Bispectral index and state entropy of the electroencephalogram during propofol anaesthesia
*E-mail: micheleiannuzzi{at}libero.itEditor—We read with interest the study by Bonhomme and colleagues,1 in which some interesting differences from our published data2 have emerged. We agree that Bland–Altman analysis appears to be the right statistical test to perform in an attempt to determine the degree of agreement between two measurement techniques. The only two studies that used the Bland–Altman analysis to compare BIS and SE have been published by Bonhomme and our group. We found a good comparability (mean difference 0.1) between the two (the upper and lower limits of agreement were –19.9 and 19.6). Bonhomme used the same type of analysis on data pairs averaged over 1 min over the entire period and reported a mean difference of 2.5 and similar upper and lower limits of agreement (–19.5 and 24.6). We agree that this may be related to differences in study design and in the way data were analysed. The patients in our study were not premedicated while the patients of Bonhomme received alprazolam 0.5 mg 1 h before surgery. The two studies differed also regarding the temporal sequence of the propofol target infusion (our study: initial target propofol concentration of 1 µg ml–1 increased by 1 µg ml–1 every 4 min, up to 6 µg ml–1; Bonhomme: initial target propofol concentration of 2.5 µg ml–1 increased by 0.5 µg ml–1 every 4 min until obtaining the target BIS value). Regarding the condition of steady state we have some concerns and do strongly believe that the difference of the results can depend on it.
In our study we tried to stress the steady-state concept either from the pharmacokinetic point of view by using Schniders pharmacokinetic model with a time peak effect of 1.6 min and increased the concentration every 4 min to obtain a steady state as previously published by Struys and colleagues3 and from the pharmacodynamic point of view by obtaining values of BIS between 40 and 60 for hypnosis and by using an estimate of the propofol concentration at the site effect. We believe that a higher mean difference reported by Bonhomme and colleagues can be attributed to the fact that patients received alprazolam and propofol, used a different pharmacokinetic model which requires a 15 min interval to obtain a steady-state condition between the site effect and blood, and because they did not refer to a site effect concentration of propofol. We believe that some patients despite values of BIS of 40–60 may have not been at steady-state conditions and this summed to interpatient variability and intermeasurement variability may have produced larger mean differences. Bonhomme considered also values recorded during nociceptive stimulation to evaluate EMG activity. Despite the fact that with Xp version BIS and SE EMG artifacts should be nearly excluded, performing laryngoscopy as a test to assess nociception is not the standard, some concern is also raised from an ethical point of view. If the final aim of our research efforts is to obtain the best anaesthetic practice and monitoring technique to avoid awareness, we should not perform laryngoscopy in a patient that could possibly, in our opinion, have some degree of consciousness.
We agree that Bonhomme by averaging data offered the advantage of getting rid of short and possibly removed delay time attributable to time variability and the effect of different sampling rates by the acquisition software. We do agree that with the limits of agreement in the two studies being large, in clinical practice the use of SE and BIS with the expectation of the same profiles of response to different events may not be appropriate, and judicious clinical assessment of the patient during the whole procedure is imperative despite fascinating technological implementation.
Naples, Italy
Editor—We would like to thank Professor Iannuzzi for his interest in our paper.1 It is reassuring to read that we agree on the main conclusion of the study, namely that we cannot expect the same profile of BIS and SE in response to different events during general anaesthesia. This conclusion stresses the need for further studies addressing the possible reasons for explaining those differences: beside scale, algorithm and delays for calculation differences, which are well known, differences in the relationship with the hypnotic level, as well as the nociceptive–anti-nociceptive balance, and the influence of muscle relaxation still need to be better defined. Using SE simply as BIS is therefore not appropriate. We also mainly agree on the possible reasons for discrepancies between the results of his and our study. However, we would like to highlight a few points to avoid any misinterpretation or confusion.
