BJA Advance Access originally published online on August 17, 2007
British Journal of Anaesthesia 2007 99(5):686-693; doi:10.1093/bja/aem231
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Narcotrend-assisted propofol/remifentanil anaesthesia vs clinical practice: does it make a difference?
1 Department of Anaesthesiology
2 Department of Legal Medicine, Charité—Universitätsmedizin Berlin, Campus Charité Mitte, Schumannstr., 20/21, D-10117 Berlin, Germany
3 Department of Psychology, University of Michigan, Ann Arbor, MI, USA
* Corresponding author. E-mail: ingrid.rundshagen{at}charite.de
Accepted for publication June 19, 2007.
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Abstract |
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Background: The Narcotrend is a computer-based EEG monitor designed to measure the depth of anaesthesia. The aim of the present study is to test the hypothesis that the intraoperative level of anaesthetic depth differs if decision-making is guided by Narcotrend monitoring or not.
Methods: Forty-eight patients undergoing elective surgery were randomized to receive a Narcotrend-controlled propofol/remifentanil anaesthetic regimen or standard clinical practice. In the EEG group, anaesthesia was adjusted to achieve a Narcotrend level of D2–E0, which is recommended for moderate to deep anaesthetic depth for surgery. EEG values were recorded continuously every 20 s in both groups. Depending on data distribution, group comparisons of the EEG parameters, propofol plasma concentration, and recovery characteristics were performed by analysis of variance for repeated measurements or non-parametric statistics.
Results: About 62 (SD 29)% of the Narcotrend values were within the target level in the EEG group during maintenance of anaesthesia; this was true for 64 (26)% of the data in the non-EEG group. The variance of the Narcotrend data was significantly lower in the EEG group compared with the non-EEG group [median: 0.4 (range: 3.5) vs 0.6 (2.5); P = 0.048]. There was no difference in propofol or remifentanil dosage, propofol plasma concentrations, and time for extubation. Ten minutes after extubation, visual analogue scores for nausea indicated a lower incidence in the Narcotrend group [7 (15) vs 24 (34); P = 0.005].
Conclusions: Guidance of anaesthesia with the Narcotrend-monitor leads to fewer deviations from a defined target than clinical assessment of anaesthetic depth only. This results in lower scores of nausea in the immediate period after anaesthesia.
Keywords: anaesthesia, depth; anaesthesia, general; anaesthetics i.v., propofol; analgesics opioids, remifentanil; equipment, monitors
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Introduction |
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The effects of anaesthesia on the EEG were described as early as in the first half of the last century. The advances in computer and monitor technology of the last decade have enabled a widespread clinical use of EEG monitors to quantify certain aspects of depth of anaesthesia. Still there is an ongoing discussion among clinicians and scientific experts about the usefulness of such monitors.1 2 Several studies indicate potential benefits of anaesthetic depth monitors, such as faster recovery, less consumption of anaesthetics, or reduced postoperative nausea and vomiting (PONV).3–6 However, other studies do not show any difference in EEG-guided anaesthesia in comparison with standard protocols.7 8 There is no clear explanation for the controversies among these studies.
The studies investigating the usefulness of EEG monitors focused mainly on recovery characteristics. However, from a scientific perspective, one would hypothesize that a difference in recovery is related to a difference during anaesthesia. That is, if anaesthesia is assisted by an EEG monitor, the level of anaesthesia should be different from a situation when an anaesthetic regimen is based solely on clinical experience. This hypothesis needs to be proven. Conclusions about a causal relationship between the use of an EEG monitor and the outcome parameters would require documentation of a different level of anaesthetic depth. Because the EEG monitors in use today have an integrated online documentation of the measured parameters (time interval: 5–20 s), it is now possible to address this question more precisely.
The Narcotrend (MonitorTechnik, Bad Bramstedt, Germany) is an EEG monitor, which is becoming more widespread in clinical application today to quantify the depth of anaesthesia. The Narcotrend algorithm is based on pattern recognition of the raw EEG and classifies the EEG epochs in different stages from A (awake) to F (increasing burst suppression to electrical silence); altogether 14 different stages are differentiated (Table 1).9 In the present study, we used the Narcotrend to guide the depth of anaesthesia. In parallel, we monitored the bispectral indexTM (BIS; Aspect Medical System, Newton, MA, USA), which has been used in numerous studies and is considered a standard measure to assess anaesthetic depth. The BIS is an empirically derived multifactorial EEG measurement.10 BIS is a dimensionless number between 0 and 100; values below 60 are associated with a low probability of consciousness.
