Bispectral index as a predictor of sedation depth during isoflurane or midazolam sedation in ICU patients.
Bispectral index (BIS) is used for monitoring anaesthetic depth with inhaled anaesthetic agents in the operating room but has not been evaluated as a monitor of sedation depth in the intensive care unit (ICU) setting with these agents. If BIS could predict sedation depth in ICU patients, patient disturbances could be reduced and oversedation avoided. Twenty ventilator-dependent ICU patients aged 27 to 80 years were randomised to sedation with isoflurane via the AnaConDa[R] or intravenous midazolam. BIS (A-2000 XP, version 3.12), electromyogram activity (EMG) and Signal Quality Index were measured continuously. Hourly clinical evaluation of sedation depth according to Bloomsbury Sedation Score (Bloomsbury) was performed. The median BIS value during a 10-minute interval prior to the clinical evaluation at the bedside was compared with Bloomsbury. Nurses performing the clinical sedation scoring were blinded to the BIS values. End-tidal isoflurane concentration was measured and compared with Bloomsbury. Correlation was poor between BIS and Bloomsbury in both groups (Spearman's rho 0.012 in the isoflurane group and -0.057 in the midazolam group). Strong correlation was found between BIS and EMG (Spearman's rho 0.74). Significant correlation was found between end-tidal isoflurane concentration and Bloomsbury (Spearman's rho 0.47). In conclusion, BIS XP does not reliably predict sedation depth as measured by clinical evaluation in non paralysed ICU patients sedated with isoflurane or midazolam. EMG contributes significantly to BIS values in isoflurane or midazolam sedated, non paralysed ICU patients. End-tidal isoflurane concentration appeared to be a better indicator of clinical sedation depth than BIS.
Key Words: bispectral index, BIS XP, correlation, electromyogram, AnaConDa
Optimal sedation of intensive care unit (ICU) patients requires frequent evaluation of the patient's sedation level in order to avoid under- and oversedation, as well as prolonged ventilator care (1,2). Many sedation scales require frequent patient stimulation for evaluation--a disturbance that may arouse and agitate the patient and possibly contribute to subsequent psychological sequelae. While undersedation is normally easily observed, an objective, non-invasive method able to discriminate between adequate and excessive sedation without disturbing the patient would be useful.
Bispectral index[TM] (BIS) non-invasively measures the raw frontal encephalogram (EEG). The EEG is processed and converted to a scale between 0 and 100. Ideally, a score of 0 equals an isoelectric EEG and a score of 100 indicates a fully awake patient (3). BIS has gained acceptance as a non-invasive, objective monitor of anaesthetic depth during surgery and several studies describe its benefits in the operating room (4,7). Good correlation between responsiveness and BIS levels in healthy volunteers has been described during sedation with isoflurane, midazolam and propofol (8).
BIS has also been considered for monitoring critically ill patients during intensive care (9-15). Studies of correlation between BIS and clinical sedation scales during intravenous sedation in the ICU have shown varying results (10-13). In earlier versions BIS values were more or less influenced by electromyogram (EMG) artifacts (12,16). According to the manufacturer, software and electrodes have been modified to reduce such artifacts in the latest available version (BIS[TM] A-2000 XP, version 3.12 and BIS[TM] QUATRO sensor) (3).
The use of isoflurane for sedation of ventilator-dependent ICU patients has been described previously (17-20). Comparisons with midazolam and propofol in adults have shown acceptable sedation effects and short wake-up times, without reported tolerance or withdrawal symptoms (17-19). A new, simplified method of administering isoflurane without a vaporiser, with low agent requirements and low ambient isoflurane levels may facilitate greater use of isoflurane for sedation in the ICU (19,20).
While studies of previous and current versions of BIS during intravenous sedation in the ICU have not been conclusive, the reliability of BIS for inhaled agents in the OR setting makes it reasonable to examine whether the latest BIS version would be helpful in monitoring ICU patients receiving inhaled agents. BIS has not previously been evaluated for assessing sedation depth in critically ill patients sedated with isoflurane. If BIS could effectively predict clinical sedation depth in these patients, unnecessary arousal of the patient by repeated clinical evaluation of sedation depth could be reduced.