First, as depth of anaesthesia monitoring ideally consists in measuring the dose–concentration–response relationship between the administered anaesthetic agents and their pharmacodynamic effect, we believe that choosing a pharmacodynamic end-point such as a target BIS value between 40 and 50 to define a steady state is better than choosing a pharmacokinetic end-point such as a fixed effect–site concentration of propofol. Indeed, the same effect–site concentration of propofol may have different pharmacodynamic effects in different individuals. The 40–50 window is classically considered as a well-defined moderate hypnotic state, where synchronized–fast–slow activity of the EEG is the main determinant of BIS calculation. It is important to mention that, although using the model of Marsh, we were waiting for the equilibration between plasma and effect–site concentration of propofol before considering BIS and SE recording at steady state. This might not have appeared clearly enough in the method section of our manuscript. We can therefore reasonably consider our steady state as a real one. Noteworthy, when looking at the results of the Bland–Altman analysis presented in Table 2 of our paper, it appears that the mean difference between BIS and SE at steady state is not significantly different from 0 either in paralysed and non-paralysed patients, and that the limits of agreement are in the range of 20 U. This is in agreement with the results of Iannuzzi.2
Second, although the Xp version of the BIS monitor has been designed to better eliminate EMG artifacts, the possibility of EMG contamination during BIS calculation cannot be excluded. Vivien and colleagues4 have demonstrated that overestimation of BIS in sedated intensive care unit patients may be revealed by the administration of neuromuscular blocking agents. Although neuromuscular blocking agents may deepen anaesthetic depth through the limitation of muscle-emerged ascending inputs to the brain, EMG contamination is still possible.
Third, in our opinion, laryngoscopy can be considered as a relatively standardized nociceptive stimulus, provided it is performed during a predefined constant length of time and always by the same investigator. Laryngoscopy, algometry and tetanic stimulation are the three most frequently used standardized stimuli in pharmacodynamic studies. Of course, we do not recommend performing systematically a test laryngoscopy to assess the nociceptive–anti-nociceptive balance in patients. The aim in our study was to elicit a frank response of BIS and SE to a standardized nociceptive stimulation, and compare both responses. Tracheal intubation was not performed at that time because it would have introduced too much variability in terms of intensity and duration of the nociceptive stimulation. A 20 s duration seemed to be a good compromise between the need of a significant nociceptive stimulus and the risk of patients experiencing an unpleasant event. We found that, in non-paralysed patients, the limits of agreement between BIS and SE are large in those circumstances. This might not have been the case if opioids had been administered, but the response evoked by the nociceptive stimulation would probably have been truncated. Patients were systematically interviewed during the postoperative period and none of them reported any episode of unpleasantness. We acknowledge that this ethical issue is of importance, and Iannuzzi was right when mentioning it.
Finally, the definition of acceptable limits of agreement between two methods of measurement is empirical and debatable. It must be based on the magnitude of the variable scales and on the clinical relevance of the chosen interval. In our opinion, as BIS and SE scales are close to 100 U large limits of agreement of plus or minus 10 sound reasonable. We have seen that BIS and SE differ more than that in several circumstances.
Liege, Belgium
*E-mail: vincent.bonhomme{at}chu.ulg.ac.be
References
1 Bonhomme V, Deflandre E, Hans P. Correlation and agreement between bispectral index and state entropy of the electroencephalogram during propofol anaesthesia. Br J Anaesth 2006; 97:340–6
2 Iannuzzi M, Iannuzzi E, Rossi F, Berrino L, Chiefari M. Relationship between Bispectral Index, electroencephalographic state entropy and effect-site EC50 for propofol at different clinical endpoints. Br J Anaesth 2005; 94:492–5
3 Struys M, Versichelen L, Byttebier G, et al. Clinical usefulness of the bispectral index for titrating propofol target effect-site concentration. Anaesthesia 1998; 53:4–12[CrossRef][Web of Science][Medline]
4 Vivien B, Di Maria S, Ouattara A, et al. Overestimation of Bispectral Index in sedated intensive care unit patients revealed by administration of muscle relaxant. Anesthesiology 2003; 99:9–17[CrossRef][Web of Science][Medline]
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