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This prospective, randomized study was performed in order to address the question of whether the depth of anaesthesia during surgery, measured by the Narcotrend, and the BIS at 20 s intervals, is related to differences in recovery characteristics. A propofol–remifentanil-based anaesthetic regimen was guided by Narcotrend monitoring in one group of patients. In the other group, anaesthesia was guided by clinical parameters only. Recovery parameters were time of extubation and visual analogue scale (VAS) for fatigue and nausea after operation. Our hypotheses were (i) that the depth of anaesthesia during surgery differed between the two groups; (ii) that the EEG-assisted group stayed closer to the defined EEG target level; and (iii) that recovery parameters differed between the two groups.
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Methods |
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Study design
A prospective, randomized study was performed to explore the level of anaesthesia during EEG-guided and standard clinical practice during propofol/remifentanil anaesthesia in relation to recovery characteristics.
Patients
After approval from the institutional ethics committee and written informed consent had been obtained, 48 patients undergoing elective surgery were enrolled in the study (Table 2). Exclusion criteria were neurological diseases, consumption of medication affecting the central nervous system, cardiac surgery, or neurosurgery. Also excluded were patients with a history of drug dependence, alcoholism, pregnancy, or a known intolerance of the used drugs.
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Anaesthesia
Midazolam (0.1 mg kg–1 orally) was given 45 min before operation. Upon arrival in the operating room, an 18-gauge catheter was inserted in a peripheral forearm vein. Anaesthesia was induced with remifentanil 0.5 µg kg–1 min–1 as a continuous infusion. One minute later, the target-controlled infusion (TCI; infusion pump IVAC P6003, ALARIS Medical Systems, Inc., San Diego, CA, USA) with propofol was added with an estimated plasma concentration of 3 µg ml–1. The patients received rocuronium 0.6 mg kg–1 before endotracheal intubation was performed. Propofol and remifentanil were used for maintenance of anaesthesia. All patients were treated by one experienced consultant anaesthetist. FIO2 was kept at 0.3. In the case of one-lung ventilation, FIO2 was kept for 10 min at 1.0 after installing one-lung ventilation and then reduced to 0.5, if blood gas analysis was acceptable. All patients received novaminsulfone 2 g for 20 min before and piritramide 7.5 mg for 5 min before the suggested end of surgery. Early postoperative pain in the post-anaesthetic care unit (PACU) was treated on demand with either piritramide or morphine. Rescue medication in the case of nausea was metoclopramid.
Randomization
Patients were randomly allocated to one of the two groups. The patients of the EEG group received EEG-assisted general anaesthesia. The EEG target level was the stage D2–E0 of the Narcotrend classification. In case the target level was left, the instruction was first to adapt the stepwise propofol TCI concentration (±0.5 µg kg–1 min–1) followed by changing the remifentanil infusion regimen (±0.1 µg kg–1 min–1). In the patients of the non-EEG group, anaesthesia was guided by clinical parameters according to the individual decision of the anaesthetist. The anaesthetist was blinded to the screen of both EEG monitors.