We hypothesised that BIS might correlate better with sedation level for ICU sedation with isoflurane than with intravenous agents. We therefore compared BIS and clinical sedation level during goal-directed sedation of 10 isoflurane-sedated patients and 10 midazolam-sedated patients. Our primary aim was to investigate if the latest available version of BIS could predict clinical sedation depth well enough to replace patient stimulation in measuring sedation depth.
MATERIALS AND METHODS
The study was conducted at the General Intensive Care Unit at the Karolinska University Hospital Solna, Stockholm, Sweden. The study was approved by the local Ethics Committee for Human Research.
Patients aged 18 to 80 years requiring ventilator support and with an expected need for sedation longer than 12 hours were eligible for the study. Patients were excluded if there was any evidence of overt encephalopathy or intracranial pathology, due to possible difficulty evaluating sedation depth or the potential risk of increasing intracranial pressure with isoflurane administration. Other exclusion criteria were family history of malignant hyperthermia, need for dialysis at inclusion, pregnancy, or continuous sedation administered for more than 18 hours prior to inclusion. After approval from the next of kin, patients were randomised to receive isoflurane sedation via the Anesthetic Conserving Device (ACD/AnaConDa[R], Sedana Medical AB, Sundbyberg, Sweden) or intravenous midazolam until extubation or for a maximum of four days.
The ACD is a modified heat-moisture exchanger, with properties facilitating simplified administration of isoflurane via a syringe pump and rebreathing (approximately 90%) of isoflurane. The sedation efficacy, agent saving property and environmental safety of this device in the ICU setting have been reported previously (19,20).
After inclusion, the patient's forehead was wiped with alcohol and a BIS[TM] QUATRO (Aspect Medical Systems, Newton, MA, U.S.A.) four-electrode sensor was applied by one of the investigators. The sensor was connected to a BIS[TM] A-2000 XP (version 3.12) (Aspect Medical Systems, Newton, MA, U.S.A.) monitor and an automatic impedance test was performed, according to the manufacturer's instructions. After acceptable impedance had been confirmed by the BIS monitor, the monitor screen was covered to avoid nursing staff bias during clinical evaluation of sedation depth.
Other sedatives were terminated and the study sedative administered. Isoflurane was initially infused to the ACD according to the manufacturer's recommended infusion rate to obtain an end-tidal concentration of 0.5% (1.0-3.5 ml/hour). Midazolam was initially infused in the dose range 0.02-0.05 mg/kg/hour. Infusion rates were thereafter adjusted by the patient's nurse in order to achieve the desired sedation level. When needed, a bolus dose of sedative was given. In the isoflurane group a bolus was given by increasing the infusion rate to 10 ml/hour for two minutes, while a bolus of 0.02-0.05 mg/kg was given in the midazolam group. If these bolus doses were not sufficient or if the patient needed a rapid increase of sedation, propofol 0.5 mg/kg was given. Morphine was given intermittently or by infusion for analgesia, according to patient need as assessed by the treating ICU physician.
Bloomsbury Sedation Score (Bloomsbury) shown in Table 1, is the standard sedation scale used in our General ICU (21) and therefore was incorporated in the study design.
Bloomsbury range -1 to +1 was considered the target interval for sedation unless the treating ICU physician specifically considered another interval desirable. Bloomsbury -2 and -3 imply deep sedation, indicating response only to painful stimuli or no response to pain. Sedative administration was increased or decreased by nursing staff in order to reach the target interval. Sedation level using Bloomsbury was assessed hourly by the patient's nurse.
BIS, frontal EMG and Signal Quality Index (SQI) were measured with the BIS monitor continuously during the sedation period and downloaded for analysis. The EMG values are expressed in decibels (dB) and reflect frontal muscular activity. The manufacturer recommends that BIS readings be interpreted with caution when EMG values are above 50 dB due to a risk of excessive EMG affecting BIS values (3). The SQI is presented as a value between 0 and 100, where SQI values above 50 are stated to indicate reliable BIS readings (3).
