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Ann Thorac Surg 2009;88:144-150. doi:10.1016/j.athoracsur.2009.03.074
© 2009 The Society of Thoracic Surgeons
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Original Articles: Pediatric Cardiac

Assessment of the Level of Sedation in Children After Cardiac Surgery

Adelaida Lamas, MD, PhDa, Jesús López-Herce, MD, PhDa,*, Luis Sancho, MD, PhDa, Santiago Mencía, MD, PhDa, Ángel Carrillo, MD, PhDa, Maria José Santiago, MDa, Vicente Martínez, PhDb

a Pediatric Intensive Care Unit, Hospital General Universitario Gregorio Marañón, Madrid, Spain
b Centro de Estudios Sociosanitarios (CESS), Castilla-La Mancha University, Cuenca, Spain

Accepted for publication March 25, 2009.

Abbreviations and Acronyms BIS = bispectral index; CG = control group; CS = COMFORT scale; CSG = cardiac surgery group; DBP = diastolic blood pressure; EEG = electroencephalogram; HR = heart rate; MLAEPs = middle latency auditory evoked potentials; MV = mechanical ventilation; PICU = Pediatric Intensive Care Unit; ROC = receiver operating characteristic curves; SBP = systolic blood pressure; RS = Ramsay Scale; SQI-BIS = signal quality index of Bispectral index

* Address correspondence to Dr López-Herce Cid, Sección de Cuidados Intensivos Pediátricos, Hospital GU Gregorio Marañón, Dr Castelo 47, Madrid, 28009, Spain (Email: pielvi{at}ya.com).


   Abstract
Top
 Abstract
Introduction
 Patients and Methods
 Results
 Comment
 Acknowledgments
 References
 
Background: There is no reference method for the evaluation of the level of sedation in children after cardiac surgery. The utility of the bispectral index and middle latency auditory evoked potentials has not been evaluated.

Methods: The bispectral index, middle latency auditory evoked potentials, Ramsay scale, and COMFORT scale were used for assessment of the level of sedation in critically ill children after cardiac surgery and other surgical procedures. The measurements with these four methods were recorded simultaneously once a day for five days. The level of sedation was categorized in two levels, moderate or deep, according to the values obtained from each method. Correlations and agreements among the methods and the best bispectral index and middle latency auditory evoked potential values that discriminated between the two levels of sedation were calculated.

Results: Thirty-two children after cardiac surgery were included in the study, together with eighteen children after other surgical procedures who formed the control group. In each group, the correlation and agreement between the four methods varied between moderate and good. In the cardiac surgery patients, when the level of sedation was determined by the Ramsay scale, the best values of bispectral index and middle latency auditory evoked potentials that discriminated between the two levels of sedation were 63.5 and 37.5, respectively, and these values predicted the level of sedation correctly in 84.4% of the patients with each method.

Conclusions: Bispectral index and middle latency auditory evoked potentials could be useful to assess the level of sedation in children after cardiac surgery.


   Introduction
Top
 Abstract
 Introduction
Patients and Methods
 Results
 Comment
 Acknowledgments
 References
 
Sedation is a fundamental part of the treatment of critically ill children [1]. However, oversedation can cause cardiovascular depression and prolongation of mechanical ventilation (MV), with the risk of superinfection [2], lung damage [3], and neuromuscular disturbances [4]. In patients in the postoperative period of cardiac surgery who require MV, inadequate sedation can lead to hypoxemia, hypercapnia, hypertension, asynchrony with the ventilator, and a risk of myocardial ischemia [5–7].

At the present time, there is no reference method for the evaluation of the level of sedation in critically ill patients [8]. Clinical scales are the most widely used methods [9], though methods derived from the electroencephalogram (EEG), such as the bispectral index (BIS) or middle latency auditory evoked potentials (MLAEPs), have been introduced in recent years [10]. These methods provide an alternative method of automatically and continuously monitoring the level of sedation. They were initially applied in adults [11, 12] and children [13, 14] during anesthesia and recently they have also been used in critically ill adults [15–17] and children [18–24]. Some studies have evaluated the utility of BIS [25] and MLAEPs [26] during cardiac surgery; however, we have found no published studies analyzing the utility of BIS and MLAEPs in children during the postoperative period of cardiac surgery in the pediatric intensive care unit (PICU).