Power analysis
In order to estimate the required sample size that yields a power of at least 80%, the mean difference reported in Kreuer and colleagues4 was utilized. They report mean differences of at least 1 SD for the differences in time until extubation between two intervention groups (Narcotrend or BIS-guided) and a standard practice group (mean differences of 5.6 and 6.0 with a pooled SD estimate of 5.3; d = 1.1). According to Cohen,11 this defines a large effect size. Given an 
= 5% and d = 1.0, the power table estimates the required sample size to be 13 subjects per group. Given the similarity in design, a non-significant difference in extubation time between the EEG and control group in the present study would indicate a substantial difference from the findings of Kreuer and colleagues.4
Electroencephalogram
EEG recording was performed using the Narcotrend monitor (NarcotrendTM Monitor, Version 2.0 AF, MonitorTechnik, Bad Bramstedt, Germany) and the BIS monitor (A-2000TM, Version 2.21, Aspect Medical Systems, Inc., Natick, MA, USA) in parallel. Timers on both monitors were adjusted to match each other. For EEG recordings, the skin was prepared with an abrasive paste (OmniPrep® Paste, Weaver and Company, Aurora, CO, USA) to obtain impedances < 5 
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For recording the Narcotrend-EEG (NARC), Ag/AgCl self-adhesive electrodes (Blue Sensor Medicotest S/A, Istykke, Denmark) were used. Electrodes were placed on the front of the head according to the manufacturer's recommendation. For recording the BIS, the BIS Standard Sensor was used and also placed according to the recommendations of the manufacturer. Electrode positioning was optimized until the impedances were < 5 . EEG recordings were started 5–10 min before induction of anaesthesia, and the patients were asked to close their eyes. A comfortable head position was arranged in order to relax the neck muscles. EEG recording was continuously performed until the patients fulfilled the clinical criteria for extubation. BIS and Narcotrend values, measured every 20 s, were stored for off-line analyses.
Plasma concentration of propofol
At baseline, after intubation, at skin incision, and at extubation, blood samples for propofol analysis were withdrawn into EDTA tubes and refrigerated until centrifugation within a few hours. Plasma was then transferred into storage tubes, with no extra contents, and frozen at –40°C until analysis. The propofol concentration in plasma was determined by high-performance liquid chromatography with a photodiode array detector (HPLC-DAD). Acetonitrile 200 µl was added to 200 µl of serum for protein precipitation. After 5 min vortexing and centrifugation, 50 µl of the supernatant was injected for HPLC. The detection wavelength was 219 nm with a bandwidth of 10 nm. The method was calibrated and validated according to the guidelines of the Society of Toxicological and Forensic Chemistry. The limits of detection and of quantification were 0.05 and 0.15 µg ml–1, respectively. Details about the used instruments and HPLC conditions used have been described elsewhere.12
Vital parameter, VAS, and miscellaneous
Heart rate, pulse oximetry readings, rectal temperature, and end-expiratory CO2 were measured continuously (Ohmeda Modulus® CD; Madison, WI, USA). Systolic and diastolic arterial pressure was measured during intubation and skin incision every minute and during maintenance every 5 min. In case a patient needed an arterial line, arterial pressure was recorded continuously. All data were stored for off-line analyses. The total dose of propofol and remifentanil, the change of the anaesthetic regimen, the total time of anaesthesia, and the time between the end of anaesthesia and extubation were registered.
The patients used a VAS (100 mm) to assess nausea and fatigue, during their stay at the PACU, after 10, 30, and 90 min. At the first postoperative day, the patients were interviewed about whether they had any memory during anaesthesia.13
Statistics
For statistical calculation, the Narcotrend classification was transferred into a numeric scale (Table 1). To analyse differences in the EEG data between the two groups, three periods of anaesthesia were defined: induction (1 min before to 3 min after intubation), skin incision (1 min before to 3 min after skin incision), and maintenance (5 min after skin incision till 5 min before skin closure). The means and the variances of the Narcotrend and BIS values were calculated for each patient during these periods. Similarly, vital parameters were calculated. For descriptive purpose, the percentage of EEG data at the target level was calculated.
The effects of patients' characteristics were tested using analysis of variance (ANOVA) (a posteriori Scheffé test). As EEG variables are known to be age-dependent, we controlled for age effects statistically using, for each EEG variable, the standardized residuals of the regression of EEG parameter on age as dependent variable.14 Thus, the variability for older patients and for younger patients was smaller after controlling for age. The Kolmogorov–Smirnov test was applied to test the normality assumption of all distributions. Inter-group comparison of the Narcotrend and BIS data (calculated grand means and variances) was assessed separately by ANOVA (repeated measurements: three time periods; two intervention groups) or by non-parametric statistics depending on the distribution of the data. Similarly, vital parameters, the plasma concentrations of propofol, the VAS for nausea and fatigue were assessed. Student's t-test or Mann–Whitney U-test were used to compare the two groups with respect to the time of anaesthesia, the total dose of propofol and remifentanil, the time of extubation, and the dose adaptations. Learning effects were tested using linear trend component analysis including either all patients or separately for the two groups. P 
0.05 was adopted for the level of significance for all statistical tests.