As BIS values at the time of clinical assessment are likely to reflect the effect of the stimulation rather than the general pre-stimulation state, and because the aim of the study was to evaluate BIS as a predictor of clinical sedation depth, the median value of BIS over 10 minutes prior to the hourly clinical assessment was used in the analysis. All Bloomsbury observations with complete BIS data over the 10 minutes prior to clinical assessment were used for analysis. The BIS monitor was set in the "ICU mode", meaning that each presented online BIS value represents processed EEG information from the previous 30 seconds. (In the "Anaesthesia mode" sampling time is 15 seconds).
Due to considerable variability in displayed BIS values noted by the investigators, BIS minute fluctuation was calculated as the mean difference between the maximum and minimum of the 60 BIS values displayed each minute.
Inspired and end-tidal fraction of isoflurane were measured continuously (Datex-Ohmeda AS 3 Compact, Datex-Ohmeda, Helsinki, Finland) and noted hourly in the isoflurane group.
As Bloomsbury is an ordinal scale, correlations between Bloomsbury and pre-stimulation BIS and between Bloomsbury and end-tidal isoflurane concentration were analysed with Spearman's rank sum. Individual coefficients for correlation between BIS and Bloomsbury were calculated in order to respect multiple observations in each individual. Correlation between EMG and BIS was analysed with Spearman's rank sum. Mean BIS variability was compared between groups with Student's t-test. Comparison of the proportions of episodes with BIS above 60 during deep sedation (Bloomsbury -2 and -3) for the different drugs was tested by logistic regression applying the Generalised Estimating Equation (GEE) method to account for repeated measurements within individuals. Morphine requirement was compared between groups with Student's t-test.
Patient characteristics are described in Table 2. Age, APACHE II and diagnoses were similar in both groups.
Sedatives used prior to inclusion (3-15 hours of sedation) were propofol (4/10 in isoflurane group, 1 7/10 in midazolam group, M) or midazolam (6/10 in isoflurane group, 3/10 in midazolam group). For the final analysis there were 277 paired observations of Bloomsbury and BIS in the isoflurane group and 331 paired observations in the midazolam group. Due to a technical error, BIS data from two of the isoflurane sedated patients could not be retrieved (Patients 16 and 19).
Poor correlation between BIS and Bloomsbury was found in both groups (Figure 1). The correlation coefficient (Spearman's rho) was 0.012 in the isoflurane group and -0.057 in the midazolam group, with the individual coefficients presented in Table 3.
A significant correlation between Bloomsbury and end-tidal isoflurane was noted (Spearman's rho=0.47, P<0.0001) (Figure 2).
There was a strong correlation between BIS and EMG (Spearman's rho=0.74) in both groups (Figure 4). Mean SQI was generally high (Table 3), and EMG was within the range stated as acceptable by the manufacturer', with mean EMG levels not exceeding 50 dB in any patient (Table 3).
The mean minute BIS fluctuation ranged between 7.5 and 16.5 percent/minute (Table 3) and did not differ between groups (P=0.4).
There was a trend towards a greater proportion of BIS values above 60 during clinically scored deep sedation (Bloomsbury -2 and -3) in the midazolam group (43% of 186 observations) than in the isoflurane group (16% of 178 observations) with an odds ratio (isoflurane versus midazolam) adjusted for repeated measurements of 0.77 (95% CI: 0.54-1.09, P=0.14).
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There was a trend towards higher morphine requirement in the midazolam group compared with the isoflurane group (4.4 mg/hour vs. 2.5 mg/hour, P=0.11).
Usefulness in the ICU
Our study indicates that BIS does not reliably predict sedation depth compared to clinical assessment during isoflurane or midazolam sedation in critically ill, non-paralysed patients. The poor predictive value of BIS is demonstrated with raw data in Figures 1 and 2 by the considerable overlap of BIS values across the range of Bloomsbury scores. In fact, none of the patients demonstrated good correlation between BIS values and Bloomsbury, with both positive and negative correlation coefficients in both groups (Table 3). In the isoflurane group individual correlation coefficients ranged between 0.0966 and -0.3123, in the midazolam group coefficients ranged between 0.4454 and -0.5121 (Table 3). While avoiding disruptive stimuli during sedation scoring would probably be beneficial, BIS monitoring in non-paralysed ICU patients receiving isoflurane or midazolam sedation does not seem sufficiently reliable to replace clinical scoring.
