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 Table of Contents  
ORIGINAL ARTICLE
Year : 2017  |  Volume : 4  |  Issue : 2  |  Page : 77-83

A comparative study between the bispectral index and the clinical score in the evaluation of sedation level in critically ill, mechanically ventilated patients


1 Department of Critical Care Medicine, Alexandria University, Alexandria, Egypt
2 Department of Critical Care Medicine, Cairo University, Cairo, Egypt

Date of Submission21-Nov-2016
Date of Acceptance16-Mar-2017
Date of Web Publication12-May-2017

Correspondence Address:
Amr Abouelela
ICU Department, PO Box 946, KFMMC, Dhahran
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/roaic.roaic_104_16

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  Abstract 


Background
The 2013 sedation guidelines from the Society of Critical Care Medicine recommend using sedation scales to optimize sedation therapy. However, scales such as the Richmond Agitation–Sedation Scale (RASS) require subjective evaluation that may lead to variability in measurement and consequent clinical actions. The bispectral index (BIS) is an objective measure of the level of consciousness that is often used as anesthetic tool, and its use has broadened to include the ICU. Published data are still insufficient to warrant routine use of the BIS in critically ill patients.
Aim
The aim of this study was to compare the RASS with the BIS in measuring the level of sedation.
Patients and methods
This was a prospective study performed on 60 critically ill, adult, mechanically ventilated patients receiving sedation infusion. The BIS monitor was attached continuously to all patients for 3 days, and the digital readings were recorded every 4 h. The RASS was assessed and recorded at the same time. Statistical analyses were performed to assess the correlation between the two used modalities over time. Patients receiving muscle relaxants were excluded from the study. In addition, patients with head injuries or patients known to have any focal brain disease or abnormal electrical brain activities such as epilepsy were excluded.
Results
A total of 1080 readings comparing the RASS with the BIS were obtained from the 60 patients included in the present study over 3 days. No serious side-effects due to the BIS electrodes placed for 3 days were recorded. The correlation between RASS scores and BIS values was positive and statistically significant. The overall correlation scores in the 3 days were as follows − r=0.652, P less than 0.001 on day 1; r=0.685, P less than 0.001 on day 2; and r=0.686, P less than 0.001 on day 3. The correlation was statistically significant with the use of midazolam but not with dexmedetomidine.
Conclusion
An overall statistically significant, positive correlation between the BIS and the RASS was present. The BIS can be used as an adjunct for scoring.

Keywords: bispectral index, neuromonitoring, Richmond Agitation–Sedation Scale, sedation score


How to cite this article:
Abouelela A, Abdelazim MS. A comparative study between the bispectral index and the clinical score in the evaluation of sedation level in critically ill, mechanically ventilated patients. Res Opin Anesth Intensive Care 2017;4:77-83

How to cite this URL:
Abouelela A, Abdelazim MS. A comparative study between the bispectral index and the clinical score in the evaluation of sedation level in critically ill, mechanically ventilated patients. Res Opin Anesth Intensive Care [serial online] 2017 [cited 2017 Jun 27];4:77-83. Available from: http://www.roaic.eg.net/text.asp?2017/4/2/77/206149




  Introduction Top


Agitation and anxiety occur frequently in critically ill patients, especially those who are mechanically ventilated, and are associated with adverse clinical outcomes [1],[2],[3]. Sedatives are commonly administered to ICU patients to treat agitation and its negative consequences [4]. Sedatives should be titrated to maintain either light (i.e. patient is arousable and able to purposefully follow simple commands) or deep sedation (i.e. patient is unresponsive to painful stimuli) according to the patient’s clinical condition and the expected duration of stay in the ICU. Multiple studies have demonstrated the negative consequences of prolonged, deep sedation and the benefits of maintaining lighter sedation levels in adult ICU patients [5],[6]. The use of sedation scales and sedation protocols designed to minimize sedative use is associated with improved ICU patient outcomes, including a shortened duration of mechanical ventilation, ICU and hospital length of stay, and decreased incidences of delirium and long-term cognitive dysfunction [7].

The Richmond Agitation–Sedation Scale (RASS) and the Sedation–Agitation Scale (SAS) are the most valid and reliable sedation assessment tools for measuring the quality and depth of sedation in adult ICU patients. The RASS and the SAS yielded the highest psychometric scores (i.e. inter-rater reliability and validation). Both scales demonstrated a high degree of inter-rater reliability, which included ICU clinicians [8]. Both scales were able to discriminate different sedation levels in various clinical situations [9]. In addition, the RASS consistently provided a consensus target for goal-directed delivery of sedative agents, demonstrating feasibility of its usage [10].