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Results |
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Patients' characteristics
Out of 48 patients, the data of 44 patients were included in the final analyses. Two patients were excluded due to problems in the organization. In one patient, benzodiazepine abuse became evident retrospectively. In another patient, the impedance of the electrodes remained too high, but in order to avoid a time delay in the operating theatre schedule, the study had to be stopped. Detailed results for both groups are listed in Tables 2 and 3. Except for age, there was no difference between the patients’ characteristics. Four patients of the EEG group and five of the non-EEG group required surgery because of malignoma; none of them received preoperative radiation or chemotherapy.
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Electroencephalogram
The artifact rate was 7.0% for the Narcotrend and 2.8% for the BIS over a total time period of 5434 min during EEG recording, when the EEG monitors did not calculate an index. After exclusion of artifacts, 14 730 Narcotrend values and 15 136 BIS values were used for the calculation of the grand means over the defined time periods.
The calculated means of the Narcotrend data and the BIS data showed a normal distribution. EEG baseline levels did not differ before anaesthesia was started [Narcotrend: 1.21 (0.51) vs 1.5 (0.86); BIS: 93.9 (3.61) vs 93.8 (3.11)]. Two-factorial ANOVA did not reveal a significant main effect between the EEG and the non-EEG group over time (F = 0.22, P = 0.639). For descriptive purposes, post hoc analyses did not show any difference between the groups at any of the three time points. The calculated variances of the Narcotrend data and the BIS data showed skewed distributions; therefore the Mann–Whitney U-test was used for group comparison. Randomization had a significant effect on the calculated variance of the Narcotrend data [median: 0.4 (range: 3.5) vs 0.6 (2.5); P = 0.048], indicating a higher variance in the non-EEG group during maintenance of anaesthesia. The variances of the BIS data did not differ between the EEG and the non-EEG group.
In the EEG group, 62 (29)% of the Narcotrend values were within the target level (D2–E0) during maintenance of anaesthesia, and 64 (26)% in the non-EEG group, respectively (Fig. 1). When defining a tolerance level (D1-E1), that is, accepting one level below or above the target level, in 95 (8)% the Narcotrend data were within this tolerance level in the EEG group. However, in the non-EEG group, 87 (20)% were within this level.
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Learning effects
There was no linear trend over time in any of the Narcotrend parameters (Fig. 2A and B). However, with respect to BIS data, the variance showed a significant negative trend over time for skin incision (r = –0.657; P = 0.001) in the EEG group, that is, the later patients in this group were treated the less variance of BIS data occurred during skin incision.
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Propofol plasma concentration
The propofol plasma concentration was slightly higher in the EEG group compared with the non-EEG group at intubation and at skin incision. ANOVA did not reveal significant group differences at any time point. Plasma concentrations were as follows: intubation, 3.7 (1.6) vs 2.9 (1.4) µg ml–1; skin incision, 3.4 (1.5) vs 3.1 (1.2) µg ml–1; extubation, 1.5 (1.3) vs 1.5 (1.4) µg ml–1; 10 min after extubation, 1.5 (1.6) vs 1.0 (0.9) µg ml–1; 90 min after extubation, 0.9 (1.3) vs 0.7 (1.0) µg ml–1.
VAS: vital parameters, VAS, and miscellaneous
There was no difference between the two groups with respect to the total dose of propofol or remifentanil, the number of changes of the anaesthetic dose, the total time of anaesthesia, and the time until extubation, even though all patients except one was extubated earlier in the EEG group (Table 4, Fig. 3). Averaged temperature did not differ between the two groups over the whole time of anaesthesia [36.2 (0.5)°C in each group].