One explanation for the poor correlation between BIS and Bloomsbury could be that these two methods measure essentially different aspects of sedation. Sedation scales measure degree of responsiveness to external and at times somewhat noxious stimuli, while BIS has been developed empirically to measure more specifically the level of pharmacologically induced hypnosis and a related risk of awareness. Responsiveness and awareness during pharmacologically induced hypnosis are not identical phenomena (22,23).
During deep sedation scored as Bloomsbury -2 to -3 (no response or response only to noxious stimuli), median pre-stimulation BIS values were above 60 in 43% of observations in the midazolam group and in 16% of clinically observed deep isoflurane sedation (P=0.14). In general anaesthesia practice, a BIS value of 60 is considered a threshold above which the risk of awareness is increased (24,25). High BIS scores during relatively deep sedation in ICU patients have been described previously (26). Determination of whether any patients with elevated BIS during deep sedation had recall from events during these episodes, or a high incidence of psychological sequelae after intensive care was not an objective of this study, but these questions deserve further exploration. It may be that the general use of opioid infusions contributes to this apparent discrepancy (4).
An alternative explanation for the lack of correlation between BIS and clinical sedation scoring in sedated ICU patients would be an essential difference in EMG contribution to BIS in sedated critically ill patients compared with patients under general anaesthesia, the population in which the original BIS algorithm was constructed.
Correlation between BIS values and facial EMG in the XP version of BIS has recently been described during intravenous sedation of ICU patients (26-28). Vivien et al demonstrated a mean BIS value reduction of 23% in sedated ICU patients when muscle relaxants were administered (27) and concluded that EMG disturbances may falsely elevate BIS values, resulting in a risk of oversedation when BIS is used for titrating sedation in ICU patients not routinely receiving muscle relaxants. Messner et al demonstrated with the BIS A-1000 monitor that the administration of a muscle relaxant in non-sedated healthy subjects led to the elimination of facial EMG and lowered BIS to values in the range of 33 to 64 and concluded that the use of muscle relaxants may result in falsely low BIS values by eliminating EMG activity, thus increasing the risk of awareness (29) and obviating the benefit for which BIS was developed. Our results with the latest available version of BIS during isoflurane sedation confirm previous findings of a close correlation between facial EMG and BIS values during intravenous sedation (27,28). BIS value interpretation (with the BIS XP version) is thus confused by EMG contribution in critically ill patients not receiving muscle relaxants, the significance and importance of this EMG contribution not being clear.
Good experience during general inhalational anaesthesia led us to believe that BIS might perform better in the ICU with isoflurane use than with intravenous agents. To our knowledge, this is the first study examining such correlation during sedation with an inhalational agent and we did not find evidence that BIS correlates better to clinical sedation depth with inhaled sedation than with intravenous sedation.
End-tidal concentration of inhaled agents is used daily in the OR as an indicator of anaesthetic depth. Analysis of correlation between sedation level and end-tidal isoflurane levels showed that end-tidal isoflurane concentration in fact appears to be better than BIS as a predictor of sedation depth (Figure 3). To our knowledge correlation between end-tidal concentrations of isoflurane and clinical measures of sedation depth in the critically ill has not been described earlier. However, correlation was not sufficiently strong (Spearman's rho=0.47) for end-tidal isoflurane concentration to replace clinical scoring in evaluation of sedation depth in ICU patients.
