1Department of Anesthesiology, Stanford University School of Medicine, Stanford, CA, USA. 2Palo Alto VA Health Care Center, Palo Alto, CA, USA*Corresponding author: Department of Anaesthesiology and Intensive Care Medicine, University of Bonn, Sigmund Freud Str. 25, D-53105 Bonn, Germany
Accepted for publication: April 5, 2001
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Abstract |
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Br J Anaesth 2001; 87: 5057
Keywords: monitoring, depth of anaesthesia
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Introduction |
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Approximate entropy is a novel EEG measure which quantifies the regularity of a data time series such as the EEG.4 Approximate entropy quantifies the predictability of subsequent amplitude values of the EEG, based on the knowledge of the previous amplitude values.
We report two patients in which we measured the BIS and approximate entropy at the time of onset of burst suppression of the EEG.
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Patient 1 |
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Patient 2 |
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After having established a propofol target concentration of 2 µg ml1 at the time 10:17, the propofol target concentration was increased to 4 µg ml1 at the time 10:37 (see Fig. 2). The BIS decreased to 40, the approximate entropy decreased to 0.7. At 10:57, the propofol target concentration was increased to 8 µg ml1. The BIS remained unchanged at 40, the approximate entropy further decreased to 0.4. The suppression ratio calculated by the Aspect monitor showed beginning appearance of burst suppression pattern (Fig. 2).
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Discussion |
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The higher variations of the approximate entropy values compared with the BIS values at low propofol target concentrations in Patient 1 are because approximate entropy is calculated without artifact rejection, while an artifact rejection mode is incorporated in the BIS algorithm.
In the first patient, burst suppression appeared after increasing propofol target concentration from 6 to 9 µg ml1. Although, burst suppression ratio is a part of the BIS algorithm,1 6 this did not change the BIS. As the suppression part of the burst suppression pattern is classified as highly regular, the approximate entropy algorithm correctly classifies increasing burst suppression ratio as increasing anaesthetic drug effect.7 The decrease of approximate entropy indicated the increased pharmacodynamic action.
In the second patient, the BIS remained unchanged despite an increase of the propofol target concentration from 4 to 8 µg ml1. The suppression ratio calculated by the Aspect monitor (Fig. 2) failed to indicate additional CNS suppression. The change in approximate entropy value indicated the pharmacodynamic situation. At this time, quantification of the phase coupling by the BIS appeared less sensitive to drug effect than quantification of regularity by approximate entropy.
With an older BIS version (version 3.12) and a combination of sufentanil, nitrous oxide, and isoflurane Detsch and colleagues recently reported paradoxical increases of BIS when isoflurane concentrations was increased.8 This was noted in the same range of BIS values between 30 and 40, where we found that the more recent BIS version (version 3.22) gave a value of BIS that failed to indicate increased propofol effect.
BIS values between 40 and 60 have been suggested as clinically adequate during surgery. The range of uncertainty between phase coupling and burst suppression ratio is at BIS values between 30 and 40. Targeting BIS between 40 and 60, a BIS value between 30 and 40 is always a too deep state of anaesthesia. The cases we report demonstrate that, based on the BIS alone, it may be difficult to determine how much too deep the state of anaesthesia is, which is clinically relevant information. Choosing the suppression ratio as a second trend line on the Aspect monitor, or visual inspection of the raw EEG on the screen for burst suppression pattern would have been helpful to provide a more exact depth of anaesthesia at a BIS value between 30 and 40, particularly in the first patient.
We only noted this problem with the BIS with these specific degrees of propofol drug effect at the onset of burst suppression.
When BIS values are between 30 and 40, an increased drug pharmacodynamic effect may not be adequately reflected by the BIS value. The calculation of the burst suppression ratio may be helpful, as new evidence of burst suppression or an increasing burst suppresion ratio with unchanged BIS can highlight this problem.
Approximate entropy, a measure sensitive to signal regularity, indicated EEG changes after the occurrence of burst suppression more readily than the BIS.
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References |
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