© The Board of Management and Trustees of the British Journal of Anaesthesia 2004
CORRESPONDENCE |
Does bispectral analysis add anything but complexity? BIS sub-components may be superior to BIS for detection of awareness
Editor—We read with interest the analysis of Miller and colleagues1 who found that SyncFastSlow, the bispectral parameter of the bispectral index score (BIS), is not superior to the analogous power spectral parameter. In their study, they calculated PowerFastSlow, the logarithmic ratio of the power of high frequency components (40–47 Hz) and total frequency content (1–47 Hz). In their analysis, prediction of an awake or anaesthetized state was mainly attributable to the change in high frequency content of the EEG. We wish to congratulate the authors for their interesting findings.
The BIS algorithm contains an additional parameter, the Beta Ratio, which is the logarithmic ratio of high frequency components (30–47 Hz) and classic EEG frequency components (11–20 Hz).2 Inspired by the findings of Miller and colleagues, we decided to evaluate the performance of Beta Ratio and SyncFastSlow in the separation of awareness from unconsciousness. For this purpose, we re-analysed a previously reported study that had produced a very challenging set of EEG data. These data were from 40 patients with ASA physical status I or II who underwent elective surgery under general anaesthesia, with a period of intended awareness after intubation.3 Patients were randomized to receive: (i) sevoflurane/remifentanil (
0.1 µg kg–1 min–1); (ii) sevoflurane/ remifentanil (
0.2 µg kg–1 min–1); (iii) propofol/remifentanil (0.1 µg kg–1 min–1); or (iv) propofol/remifentanil (0.2 µg kg–1 min–1). Every 30 s after the start of the remifentanil, patients were asked twice to squeeze the investigator's hand. Awareness was defined as a verified (i.e. repeated) response to command. Sevoflurane or propofol were given until loss of consciousness (LOC1). Anaesthesia was increased and Tunstall's isolated forearm technique4 was used during neuromuscular block with succinylcholine. After tracheal intubation, propofol or sevoflurane were stopped until return of consciousness (ROC1). Propofol or sevoflurane were re-started to induce LOC2. After surgery, drugs were discontinued, and ROC2 was observed. Monitoring included standard anaesthesia parameters, EEG, Patient State Index (PSI), and BIS (Aspect A-1000, BIS version 3.3, Aspect Medical Systems Inc., Newton, MA, USA) derived from recommended electrode positions. EEG electrode impedances were <5 k
. The high pass was set at 0.25 Hz, no low pass was used, and the notch filter (50 Hz) was enabled. The EEG was continuously digitized at 256 Hz per channel and simultaneously with the other monitoring parameters recorded on a personal computer. Beta Ratio and SyncFastSlow, two sub-components of the BIS2 were calculated from the digitized EEG from a 15 s time window with a 15 s averaging interval for parameter smoothing. For analysis, values at the last command before and at LOC and ROC were used (Fig. 1). Prediction probability (Pk)5 of the Beta Ratio and SyncFastSlow were calculated and compared to the previously reported Pk of BIS. Statistical analysis was performed with two Excel Macros, PKMACRO and PKDMACRIO, provided by Warren D. Smith Ph.D. (Professor, Biomedical Engineering Program, California State University, Sacramento, CA, USA). Pk values were compared using t-scores for paired data. We found that the Pk of the Beta Ratio was 0.825 (0.023) (Pk (SE)), and Pk for SyncFastSlow was 0.733 (0.028). Interestingly, Pk of both subcomponents was higher than Pk of BIS (0.685 (0.029)). Thus, we conclude that for the separation between awareness and unconsciousness, SyncFastSlow, the bispectral component of BIS related to the complexity of the EEG signal, may not add much information. In addition, in our data only the spectral component Beta Ratio separated awareness from unconsciousness to a greater degree than the composed parameter, BIS.
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Munich, Germany
Editor—We thank Dr Schneider and his colleagues for their interest in our work. We are also encouraged to see that the conclusions obtained from their analysis and data are very similar to our conclusions—namely that bispectral analysis of the EEG does not contribute greatly to the detection of loss of consciousness. The bispectral analysis may contribute to EEG quantification at higher concentrations of general anaesthetic. However, this remains speculation until such time as Aspect releases the details of how the three subcomponents of BIS are combined to produce the final index.
Hamilton, New Zealand
References
1 Miller A, Sleigh JW, Barnard J, Steyn-Ross DA. Does bispectral analysis of the electroencephalogram add anything but complexity? Br J Anaesth 2004; 92: 8–13
2 Rampil IJ. A primer for EEG signal processing in anesthesia. Anesthesiology 1998; 89: 980–1002[CrossRef][Web of Science][Medline]
3 Schneider G, Gelb AW, Schmeller B, Tschakert R, Kochs E. Detection of awareness in surgical patients with EEG-based indices-bispectral index and patient state index. Br J Anaesth 2003; 91: 329–35
4 Tunstall ME. Detecting wakefulness during general anaesthesia for caesarean section. Br Med J 1977; 1: 1321
5 Smith WD, Dutton RC, Smith NT. Measuring the performance of anesthetic depth indicators. Anesthesiology 1996; 84: 38–51[CrossRef][Web of Science][Medline]
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