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Analysing the absolute values of arterial pressure, there was a significant difference in variance of the diastolic and mean arterial pressure during maintenance, indicating a larger variance of both parameters in the non-EEG group (P
0.034). When age-corrected data were analysed, the difference was no longer significant. Twenty-six patients needed theoadrenaline plus cafedrin (Akrinor: 100 mg cafedrin plus 5 mg theoadrenalin/1 ml) during induction of anaesthesia without a difference between the groups: EEG group (n = 14), 0.8 (0.6) ml; non-EEG group (n = 12), 0.6 (0.3) ml [means (SD)]. Two patients of the EEG group received atropine 0.5 mg during induction. Dopamine (1–5 mg kg–1 min–1) was used in patients with peripheral vascular surgery, in order to keep mean arterial pressure > 80 mm Hg. This was done in four patients of the EEG group and two patients of the non-EEG group. For anti-hypertensive intervention, one patient received nitroglycerin spray and another patient urapidil (20 mg), both belonged to the EEG group. During extubation, four patients received clonidine (75–150 µg; two from each group). The patients of the EEG group reported less nausea than those of the non-EEG group. This effect reached statistical significance at 10 min after extubation, only. One patient of the EEG group and three of the non-EEG group received metoclopramid for nausea; none of them vomited. VAS for fatigue was similar for both groups (Table 5). In the PACU, three patients of each group received morphine, whereas 10 of the EEG group and eight of the non-EEG group received piritramide. There was no significant difference in the amount of opioids between the two groups [piritramide: EEG group, 6 (2) mg; non-EEG group, 7 (3) mg; morphine: EEG group, 5 (0) mg; non-EEG group, 8 (3) mg].
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None of the patients showed explicit memory during anaesthesia. Two patients in the EEG group remembered that they had dreamt during anaesthesia. One patient could not recall what he had dreamt about. One patient reported a dream about herself performing an operation and being unable to finish, because she woke up at the end of anaesthesia. She pointed out it was an unpleasant dream for her.
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Discussion |
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In the present study, we showed that during maintenance of propofol/remifentanil anaesthesia, the practice of anaesthesia management differs, as indicated by a smaller variance of the Narcotrend data in the Narcotrend-assisted group. The variance is the averaged square deviation of a patient's overall mean score. By squaring the deviance, the variance is more sensitive for extreme deviations from the target, which is a desired feature for a measure of the instability of the anaesthetic depth. Our analysis takes into consideration that clinically concerning differences (e.g. deviation of three EEG stages from the set range) count more than clinically less relevant deviations (e.g. deviation of one EEG stage from the set range). Thus, we demonstrated that in the EEG-guided group, the single Narcotrend value remained more stable over time and closer to the defined target level. This can be taken as an indicator that in the EEG-controlled group, the level of anaesthesia was adapted faster, and major differences from the target value were avoided. Obviously, the anaesthetist used the EEG to control anaesthetic depth and reacted slightly differently compared with when relying solely on clinical parameters and personal experience.
The defined Narcotrend target level D2–E0 was achieved in both groups to a similar extent. We did not prove our hypothesis that the level of anaesthesia, measured by mean Narcotrend and BIS values, differed between EEG-guided and standard clinical practice during propofol/remifentanil anaesthesia. With respect to the recovery parameter, we could show that nausea was reduced in the very early period after anaesthesia in the Narcotrend-assisted group. This was not a long-lasting effect, because both groups had similar scores at 30 min after the end of anaesthesia.
In the present study, two-thirds of the actual Narcotrend values were within the target level D2–E0 during maintenance of EEG-guided anaesthesia. Kreuer and colleagues4 compared Narcotrend- and BIS-guided anaesthesia with standard practice. Their data relied on EEG values manually recorded every 5 min. They set D0 as the target area. About 52.9% of the time, the anaesthesiologist was able to reach a Narcotrend level of D0 or D1. In another study by the same research group, evaluating Narcotrend stages during desflurane/remifentanil anaesthesia, 61% were within the range D0 or D1, but 13% of the values were in the range of C2, as recommended for light anaesthesia.3 In contrast to the studies of Kreuer's group, we were not able to demonstrate a significant reduction in propofol consumption and faster recovery times in the EEG-guided group. In their studies, patients undergoing minor orthopaedic surgery were included. In our study, all kinds of surgical patients participated, so surgical stimulation was probably stronger. However, the main reason, that we could not reproduce the previous results, is probably due to the different target levels. In the present study, the target level of D2–E0 was adopted, as recommended by the manufacturer as a middle range of anaesthetic depth.
In the present study, about two-thirds of the Narcotrend values were within the set area, even when the EEG was blinded. Thus, the anaesthesiologist also performed quite well relying on clinical data and experience alone, too. An unresolved question is whether the motivation of the anaesthetist biased the study results. However, we could demonstrate a learning effect of the performing anaesthetist in the EEG group, which resulted in less variance of BIS values during skin incision.