The aim of this study has been to analyze the utility of BIS and MLAEPs in children during their stay in the PICU after cardiac surgery. Another objective was to determine whether the assessment of the level of sedation with these methods is similar to that observed in critically ill children after other types of surgery.


   Patients and Methods
Top
 Abstract
 Introduction
 Patients and Methods
Results
 Comment
 Acknowledgments
 References
 
The study was approved by the local Institutional Review Board, and informed consent was obtained from the parents. Patients less than 14 years of age with MV and receiving sedation were included in the study during their stay in the PICU. The patients were divided in two groups: patients in the postoperative period of cardiac surgery (cardiac surgery group [CSG]), and patients after other types of surgery (noncardiac surgery group or control group [CG]). The study protocol was started when the child was admitted to the PICU and was completed on day 5 of the PICU admission. The exclusion criteria were the following: no need for MV, the interruption of MV within 24 hours after admission, trauma, uremia, and neurologic injury or illness.

The study protocol included assessment of the level of sedation using the modified Ramsay scale (RS) [27] (Table 1), COMFORT scale (CS) (Table 2), BIS, and MLAEPs, all of which were recorded simultaneously, once a day, for a maximum of 5 days. The BIS monitor was placed at the time of admission to the PICU and was maintained continuously until the end of the study. The MLAEPs monitor was placed only once a day. Baseline values were obtained for both monitors 5 minutes after the MLAEPs monitor was connected, when stable values were observed. The study was interrupted before day 5 if the patient was in a light level of sedation (Ramsay score 1–2 or COMFORT score of 27–40), if MV was interrupted, if death occurred, or if the patient was discharged from the PICU.


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  		Table 1 Modified Ramsay Scale
 

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  		Table 2 COMFORT Scale
 
The nurses assessed the level of consciousness using the RS and CS and they were blinded to the BIS and MLAEPs results. Physiologic variables (heart rate [HR], systolic blood pressure [SBP], and diastolic blood pressure [DBP]) were also recorded at the same time the level of sedation was assessed.

Sedation, analgesia, and neuromuscular blockers were administered in accordance with standard PICU practice. All patients received continuous infusions of opioids and (or) benzodiazepines. We used continuous pharmacologic sedation with midazolam (initial dose of 1 to 2 mcg/kg/minute) and fentanyl (initial dose of 1 to 2 mcg/kg/hour). In patients requiring muscle relaxation, vecuronium was administered at a dose of 0.1 mg/kg/hour. For periods of agitation, boluses of medication were used according to the nurse's or physician's assessment. No change in the medication and no procedures such as tracheal suction, physiotherapy, or other stimuli were performed for 15 minutes before and after the study or during the procedural protocol.

The BIS scores were recorded using a BIS XP 3.4 monitor (Aspect Medical Systems, Newton, MA). The sensors, pediatric ZIP prep three electrode for children less than 1 year of age, and Quatro ZIP prep four electrode for older children, were placed centrally 2 inches above the nose, adjacent to the eyebrow, and between the corner of eye and the hairline. Values with a signal quality index (SQI-BIS) greater than 60% were recorded.

The MLAEPs were obtained using the AAI1.5 A-Line auditory evoked potential index monitor (ALARIS Medical Systems, software 1.5, Danmeter A/S, Odense, Denmark). The A-Line electrodes (A-Line auditory evoked potentials electrodes; Danmeter A/S) were positioned at midforehead (positive), left or right forehead (reference), and at the mastoid process in children over 8 years of age or in front of the pinna of the ear in younger children. The electrodes were placed on the contralateral side to the BIS sensor. The MLAEPs were elicited with headphones producing a bilateral 70 dB click stimulus with duration of 2 ms.

In patients receiving neuromuscular blockers, an accelerometry monitor (Organon Teknika, Boxtel, the Netherlands) was positioned over the ulnar nerve each day at the beginning of the study in order to detect the train-of-four percentage of the adductor pollicis muscle. The first response in the train-of-four sequence was recorded and used as control. The patient was considered to be paralyzed if the number of movements of the thumb was 3 or less.

