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Agreement between different depth of anaesthesia monitors
- Michele Iannuzzi, Emanuele Iannuzzi, Maria Chiefari, Liberato Berrino, Francesco Rossi (25 August 2006)
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Correlation and agreement between BIS and SE
- Vincent L. Bonhomme, Pol C. Hans (25 August 2006)
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Michele Iannuzzi, MD PhD EDIC , Emanuele Iannuzzi, Maria Chiefari, Liberato Berrino, Francesco Rossi
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Dear Editor, We read with interest the results of the work of Bonhomme V et al. Some interesting differences from our published data have emerged. We do agree that the 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 an our group We found a good comparability (mean difference 0.1) between the two parameters while the upper and lower limits of agreement were -19.9 and 19.6. Bohnomme 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 do agree that the reason for 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 mcg ml-1 increased by 1 mcg ml-1 every 4 min, up to 6 mcg ml-1; Bohnomme: initial target propofol concentration of 2.5 mcg ml-1 increased by 0.5 mcg 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 at maximum 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 et al and from the pharmacodynamic point of view by obtaining values of BIS between 40-60 for hypnosis and by using an estimate of the propofol concentration at the site effect. We do believe that a higher mean difference reported by Bonhomme et al can be due to the fact that patients received a double pharmacological regimen (alprazolam and propofol), used a different pharmacokinetic model such as Marsh for propofol infusion which requires a 15 min interval to obtain steady state condition between the site effect and blood and because the authors did not refer to a site effect concentration of propofol. We do believe that some patients despite values of BIS of 40-60 may have not been in steady state conditions and this summed to interpatient variability and intermeasurement variability may have conduced to larger mean differences. This aspect should strongly be considered. Bohnomme considered also values recorded during nociceptive stimulation to evaluate EMG activity. Despite the fact that with Xp version BIS and SE EMG artefacts should be nearly excluded, performing laringoscopy 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 anesthesiological practice and.monitoring technique to avoid awareness why should we perform an unpleasant maneuver such as laringoscopy in a patient that could possibly in our opinion present some degree of consciousness. Moreover it is not standard clinical practice to perform laringoscopy before hypnotic drugs and opioid drugs have been administered, neither administering propofol and neuromuscular blocker without any opioids, in the latter case we could be in front of a patient that is not deeply anesthetised but presents a complete neuromuscular block. We do agree that Bohnomme by averaging data offered the advantage of getting rid of short and possibly removed delay time due to time variability and the effect of different sampling rates by the acquisition software. We do agree that being the limits of agreement in the two studies 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. Conflict of Interest:None declared |
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Vincent L. Bonhomme , Pol C. Hans
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Editor- We would like to thank Professor Iannuzzi for his interest on our paper. It is reassuring to read that we agree on the main conclusion of the study, namely that we can not 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 used 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 synchronised-fast-slow activity of the EEG is the main determinant of BIS calculation1-4. 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 units. This is in agreement with the results of Iannuzzi. Second, although the Xp version of the BIS monitor has been designed to better eliminate EMG artefacts, the possibility of EMG contamination during BIS calculation cannot be excluded. Vivien et al. have demonstrated that overestimation of BIS in sedated intensive care unit patients may be revealed by the administration of muscle relaxants 5. Although muscle relaxants may deepen anaesthetic depth through the limitation of muscle- emerged ascending inputs to the brain, EMG contamination is still possible. Third, to our opinion, laryngoscopy can be considered as a relatively standardised nociceptive stimulus, provided that it is performed during a pre-defined constant length of time and always by the same investigator. According to the anaesthetic literature, laryngoscopy, algometry and tetanic stimulation are the three most frequently used standardised stimuli in pharmacodynamic studies (for a review, see 6). Of course, we do not recommend performing systematically a test laryngoscopy to assess the nociceptive-anti-nociceptive balance in patients. However, tracheal intubation is often necessary in daily anaesthetic practice, and the requested laryngoscopy, in that case, followed by the observation of the evoked response of the depth of anaesthesia monitor, may provide useful information for the management of anaesthesia during the remaining time of the procedure. The aim in our study was to elicit a frank response of BIS and SE to a standardised 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 second duration seemed to be a good compromise between the need of a significant nociceptive stimulus and the risk for patients of 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 debateable. It must be based on the magnitude of the variable scales and on the clinical relevance of the chosen interval. To our opinion, as BIS and SE scales are close to 100 units 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. Vincent L. Bonhomme and Pol C.Hans References 1. Dahaba AA. Different conditions that could result in the bispectral index indicating an incorrect hypnotic state. Anesth Analg 2005; 101: 765-73 2. 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 3. Leslie K, Sessler DI, Smith WD et al. Prediction of movement during propofol/nitrous oxide anesthesia. Performance of concentration, electroencephalographic, pupillary, and hemodynamic indicators. Anesthesiology 1996; 84: 52-63 4. Vernon JM, Lang E, Sebel PS, Manberg P. Prediction of movement using bispectral electroencephalographic analysis during propofol/alfentanil or isoflurane/alfentanil anesthesia. Anesth Analg 1995; 80: 780-5 5. 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 6. Guignard B. Monitoring analgesia. Best Pract Res Clin Anaesthesiol 2006; 20: 161-80 Conflict of Interest:None declared |
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