However, scales such as the RASS require subjective evaluation that may lead to variability in measurement and consequent clinical actions, especially when there is work overload or out-of-regular duty hours when less number of physicians and nurses are available who sometimes lack enough experience. Moreover, the use of muscle relaxants in critically ill patients is not uncommon for certain clinical indications, which makes the application of clinical sedation scales an unachievable task. Clinical Practice Guidelines for the Management of Pain, Agitation, and Delirium in Adult Patients in the ICUs suggest that objective measures of brain function [e.g. the bispectral index (BIS)] be used as an adjunct to subjective sedation assessments in adult ICU patients who are receiving neuromuscular blocking agents, as subjective sedation assessments may be unobtainable in such patients [11].

The BIS is an objective (physiological) measure of the level of consciousness that is often used as anesthetic tool during surgery, and its use has broadened to include adult and pediatric patients in the ICU and the emergency department. The BIS monitor processes a modified electroencephalogram (EEG) to assess the hypnotic effects of sedatives and anesthetics, replacing the reliance on physiological variables for determining the depth of anesthesia. The BIS is a statistically based, empirically derived complex parameter. It is a weighted sum of several EEG subparameters, including a time domain, frequency domain, and high-order spectral subparameters. The BIS monitor provides a single, dimensionless number, which ranges from 0 (equivalent to EEG silence) to 100 (fully awake), making interpretation simple and available to any bedside caregiver [12]. Initially, the BIS technology was basically related to use in anesthesiology, but most of the validation and clinical studies available nowadays are related to its use in the operating room for anesthesia monitoring purposes. Only a few data are available regarding the use of the BIS in the ICU setting. Published data are still insufficient to warrant routine use of the BIS in critically ill patients [13].

The aim of this study was to compare the RASS − a reliable clinical parameter used in assessing the level of sedation in critically ill, mechanically ventilated patients − with the BIS − an objective technology to measure the level of consciousness.


  Patients and methods Top


Approval of the local research and ethics committee was obtained before the start of the present study. Written consent was obtained from the patients’ next of kin for enrollment.

This study was a prospective study carried out on 60 critically ill, mechanically ventilated, adult patients receiving a sedation infusion with either midazolam or dexmedetomidine (precedex) in the ICU at King Fahd Military Medical Complex in Dhahran, Saudi Arabia. The choice of sedation agent and the target level used was made by the attending physician, independent of the study aims. All patients received fentanyl infusions as a part of the analgesia protocol for mechanically ventilated patients.

The BIS was obtained using the BIS double-channel monitor product of Covidien (Plymoth, MN, USA), connected with the BIS Quatro (four electrodes) sensor. After electrode placement above the bridge of the nose, over the temple area and between the corner of the eye and the hairline, the BIS monitor was attached continuously in all patients included in the study for 3 days. The digital reading was recorded every 4 h, and the RASS [9] ([Table 1]) was assessed by the beside nurse and recorded at the same time in a separate sheet prepared for the purpose of this study.
Table 1 The Richmond Agitation–Sedation Scale

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Patients receiving muscle relaxants were excluded from the present study as the RASS cannot be applied in such conditions. In addition, patients with head injuries, patients with severe metabolic derangements (hepatic, uremic, anoxic encephalopathy, etc.), and patients known to have any focal brain disease or abnormal electrical brain activities such as epilepsy that might affect the BIS readings were excluded.

A sample size of 60 patients achieved 83% power to detect an average correlation coefficient between the BIS and the RASS of 0.6 using a two-sided hypothesis test with a significance level of 0.05

Statistical analysis

All statistical analyses were performed using IBM statistical package for the social sciences statistics program version 21 (SPSS; SPSS Inc., Chicago, Illinois, USA) [14]. Quantitative data are described by mean and median as measures of central tendency and SD and by minimum and maximum values as measures of dispersion. Categorical variables are summarized by frequencies and percentages. The correlation between BIS and RASS variables was tested using nonparametric Spearman’s ρ correlation coefficient. Correlation among BIS scores along different periods of time was analyzed using Pearson’s correlation coefficient due to a large sample size greater than 30, and normal distribution of the BIS was determined using the Kolmogorov–Smirnov test. The correlation coefficients were interpreted as follows: less than 0.3, small; 0.3–0.5, moderate; and greater than 0.5 as a large linear relationship between the two variables [15].