The patients were categorized in two levels of sedation (moderate versus deep) in accordance with previously published references [18, 19, 21, 28, 29]. The values for moderate sedation were BIS of 60 or greater, MLAEP of 30 or greater, COMFORT of 18 or greater, and Ramsay of 5 or less. The values for deep sedation were BIS less than 60, MLAEP less than 30, COMFORT of 17 or less, and Ramsay of 6 or greater. In order to study the influence of age on the evaluation of the level of sedation, the patients were divided into three age groups: less than 6 months, between 6 months and 2 years, and over 2 years.

The statistical analysis was performed using the SPSS statistical package for Windows (software version 13.0; SPSS Inc, Chicago, IL). Two statistical analyses were carried out, one using the data obtained from each patient on the first day and the second with the data from the whole study period. Correlations were determined using the Spearman rank correlation test (r) and the level of agreement was performed using Cohen's kappa test ({kappa}); both analyses were stratified by age, sex, and neuromuscular blockade. Spearman rank (r) values greater than 0.4 and kappa ({kappa}) values greater than 0.4 were considered to represent moderate to excellent agreement. The changes in the level of sedation during the study period were analyzed using the Pillai test. Hypotheses about means were tested by analysis of variance with a subsequent t test for pairs of means corrected by the Bonferroni and Dunnett's C methods. Data are expressed as means ± standard deviation (SD) in the case of a normal distribution. A p value less than 0.05 was considered significant in all tests.

The ability of the BIS and MLAEPs to discriminate between the two sedation levels, previously classified using the CS and RS, was tested using receiver operating characteristic (ROC) curves. The cutoff points were determined at the point of greatest sensitivity and specificity for discrimination. Logistic regression models using these cutoff points were then created to predict the sedation level of the patients (moderate versus deep) and this was compared with the results of the CS and RS.


   Results
Top
 Abstract
 Introduction
 Patients and Methods
 Results
Comment
 Acknowledgments
 References
 
Fifty patients were enrolled in the study; 32 (64%) patients in the CSG, with 56% males, and 18 (36%) in the CG, also with 56% males. The age distribution for the CSG was 40.6% (less than 6 months), 37.5% (6 months to 2 years), and 21.9% (more than 2 years of age); for the CG was 22.2% (less than 6 months), 61.1% (6 months to 2 years), and 16.7% (more than 2 years of age).

Midazolam and fentanyl were administered to all patients in both groups and muscle relaxants (vecuronium) to 40.6% in the CSG and 22.2% in the CG. There were 7 patients (21.9%) in the CSG with active pacemakers and none in the CG. The diagnoses of the patients in each group are listed in Table 3. Over the study period, observations of the four methods of assessment of the level of sedation were recorded in the CSG on 98 occasions, 48 of which were in paralyzed patients; 55 observations were recorded in the CG, 14 of which were in paralyzed patients.


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  		Table 3 Diagnoses
 
There were no differences in the levels of correlation and agreement between the first day and the whole study period. For this reason we only present the results of the analysis from the whole study period. There were nonsignificant differences in the mean values of MLAEPs and CS between the CSG and the CG: MLAEPs 28.2 ± 20.1 versus 33.1 ± 20.1 (p = 0.147) and CS 13.3 ± 5.1 versus 14.8 ± 5.9 (p = 0.096). However, the level of sedation evaluated by RS and BIS was significantly deeper in the CSG than in the CG; BIS 53.4 ± 18.9 versus 60.2 ± 20.9 (p = 0.041) and RS 6.7 ± 1.8 versus 5.4 ± 2.5 (p = 0.005). There were no differences with respect to age or sex.

At the beginning of the study, all four methods classified most patients in both groups as being in deep sedation. The evaluation of the level of sedation by each method was stable throughout the 5 days of the study. In both groups, the clinical scales classified more patients as being in deep sedation than BIS and MLAEPs (in the CSG: 56.3% by BIS, 68.8% by MLAEPs, 84.4% by RS, and 96.9% by CS; and in the CG: 55.6% by BIS, 77.8% by MLAEPs, 83.3% by RS, and 88.9% by CS).

