Multifaceted Anesthesia

Initial impressions of the clinical applicability of entropy monitoring

Dr Doug Campbell, BM, FRCA, FANZCA Clinical Senior Lecturer, University of Auckland Department of Anaesthesiology Auckland, New Zealand http://www.health.auckland.ac.nz/anaesthesiology/

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How it all started

I first became interested in entropy as applied to the electroencephalogram (EEG) after reading papers from Jamie Sleigh from University of Waikato. I went to the World Congress of Anesthesia in Montreal in 1999, and heard Hanna Viertiö-Oja from Datex-Ohmeda, Helsinki present her research with various entropy algorithms as applied to the EEG of anesthetized patients. I was impressed by the way spectral entropy tracked the various stages of anesthesia. Since then, I have followed the progress in the development of a clinical Entropy module.

My initial impression

I have had the benefit of using the Datex-Ohmeda M-Entropy module for the last couple of months. Initial impressions were favorable. The electrode was simple to place and then connect to the module. The lack of a head box is an obvious and welcome improvement. The impedance check is done automatically. Within a few seconds, a single channel EEG, state entropy (SE), and response entropy (RE) are displayed. Being part of an integrated monitor rather than a stand-alone unit is useful.

Some practical observations

The SE appears to track what I believe to be the hypnotic component of the anesthetic, and with an averaging time of 15s seems to adequately describe the baseline hypnotic state. The RE has an averaging time of only 2s, and does appear to respond quickly after a change in stimulus or a stepwise change in drug concentration that is later followed by SE. However, the RE also includes high frequency EEG and EMG components in the 32-47 Hz range. I have noticed that sometimes the RE will rise in response to a change in surgical stimulus like knife to skin that is not followed by a change in SE. I assume this might be a response to inadequate analgesia rather than inadequate hypnosis. This might be a useful addition to being able to monitor the hypnotic state but I have not seen this phenomenon often enough to be sure this is what is happening.

So far, the SE and RE seem to track changes in the hypnotic component of an anesthetic. I will be interested to see the module used in a wide spectrum of cases and anesthetic drug groups. I have not yet seen any idiosyncratic activity associated with the Entropy module.

Interesting to see what the future will bring

My initial impressions are that this monitor can reliably track the pharmacodynamic effect in the cortex in the patients I have used it on. With interest, I await further work in wider patient groups and different anesthetic groups. Obviously, the whole anesthetic community is interested in monitors of this type being able to reduce the incidence of awareness.

Auckland Hospital, New Zealand [Photo by Dr. Campbell]

City view: Auckland, New Zealand [Photo by Dr. Campbell]

Last updated: 25 March 2004 Created
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