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Department of Anesthesiology, Uniformed Services University of the Health Sciences, Bethesda, MD Department of Pediatrics, Walter Reed Army Medical Center, Washington, DC
To the Editor:
We read with interest the article by McDermott et al. (1). Finding a way to adequately measure the depth of sedation in pediatric patients is important; however, we believe that there were several problems with the study. First was the lack of specifics given on how the University of Michigan Sedation Scale (UMSS) was applied. Malviya et al. (2) states in their article that "significant physical stimulation" consists of rubbing the back or tickling the chest or axilla, but there is no indication of the type of stimulation used in this study. Since no patient had a UMSS score of 4 and the authors state that "the observers were reluctant to apply vigorous stimulation to sedated children," it is reasonable to question whether or not the UMSS was correctly applied. The fact that the observer was not blinded to the BIS score is a significant flaw in the study design. No mention was made of the quality of the BIS data; how many BIS scores were unusable because of a low signal quality index? The authors state that the BIS is easy to use. In our experience, the amount of pressure required to get a good application of the BIS sensor makes children uncomfortable, and even quiet cooperative older children have too much EMG interference to get adequate data unless they are sedated. The BIS score used for comparison was the score immediately after the UMSS was completed. Simply applying the UMSS to patients could potentially increase the BIS score, especially in noninvasive procedures. Finally, while we agree that there is a correlation between the BIS score and UMSS, I question the clinical importance of the correlation. Looking at the "all subjects correlation," it appears that 25% of the time patients with a UMSS of 3 (deeply sedated) had a BIS of >80 and more than 33% of the time they had a BIS >70. By substituting BIS for a valid sedation scale we may risk underestimating the child’s actual level of sedation. In addition, most of the patients in this study were having invasive procedures, neither BIS or a sedation scale is needed to judge the level of sedation for an invasive procedure, since the patients own response to stimulation is an adequate determinant of depth of sedation. The real question is whether BIS can be used to judge the level of sedation in pediatric patients having noninvasive studies.
References
Department of Anesthesiology, The Children’s Hospital, Denver, Colorado
In Response:
We appreciate the interest of Drs. Shields and Creamer in our article (1). The issues of study design that they raised were discussed in our paper. The application of "significant physical stimulus," as required by the University of Michigan Sedation Scale (UMSS), can be a real disadvantage of the UMSS (and other commonly used observational scoring systems) in the clinical settings in which children are sedated. We decided that procedural stimuli (e.g., dental work, colonoscopy, insertion of vascular catheters) were significant physical stimuli themselves, so during those types of procedures rubbing the back or tickling the axilla did not add further disruption. During imaging procedures, however, we quickly learned that application of physical stimuli was disruptive. This was not much of an issue during some procedures (computerized tomography scans were so brief that many data pairs were collected during the long period of recovery from pentobarbital in which application of physical stimuli was acceptable). During cardiac catheterization, however, stimulation of the child was simply unacceptable because it often caused undesired movement. Thus, as discussed in our article, the observers did not stimulate the children during most of those procedures. Had they done so, we believe the number of subjects with UMSS scores of 4 would have increased, thus improving the already significant correlation with BIS scores.
The issues of EMG activity and blinding of observers were discussed thoroughly in the article. The potential effects of EMG interference and observer bias were tested during the study and, as reported, did not affect the results. Our article did not address the issues of signal quality index and acceptance of the sensor by the patient because these were rarely problematic for us. Finally, the "all subjects correlation" (Fig. 1 in the article) included noncorrelating data pairs from subjects who received ketamine or the oral sedatives used in the dental clinic. As cited in our article, ours is not the only study to conclude that BIS cannot be used with those drugs. That’s why Figure 4 was included.
Drs. Shields’ and Creamer’s statement,"...neither BIS or [sic] a sedation scale is needed to judge the level of sedation for an invasive procedure since the patient’s own response to stimulation is an adequate determinant of depth of sedation," ignores the risks of oversedation and is an example of why the guidelines of both the American Society of Anesthesiologists (2) and the American Academy of Pediatrics (3) recommend systematic monitoring of the depth of sedation. If a child does not respond to the stimulation of an invasive procedure, is the child satisfactorily sedated or is the child under general anesthesia? Without an assessment of depth, there is no way to tell. Unintended oversedation does occur and is associated with an increased risk of adverse events, both of which can reduced by systematic assessment of depth of sedation (4).
Drs. Shields’ and Creamer’s letter highlights several of the problems associated with current methods of ascertaining the depth of sedation in children. The first problem is the requirement of most observational scoring systems to interact verbally or physically with the child in order to measure the depth of sedation. This is counterproductive in most sedation settings, because it can arouse the child to the extent that the diagnostic or therapeutic procedure cannot be carried out successfully. This is particularly problematic during imaging studies, in which an unmoving child is essential.
Another problem with current observational scoring systems is that they are, in part, subjective. For example, definitions of "significant physical stimulation" and other examiner-patient interactions are qualitative and introduce subjectivity into the scoring system. Does one speak to the child loudly or softly? How hard does one rub the back or tickle the axilla? If the stimulus cannot be quantified, neither can the response.
As Drs. Shields and Creamer point out, the BIS is not perfect either. It can’t be used with certain sedating drugs, such as ketamine and the oral sedatives described in our study. EMG activity can interfere with BIS scores, although current software and the new XP sensors are designed to filter out such interference. The BIS algorithm is based on adult EEG data and should be reformulated with neonatal data, so that questions about its applicability to infants can be resolved. The BIS does have some readily apparent advantages over observational scoring systems, however. It is easy to use and is quantitative. It does not require interaction with the patient that may disrupt the procedure for which the sedation is given. Our data support its validity. Clearly, the BIS deserves further investigation as a tool for assessment of depth of sedation in children.
References
This article has been cited by other articles:
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  T. Engelhardt, M. K. Chan, A. J. McCheyne, C. Karsli, I. Luginbuehl, and B. Bissonnette The Effect of Varying Continuous Propofol Infusions on Plasma Cyclic Guanosine 3',5'-Monophosphate Concentrations in Anesthetized Children Anesth. Analg., September 1, 2007; 105(3): 616 - 619. [Abstract] [Full Text] [PDF]
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C. Kerssens and P. S. Sebel To BIS or Not to BIS? That Is the Question Anesth. Analg., February 1, 2006; 102(2): 380 - 382. [Full Text] [PDF]
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S. Sadhasivam, A. Ganesh, A. Robison, R. Kaye, and M. F. Watcha Validation of the Bispectral Index Monitor for Measuring the Depth of Sedation in Children Anesth. Analg., February 1, 2006; 102(2): 383 - 388. [Abstract] [Full Text] [PDF]
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 S. T. Reeves, J. E. Havidich, and D. P. Tobin Conscious Sedation of Children With Propofol Is Anything but Conscious Pediatrics, July 1, 2004; 114(1): e74 - e76. [Abstract] [Full Text] [PDF]
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