The correlations between the four methods of the evaluation of the level of sedation in the CSG and in the CG were moderate to good. We performed separate analyses of the observations made in paralyzed patients and in nonparalyzed ones. The correlation of the four methods in nonparalyzed children was moderate to excellent in both groups. In paralyzed patients, there was only a moderate correlation between the clinical scales in the CSG and a moderate correlation between the two EEG-derived monitors in the CG, together with a moderate-to-good correlation between these two methods and the RS (Table 4). In the CSG there was a moderate level of agreement between the BIS and MLAEPs, RS and CS, and BIS and RS. In the CG, there was a moderate-to-good level of agreement among all methods except between the BIS and CS. No correlation was found between the four methods and the autonomic variables (HR, SBP, and DBP).


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  		Table 4 Correlations Between BIS, MLAEPs, Ramsay, and COMFORT in Paralyzed and Nonparalyzed Patients
 
In patients with active heart pacing, the BIS values increased by 10 to 15 points when it was possible to turn off the pacemaker temporarily, although systematic recording of these changes was not performed. When the pacemaker was active it was not possible to obtain values from the MLAEPs monitor as the monitor detected excessive artifacts, which disappeared on turning off the pacemaker for a few seconds.

The BIS and MLAEPs values with the best level of discrimination between moderate and deep sedation, based on classification by the RS and CS, were identified through analysis of the ROC curve. When the level of sedation was classified by the RS in the CSG, the calculated cutoff point for the BIS was 63.5 (sensitivity of 80%, specificity of 70.4%, and area under the curve of 0.785) (Fig 1) and for MLAEPs was 37.5 (sensitivity of 60%, specificity of 85.2%, and area under the curve of 0.730) (Fig 2). The logistic regression models using the calculated cutoff points for the BIS and MLAEPs were able to predict the level of sedation correctly according to the RS in 84.4% of the patients with both methods. The ROC analysis could not be performed for the level of sedation assessed by the CS in the CSG because only 1 of the 32 patients was classified as being in moderate sedation by this scale. The cutoff points were then recalculated after stratification of the patients according to the state of paralysis. In the CSG the cutoff points for the BIS and MLAEPs in nonparalyzed patients, based on the level of sedation classified by the RS, were the same as in the global group; BIS 63.5 (sensitivity of 75%, specificity of 53.3%, and area under the curve of 0.708) (Fig 1) and MLAEPs 37.5 (sensitivity of 75%, specificity of 73.3%, and area under the curve of 0.708) (Fig 2). The logistic regression models using these cutoff points for the BIS and MLAEPs were able to predict the level of sedation correctly according to the RS in 78.9% of the patients with both methods. The ROC analysis could not be performed in the control group in the general analysis, and in the analysis of nonparalyzed patients, because only 2 patients were classified as being in moderate sedation by RS and 3 patients by CS.


Figure 1
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  		Fig 1. Receiver operating characteristic curves and cutoff points for the bispectral index (BIS) when the level of sedation was classified by the Ramsay scale in cardiac surgery patients and in nonparalyzed cardiac surgery patients. In all cardiac patients (A) the area under the curve (AUC) was 0.785. The calculated cutoff point for the BIS was 63.5 (sensitivity of 80%, specificity of 70.4%). In nonparalyzed cardiac patients (B) the AUC was 0.708 and the calculated cutoff point was 63.5 (sensitivity of 75%, specificity of 53.3%).

 

Figure 2
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  		Fig 2. Receiver operating characteristic curves and cutoff points for middle latency auditory evoked potentials (MLAEPs) when the level of sedation was classified by the Ramsay scale in cardiac surgery patients and in nonparalyzed cardiac surgery patients. In all cardiac patients (A) the area under the curve (AUC) was 0.730. The calculated cutoff point for the MLAEPs was 37.5 (sensitivity of 60%, specificity of 85.2%). In nonparalyzed cardiac patients (B) the AUC was 0.708 and the calculated cutoff point for MLAEPs was 37.5 (sensitivity of 75%, specificity of 73.3%).

 

   Comment
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
Acknowledgments
 References
 
The assessment of sedation in the postoperative period of cardiac surgery is highly complex, particularly in children. The clinical scales are the most widely used methods. Due to its simplicity, the RS is preferred, although it has not been validated in children [8, 9, 30, 31]. The CS scale has been designed specifically for critically ill children on MV [20–23, 32]. However, both scales are subjective [8, 9] and they are not designed for use in patients with neuromuscular blockade [21]. Methods derived from the EEG, principally the BIS and MLAEPs, have recently been proposed but experience with these methods in children is limited [18–24].

