Entropy™ monitor requires muscle activity to generate correct values in an awake subject. It could therefore be unreliable at detecting awareness in patients who have been given neuromuscular blocking drugs. In addition, Entropy does not generate the same result each time it is presented with the same EEG.

Frequency response of EEG recordings from BIS and Entropy devices.

The BIS and Entropy systems are used as indicators of anaesthetic drug effect, and can also record EEGs in digital form. A number of studies have used such recordings for analysis, even though information about bandwidth and fidelity has not been provided by the manufacturers. In this study we consider these systems purely as EEG recording devices, and evaluate their suitability for quantitative analysis. Using a calibrated electronic testing system, we played an artificial test signal of known amplitude with frequency varying from 0.01 to 400 Hz into the BIS and Entropy EEG systems, and saved the recording in each of their supported recording modes. The amplitude of the recording at each frequency was then used to derive the frequency response curves, as well as the 3 dB and 0.5 dB points for each mode. There are important differences between the various BIS and Entropy recordings. The BIS 256 Hz and the Entropy 400 Hz recordings are broadly comparable, although Entropy has a slightly reduced response above 50 Hz and below 1 Hz. The BIS 128 Hz recording has a somewhat reduced bandwidth, due to its relatively low sampling rate. The Entropy 100 Hz recording in the Datex-Ohmeda S/5 monitor has a flawed implementation, leading to aliasing of signals over 50 Hz and potential distortion of the recording, while in the GE Carescape it has an uneven response and a narrowed bandwidth. Consequently, it is important to know which specific host monitor was used when an Entropy 100 Hz recording was made. In summary, the choice of recording device and host monitor may affect the results of some quantitative EEG analysis, and some previously published studies may need to be re-evaluated.