On-line BIS values are updated every second, with each presented value being calculated from EEG information over the past 15 or 30 seconds depending on intended use ('Anaesthesia mode" and "ICU mode"). In our study we used the "ICU mode". Despite the choice of sampling over 30 seconds the variability of displayed BIS values was relatively high; mean minute fluctuation, measured as the difference between the highest and lowest presented BIS value over each minute, was between 7.5 and 16.5 (Table 3). The manufacturers pre-programmed option of calculating BIS values over 15 or 30 seconds may be appropriate in the anaesthesia setting, for real-time detection of rapid changes in anaesthetic depth after drug boluses and changes in stimulation. With the given data, however, we speculate that sampling periods longer than 30 seconds might contribute to more stable BIS-values, perhaps more suitable during uneventful sedation periods in the ICU.
There is no universally accepted standard for how BIS should be used and interpreted during monitoring of sedation in the ICU. Including patient stimulation for BIS interpretation eliminates the potential advantage of avoiding painful stimulation that BIS offers. We chose to calculate the median BIS value over 10 minutes prior to stimulation, rather than a single value or a mean value over one minute or a mean value encompassing stimulation, methods other authors have used in evaluating BIS (12-14). In our view BIS should optimally--as most other online monitors--give valid information without requiring patient stimulation for interpretation.
One reason for using a median value over 10 minutes is that BIS values over one minute vary in the range of 10 to 15% (Table 3), making it difficult to choose a representative value. Secondly, using the median over 10 minutes reduces the impact of brief changes in BIS values, such as arousals from various stimuli in the ICU environment. Such changes in BIS may be of clinical interest, but they do not represent the baseline BIS level that potentially could replace patient stimulation for monitoring of sedation depth.
The Bloomsbury Sedation Score is not a widely used sedation scale. Our rationale for using Bloomsbury rather than a more widely known instrument was that it has been used for evaluating sedation depth in our unit for several years. Using a scale that was familiar to the nurses performing the clinical evaluation of the patients reduced misclassification and improved monitoring compliance. Bloomsbury is similar to the more commonly used Motor Activity Assessment Scale 30. Patient stimulation for evaluation is a common feature for both scales, as well as a similar, clear cutoff between adequate and deep sedation.
The distribution of observed sedation levels included relatively few ratings of too light sedation. Inadequate sedation is generally easily detected and BIS monitoring to avoid undersedation is probably superfluous. We included these data in the analysis for the sake of completeness but excluding the observations of too light sedation would not have affected correlation significantly.
We conclude that BIS XP (version 3.12) with its present performance does not reliably predict level of sedation as measured by clinical evaluation during isoflurane or midazolam sedation in non-paralysed, critically ill patients. During isoflurane sedation of critically ill patients, end-tidal isoflurane concentration correlates better to clinical scoring of sedation depth than BIS.
Supported in part by Hudson RCI (supplied the Anesthetic Conserving Devices) and Abbott Scandinavia (supplied isoflurane). Supported by the Department of Anaesthesiology and Intensive Care Medicine, Karolinska University Hospital Solna.
We thank the nurses of the General ICU at the Karolinska University Hospital for their excellent clinical care of and help in monitoring the study patients. We also thank former Department Chair Dr Lars Irestedt and Dr Per Gannedahl for valuable advice.
Accepted for publication on January 29, 2007.
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P. V. SACKEY *, P. J. RADELL ([dagger]), F. GRANATH ([double dagger], C. R. MARTLING ([section])
General Intensive Care Unit, Karolinska University Hospital Solna, Stockholm, Sweden
* M.D., Ph.D., Specialist in Anaesthesiology and Intensive Care, Department of Anaesthesiology and Intensive Care Medicine, Karolinska University Hospital Solna and Institution of Physiology and Pharmacology, Karolinska Institute.
([dagger]) M.D., Ph.D., Head, Department of Paediatric Anaesthesia and Intensive Care, Astrid Lindgren Children's Hospital and Institution of Physiology and Pharmacology, Karolinska Institute.
([double dagger]) Ph.D., Senior Statistician, Clinical Epidemiology Unit, Department of Medicine, Karolinska University Hospital Solna.
([section]) M.D., Ph.D., Consultant, Head of General Intensive Care Unit, Department of Anaesthesia and Intensive Care, Karolinska University Hospital Solna and Institution of Physiology and Pharmacology, Karolinska Institute.
