The BIS algorithm contains an additional parameter, the Beta Ratio, which is the logarithmic ratio of high frequency components (3047 Hz) and classic EEG frequency components (1120 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 kg1 min1); (ii) sevoflurane/ remifentanil (
0.2 µg kg1 min1); (iii) propofol/remifentanil (
0.1 µg kg1 min1); or (iv) propofol/remifentanil (
0.2 µg kg1 min1). 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
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: 813
2 Rampil IJ. A primer for EEG signal processing in anesthesia. Anesthesiology 1998; 89: 9801002[ISI][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: 32935
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: 3851[CrossRef][ISI][Medline]