Our study has analyzed the assessment of sedation using the BIS, MLAEPs, and the two clinical scales in the postoperative period of cardiac surgery in children. The results suggest that the BIS and MLAEPs could be useful in the evaluation of the level of sedation in children during the postoperative period of cardiac surgery. The correlation, concordance, and predictive values of the level of sedation determined by the clinical scales are similar to those found in critically ill children after other types of surgery.

The correlation of the BIS and MLAEPs with the clinical scales in children in the postoperative period of cardiac surgery in our study is similar to that found by other authors in critically ill children, although none of those studies has specifically analyzed this subgroup of children [18–24]. Very few studies have used MLAEPs in cardiac surgery. In adults undergoing cardiac surgery, Musialowicz and colleagues [33] looked at changes in the latency of the Nb wave as a function of the level of sedation during the preoperative period, operation, and the first day after surgery; they found an increase in this latency in the deepest states of sedation. There is only one study in critically ill adult patients that has evaluated the correlation between MLAEPs and the RS, reporting results similar to those found in our study [17].

The degree of correlation between the BIS and MLAEPs in our patients varied between moderate and good, and it may therefore be considered that both methods measure the level of sedation in a similar manner in children after cardiac surgery and after other types of surgery. There are no previous studies that have analyzed the correlation between the RS and CS in these patients; in our study there was a good correlation between these two clinical scales. The BIS and MLAEPs showed a better concordance with the RS than with the CS. This could be due to the confounding factor of the hemodynamic variables that are included in the evaluation by the CS and that could be affected by the use of vasoactive drugs administered in the postoperative period of cardiac surgery. In paralyzed cardiac surgery patients we found no correlation between the objective methods and the clinical scales, probably because the clinical scales are not designed to evaluate sedation in patients with muscle relaxation. As the risk of oversedation or undersedation is highest in these patients, we consider that the BIS and MLAEPs could be particularly indicated for the assessment of the level of sedation in paralyzed patients. However, the absence of correlation between the two methods in children with muscle relaxation after cardiac surgery means that our results must be interpreted with caution.

There was no correlation between the heart rate and blood pressure or between those variables and any of the methods for assessing sedation in either of the two groups; moreover, this analysis was not altered by the presence or absence of muscle relaxation. Our results agree with those reported in other studies in which the hemodynamic variables were not considered to be appropriate parameters for assessment of the level of sedation in the postoperative period of cardiac surgery [34, 35].

We have found no previous studies in children who have undergone cardiac surgery that have performed a ROC curve analysis to determine the cutoff points for the BIS and MLAEPs. In our study, the BIS and MLAEPs scores that discriminated between moderate and deep sedation, based on the RS, were the same in children after cardiac surgery and after other types of surgery. The value for the cutoff point for the BIS in both groups of our study was similar to the one published by Triltsch and colleagues [22] for critically ill patients on MV in the PICU.

Most of the patients in our study were in deep sedation and a significant percentage had muscle relaxation. In this type of patient the BIS and MLAEPs enabled us to assess the level of sedation in a continuous, simple, and noninvasive manner that was easy to interpret. However, the level of sedation is not dichotomic but rather it is a continuum and changes over time. For this reason the BIS and MLAEPs values should not be considered absolute, and the assessment of sedation must be performed through a continual analysis of the BIS and (or) MLAEPs, together with the clinical scales.

There is an acceptable level of correlation between the BIS and MLAEPs and between these methods and the Ramsay and COMFORT clinical scales. The BIS and MLAEPs could be useful tools to assess the level of sedation in children in the postoperative period of cardiac surgery. In this situation, the hemodynamic variables do not correlate with the BIS or MLAEPs methods or with the clinical scales.


   Acknowledgments
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Acknowledgments
References
 
The authors are grateful to the nurses and doctors of the pediatric intensive care unit of the Gregorio Marañon University General Hospital, Madrid, for their collaboration in performing this study and to Montserrat Solera for all her help with the statistical analysis.


   References
Top
 Abstract
 Introduction
 Patients and Methods
 Results
 Comment
 Acknowledgments
 References
 

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