Address for reprints: Dr P V Sackey, Department of Anaesthesiology and Intensive Care Medicine, Karolinska University Hospital Solna, 171 76 Stockholm, Sweden. email@example.com
TABLE 1 Bloomsbury sedation score 3 Agitated and restless 2 Awake and uncomfortable 1 Aware but calm 0 Roused by voice, remains calm -1 Roused by movement or suction -2 Roused by painful stimuli -3 Unrousable A Natural sleep TABLE 2 Patient characteristics Patient Gender I = isoflurane M = male Age APACHE * M = midazolam F = female (years) II Score I 1 F 39 14 I 2 F 64 30 I 3 F 67 17 I 4 F 44 31 I 5 F 79 32 I 6 F 41 8 I 7 M 61 20 I 8 F 72 16 I 9 M 76 30 I 10 M 58 13 M 1 F 27 23 M 2 F 58 18 M 3 M 62 15 M 4 F 65 32 M 5 M 57 18 M 6 M 31 12 M 7 F 73 26 M 8 F 67 14 M 9 M 57 17 M 10 M 80 23 Patient BIS I = isoflurane monitoring M = midazolam (hours) Main diagnosis I 1 14 Delayed extubation I 2 12 Sepsis I 3 75 Airway obstruction (tumour) I 4 60 Multiple trauma I 5 65 Sepsis I 6 19 Thoracic trauma I 7 59 Pneumonia I 8 14 Delayed extubation I 9 88 Postoperative pneumonia I 10 63 Abdominal trauma M 1 96 Sepsis/endocarditis M 2 5 Intestinal perforation M 3 17 Sepsis M 4 95 Sepsis M 5 52 Pancreatitis M 6 57 Thoracic trauma M 7 87 Peritonitis M 8 15 Delayed extubation M 9 96 Thoracic trauma M 10 3 Pneumonia * APACHE: Acute Physiology and Chronic Health Evaluation. TABLE 3 Drug doses, BIS monitor data and correlation for each patient Mean end-tidal isoflurane Patient concentration Mean morphine Correlation I = isoflurane (%) and midazolam requirement coefficient M = midazolam dose (mg/h) (mg/h) BIS-Bloomsbury I 1 0.28 0 -0.1613 I 2 0.27 4.6 0.0584 I 3 0.37 2.6 -0.0899 I 4 0.27 6.1 -0.0496 I 5 0.24 1.0 -0.3123 I 6 0.30 2.5 * I 7 0.32 2.6 -0.1652 I 8 0.13 2.4 0.0966 I 9 0.23 1.8 * I 10 0.24 1.5 -0.0626 M 1 7.6 8.6 0.3645 M 2 7.0 0.0 -0.4454 M 3 7.6 4.6 0.2498 M 4 4.2 2.6 0.2730 M 5 8.1 0.0 0.3688 M 6 8.5 8.1 0.4454 M 7 0.2 1.8 ** M 8 3.2 5.8 -0.5121 M 9 4.4 7.7 -0.1096 M 10 8.0 5.0 ** Patient I = isoflurane BIS minute Mean Signal Mean EMG M = midazolam fluctuation (%) Quality Index value (dB) I 1 13.8 93 34 I 2 12.3 95 29 I 3 14.2 94 33 I 4 13.6 96 36 I 5 14.6 93 35 I 6 * * * I 7 15.2 93 34 I 8 14.8 95 38 I 9 * * * I 10 15.4 90 34 M 1 12.8 92 29 M 2 16.4 93 46 M 3 11.9 93 35 M 4 14.4 95 39 M 5 16.5 93 35 M 6 14.8 87 36 M 7 10.5 94 44 M 8 13.7 92 44 M 9 14.9 92 33 M 10 7.5 89 44 * BIS, SQI and EMG data not retrievable due to technical error. ** Spearman's rho not calculated due to unchanged Bloomsbury sedation score.
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|Title Annotation:||intensive care unit|
|Author:||Sackey, P.V.; Radell, P.J.; Granath, F.; Martling, C.R.|
|Publication:||Anaesthesia and Intensive Care|
|Article Type:||Clinical report|
|Date:||Jun 1, 2007|
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