  Results Top


A total of 84 patients fulfilling inclusion criteria were initially recruited for the present study; 24 of them were excluded because of failure to obtain consent (seven patients), because sedation and mechanical ventilation were stopped before 3 days (nine patients), and because of technical errors in accurately recording BIS and RASS data (eight patients).

The baseline characteristics of the 60 included patients are shown in [Table 2]. The average age of patients was 53.3±17.3 years, and the majority of patients were males (48/60 males vs. 12/60 females). The indication for mechanical ventilation varied from polytrauma, pneumonia, acute respiratory distress syndrome, and shock. Of the 60 patients, 52 of them were sedated using midazolam, whereas the remaining eight patients were sedated using dexmedetomidine. All 60 patients received fentanyl infusion as an analgesic.
Table 2 The baseline characteristics of the studied patients (n=60)

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A total of 1080 readings (18 readings for each patient) comparing the RASS with the BIS were obtained from the 60 patients included in the study over 3 days. [Figure 1] shows the descriptive analysis of the RASS and the BIS over the 3 days. Half of the readings were obtained during the day shift (8 a.m., 12 mid-day, 4 p.m.), whereas the other half of the readings were obtained during the night shift (8 p.m., 12 midnight, 4 a.m.) ([Table 3]). The RASS assessment was performed mainly by bedside nurses; however, during certain occasions a double-check was performed by the physicians as well, especially during the morning rounds, and no significant difference was noticed between the nurses’ and physicians’ assessments. The trend of the readings was quite logical − higher BIS readings correlated with light sedation, and the readings decreased gradually when the RASS score decreased from −1 to −5. Apart from the readings of day 1, RASS −2 showed unexplained low BIS readings, which can be explained with a very low number of patients falling in this category (four patients in the day shift and two patients in the night shift).
Figure 1 Descriptive analysis of the Richmond Agitation–Sedation Scale (RASS) and the bispectral index (BIS) over 3 days

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Table 3 Descriptive analysis of the Richmond Agitation–Sedation Scale and the bispectral index over 3 days

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No serious side-effects due to the BIS electrodes were recorded over the 3 days. In some patients, the leads had to be changed when patient movement or excessive sweating dislodged the leads. Only simple skin redness was seen in some patients after removal of the leads, but it was not clinically significant and improved with no intervention.

The correlation between RASS scores and BIS values is shown in [Table 4] and [Figure 2]. A statistically significant correlation (P<0.001) was obtained with all the readings at different times over the 3 days. The overall correlation values found during the 3 days were r=0.652, P less than 0.001 on day 1; r=0.685, P less than 0.001 on day 2; and r=0.686, P less than 0.001 on day 3.
Table 4 Correlation between the bispectral index and the Richmond Agitation–Sedation Scale over 3 days

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Figure 2 Correlation between the bispectral index (BIS) and the Richmond Agitation–Sedation Scale (RASS)

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The correlation between RASS scores and BIS values with the use of different sedatives is shown in [Table 5]. Although the correlation was highly consistent and statistically significant with midazolam, with a good positive correlation between the BIS and the RASS (P<0.001*), the situation was different with dexmedetomidine, especially on day 1. The correlation was not significant during the day shift, and there was a negative correlation in the night shift. The correlation was statistically significant and positive again on day 2 and day 3 during the day shift and night shift separately, whereas the overall correlation was not statistically significant.
Table 5 Correlation between the bispectral index and the Richmond Agitation–Sedation Scale with different sedatives

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  Discussion Top


The clinical requirement of appropriate sedation during mechanical ventilation is well established − not only to relieve stress, anxiety, pain, and suffering but also to facilitate patient–ventilator interaction and synchrony, as well as to minimize the possible adverse effects related to mechanical ventilation. It also helps in preventing unnecessary removal of invasive devices such as central line and endotracheal tube, and might have a positive impact on patient outcome [16]. However, there is no clear consensus regarding standard definitions for, and assessment of, appropriate sedation. This distinction is important, because over-sedation may also be associated with increased morbidity [17],[18].

Since publication of the 2002 Society of Critical Care Medicine guidelines, various studies on the BIS and sedation scales have been carried out. Specifically, correlations of the BIS and clinical scores for sedation have demonstrated wide-ranging degrees of correlation [19],[20]. These studies have used various methods and protocols, different models of the BIS machines, and different cutoff points for data collection, thus making comparisons of results a quite challenging task. Moreover, some researchers comparing the BIS with sedation scales have also reported a similarly poor correlation in ICU patients, and the conflict is not yet solved [21],[22].