Vakkuri and colleagues5 have used the automatic documentation of an EEG device to address the question of anaesthetic depth intraoperatively, too. They showed in their multicentre study, which was performed as an utility study for an EEG device based on spectral entropy, that entropy values were higher during anaesthesia, whereas propofol consumption was less in the EEG-monitored group. This resulted in a shorter time delay in the early recovery parameters in the entropy group. Consumption of alfentanil was similar in both groups. We recruited a limited number of patients, whereas Vakkuri and colleagues investigated a total number of 368 patients. Our aim was to address the question of anaesthesia practice. Both studies taken together indicate to some extent that EEG monitoring changes anaesthetic guidance during surgery.
Kreuer and colleagues15 showed a sufficient correlation between BIS and Narcotrend index, a recently developed index based on the Narcotrend EEG classification. The authors demonstrated that a simple transfer from BIS to Narcotrend values is not adequate in all ranges. This may explain why we were able to demonstrate a difference in anaesthetic practice indicated by the variance of the Narcotrend values but not for the BIS. On the other hand, we demonstrated a learning effect over time for the period of skin incision suggesting improved precision with the BIS data only. We used a Narcotrend-controlled design, and the BIS monitor was used in parallel, because it has been regarded as a standard monitor to assess anaesthetic depth. The Narcotrend monitor did not calculate values about 7% of the time, whereas the BIS showed an artifact rate of about 3%. For both monitors, the algorithms are private property and are incompletely published. Therefore, it remains unclear whether one monitor is more sensitive to external influences, or the other monitor still calculates indexes in spite of artifacts. However, an artifact rate of > 3% implies a clinical limitation for both monitors.
A recent meta-analysis performed by Liu6 showed that in ambulatory surgery patients, BIS monitoring reduced the risk of PONV about 6% in the PACU. The author suggested that a reduced risk of nausea and vomiting was probably due to a dose reduction in the used general anaesthetics. Nausea and vomiting has not been investigated in relation to Narcotrend-guided anaesthesia before. We could show a short-lasting effect in our patients receiving propofol/remifentanil. We were not able to demonstrate a reduction in the dose of the used anaesthetics. Therefore, it seems reasonable that the difference in practising anaesthesia, as indicated by the smaller variance of the Narcotrend data during maintenance of anaesthesia, was more likely to cause reduced nausea in our patients.
By chance, our patient groups differed in age. The oldest patient of the EEG group was 70 yr and of the control group 78 yr. Age effects were taken into account using residuals of the regression of all dependent variables on age instead of the original variables. The variability for older patients and for younger patients was smaller after controlling for age. Therefore, it seems unreasonable that the effects we demonstrate here are related to age at all.
Our findings have some impact on future studies. (1) EEG data during anaesthesia, recorded on-line, should be considered, in order to establish a causal relationship between EEG variables and outcome parameters. (2) Further studies should clarify which is an adequate Narcotrend level during surgical anaesthesia. During transition from consciousness to unconsciousness a huge inter-individual variability of the Narcotrend index, a numerical index based on the Narcotrend classification, was shown.16 17 (3) Further studies are needed which focus on the motivation and the experience of the performing anaesthesiologist. Hadzidiakos and colleagues18 have addressed this question in part in their recently published study, showing that experienced anaesthesiologists performed better when clinical assessment of anaesthetic depth was compared with electrophysiological variables. Pavlin19 pointed out, too, that inexperienced junior residents were more likely to lessen anaesthetic depth when using a BIS monitor.
In conclusion, we were able to demonstrate a change in practicing anaesthesia when the anaesthesiologist used the Narcotrend to quantify anaesthetic depth during surgery. Further studies are warranted to clarify the impact of Narcotrend-monitoring on performing anaesthesia and on patient outcomes.
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Acknowledgements |
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The study has been supported by Astra Zeneca, Wedel, Germany and by an institutional research grant of the University.
The results have been presented in part at the ‘Haupstadtkongress für Anästhesie und Intensivmedizin’ in Berlin, Germany, 2005 and at the Annual Meeting of the American Society of Anesthesiologists in Atlanta, USA, 2005.
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References |
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- Ingrid Rundshagen
- British Journal of Anaesthesia, 10 Jan 2008 [Full text]
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