In the present study, the correlation between RASS scores and BIS values showed a statistically significant positive correlation (P<0.001), which was obtained with all the readings. None of the previous studies investigated the difference in correlation between daytime and night-time. Our decision to investigate this point was based on a theoretical assumption that the clinical assessment overnight could be less accurate because of shortage of nursing staff or less experience; however, the results showed that this assumption is not true, and the correlation is similar all over, denoting good assessment of the RASS in the night shift with significant positive correlation with the BIS. In addition, we assumed that the accuracy of BIS readings could be reduced over time, which will affect the correlation over the second and third day, but again this was not applicable as the strong positive correlation was the same on all 3 days. This can be simply explained by the fact that the leads of the BIS were checked regularly and changed whenever indicated, which provided us with accurate measurements over the 3 days of the study.

Our results are in agreement with many similar studies. Arbour et al. [23] studied 40 ICU patients requiring mechanical ventilation and sedation, and their patients were monitored using the SAS and the BIS. A moderate positive correlation between BIS and SAS values was obtained, with an r value of 0.502 (P<0.0001), and they concluded that in situations where clinical assessment is equivocal BIS monitoring may have an adjunctive role in sedation assessment [23].

In another study, in concordance with our study, Olson et al. [24] investigated the possible benefit of including the BIS tool as an adjunct to the clinical score. Their study was carried out on 67 mechanically ventilated, adult patients receiving continuous intravenous sedation with propofol. They found that patients in the BIS-augmentation group (monitored by clinical scores and the BIS) received significantly less propofol (93.5 vs. 157.8 ml, respectively; P<0.015), had lower infusion rates (14.6 vs. 27.9 µg/kg/min; P=0.003), and the BIS-augmentation group regained consciousness much earlier than those in the Ramsay-alone group (1.2 vs. 7.5 min; P<0.0001) [24]. The results also highlight the potential benefit of including the BIS in monitoring, especially with the use of propofol, as it was the only sedative used in this study, As it is well-known that it is a short-acting drug, we should expect high fluctuations in the clinical score assessment over time, and this may explain the advantage of including the BIS to provide more consistent monitoring of the level of sedation in such peculiar groups of patients.

On the other hand, Trouiller et al. [21] conducted a study on 62 mechanically ventilated patients requiring intravenous sedation and analgesia for up to 24 h. Paired measurements of the BIS and sedation measured using the adaptation to intensive care environment score were obtained every 3 h until awakening. At least one discordant assessment was observed in 52 (83.9%) patients. The median individual discordance ratio was 32% (14.3–50.0%) [21].

Moreover, a poor correlation between BIS values and SAS scores was observed in the study by LeBlanc et al. [22], which was designed to assess the degree of correlation between BIS values, scores on the SAS, and steady-state serum concentrations of lorazepam. They found that the wide range of BIS values within a given SAS score is still of concern and may complicate its clinical application. In evaluating the clinical predictability of the BIS in that patient population, the BIS value was discordant with the SAS assessment in many cases. One patient had an SAS score of 5 and a BIS value of 78 on study day 1, and then a SAS score of 3 and a BIS value of 97 on the subsequent day. We are not sure whether the poor correlation was related to the use of lorazepam or not, as this is may be the only study performed using lorazepam for sedation during BIS neuromonitoring [24].

It is not known whether the use of different sedation agents can affect the accuracy and validity of using the BIS in critically ill patients. In the present study, the correlation was very consistent and statistically significant with midazolam, with a good positive correlation between BIS readings and RASS values. Although it was not statistically significant with the use of dexmedetomidine and no clear correlation could be documented between BIS readings and RASS values, this can be related to different physical properties and clinical action of this drug and also be easily explained by the very few number of patients on dexmedetomidine (8/60), which makes any statistical analysis unachievable and inaccurate.

In conclusion, we found an overall statistically significant, positive correlation between the BIS and the RASS. The BIS can be used as an adjunct or alternative to clinical scores. The correlation was valid and clear with the use of midazolam, whereas the correlation with the use of dexmedetomidine was not clear because of the small number of patients, and this should be examined with a larger sample size.

The main limitations of this study are as follows. Only one sedative drug − midazolam − was used in the majority of patients, with a very small number of patients on dexmedetomidine. Other important drugs such as propofol were not included at all, which makes the results valid only with regard to midazolam. There was no enough power to detect correlations with respect to other agents. Finally, patients included in the present study received fentanyl, and the sedative effects of opioids can affect the BIS reading. However, opioids are routinely administered to critically ill, mechanically ventilated patients, and this condition is unavoidable in all other similar studies.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]



 

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