(Circulation. 2001;103:2483.)
© 2001 American Heart Association, Inc.
Clinical Investigation and Reports |
From the University of Washington (F.C., B.W.D., F.M.L., G.L.R.) and Agilent Technologies (D.B.J., D.S., T.D.L., B.C.), Seattle, Wash; Harvard Medical School (C.I.B.), Boston, Mass; Childrens Hospital and Health Center (J.C.P., A.A.Z.), San Diego, Calif; and the University of Iowa (D.L.A.), Iowa City.
Correspondence to Frank Cecchin, MD, Childrens Hospital and Medical Center, 4800 Sandpoint Way, NE, Seattle, WA 98105. E-mail fcecch{at}chmc.org
| Abstract |
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Methods and
ResultsChildren aged
12 years who
either developed arrhythmias or were at risk for developing
arrhythmias were studied. Two sources were used for the
database: children whose rhythms were recorded prospectively via a
modified AED and children who had arrhythmias captured on paper
and digitized for subsequent analysis. The rhythms were divided
into 5-second strips, classified by 3 reviewers, and then assessed by
the AED analysis algorithm. A total of 696 five-second rhythm
strips from 191 children (81 female and 110 male) aged 1 day to 12
years (median 3.0 years) were analyzed. There was 100%
specificity for nonshockable rhythms. Sensitivity for
ventricular fibrillation was
96%.
ConclusionsThere was excellent AED rhythm analysis sensitivity and specificity in all age groups for ventricular fibrillation and nonshockable rhythms. The high specificity and sensitivity indicate that there is a very low risk of an inappropriate shock and that the AED correctly identifies shockable rhythms, making the algorithm both safe and effective for children.
Key Words: defibrillation pediatrics arrhythmia
| Introduction |
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The algorithms in use with current AEDs were derived by using rhythm databases recorded from adults. Children differ from adults as to the types and characteristics of shockable and nonshockable rhythms. The lower incidence of VF indicates that they are more likely to have nonshockable rhythms than are adults. The characteristics of these nonshockable rhythms will be different because children have faster sinus and supraventricular tachycardia rates than do adults. Theoretical concerns about the capacity of the AED to detect VF in pediatric patients exist because of the smaller cardiac mass in children. Although AEDs have not been fully tested in children, the available data suggest excellent specificity.6 7
The purpose of the present study was to create a database of recordings of shockable and nonshockable rhythms from children. This database was used to test an AED patient analysis system for accuracy in determining a shock decision for pediatric rhythms.
| Methods |
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12 years. First, we performed a
prospective clinical study in which rhythms were recorded via a
modified AED in children at risk of arrhythmias. Second,
prerecorded ECG strips of infrequently observed shockable
arrhythmias were digitized for subsequent
analysis.
Recorded Rhythms
Children at risk for developing arrhythmias
were enrolled prospectively at 4 pediatric care centers (Childrens
Hospital and Medical Center, Seattle, Wash; Childrens Hospital of
Iowa, Iowa City; Childrens Hospital, Boston, Mass; and Childrens
Hospital and Health Center, San Diego, Calif). Institutional review
board approval was obtained from each institution in addition to
informed assent and consent from each study participant and parent.
Rhythms were recorded in the following settings: electrophysiology
laboratory, intensive care unit, and cardiac surgical operating
room.
An AED (ForeRunner, Agilent Heartstream) was modified to function as a 30-minute loop recorder with a wide bandwidth (0.2 to 80 Hz) ECG recording system identical to a fully functioning AED and similar to a standard 12-lead ECG. The defibrillation capability of the device was disabled. Defibrillation pads were used to record the rhythms. Pad size depended on the childs chest size and clinical setting; either the standard 100-cm2 adult size (DP1, Agilent Heartstream) or a smaller 43-cm2 pediatric version (M3717A, modified to connect to a ForeRunner, Agilent Heartstream) was used. Pad position was determined by the clinical setting, with the preferred pad position being anterior-anterior. The other pad position was anterior-posterior, and some patients required a more side-to-side configuration. In some instances, monitoring electrodes were used if it was not possible to apply defibrillation pads. Some of the recorded supraventricular and ventricular tachycardia (VT) rhythms were paced rhythms, and the pacing artifact was filtered for the reviewers. Paced rhythms were 5% of the total, and 59% of these were in the unspecified VT group.
Digitized Rhythms
VF and VT paper recordings were acquired
retrospectively from 11 centers via solicitation through letters mailed
to a registry of pediatric electrophysiologists. Recordings
came from both in-hospital and out-of-hospital sources and were
converted into a digital format by scanning, image manipulation, and
data processing.
Rhythm Strip Classification
AED algorithm performance was evaluated for
both sensitivity and specificity. Sensitivity refers to the ability of
the device to detect shockable rhythms. Specificity refers to the
ability of the device to detect nonshockable rhythms.
The American Heart Association (AHA) 1997 recommendations for classification and performance goals were used.8 This is intended for the assessment of AEDs developed for adults. No AED standards are available for children. Rhythm groups are organized in 3 broad categories that are based on the likely benefit of defibrillation for that rhythm group: (1) Shockable rhythms are lethal rhythms unless a shock is delivered very quickly. (2) Intermediate rhythms are those for which the benefits of immediate electric countershock are limited or uncertain. No performance goals have been established for this category. (3) The final category is nonshockable rhythms, which are benign (or normal) rhythms that must not be shocked, especially in children who have a pulse, because no benefit will follow and deterioration in rhythm may result. To maximize safety in the event of misapplication of the device/electrodes, asystole is included in this group. The following definitions refer to a 5-second ECG strip.
Shockable Rhythms
For VF, complexes show only ventricular
origin and rapidly changing morphology. The amplitude is
200
µVpp for
5 of the complexes, and there are
12 complexes
100 µVpp (peak to peak).
Rapid VT involves polymorphic VT and
ventricular flutter with rates
250
bpm.
Intermediate Rhythms
For intermediate VT, complexes show only
ventricular origin but do not satisfy the criteria for
rapid VT.
Low rate/amplitude VF includes low-rate or low-amplitude VF or electrical activity of unknown etiology. The rhythm does not satisfy criteria for asystole, VF, or idioventricular classes.
Nonshockable Rhythms
For sinus rhythm, complexes show an atrial origin and
do not qualify for supraventricular arrhythmia
(SVA) class.
For SVA, complexes show a supraventricular origin with or without atrioventricular block and bundle-branch block. This includes atrial flutter and AF, sinus arrhythmia with or without premature atrial contractions, junctional rhythms, and supraventricular tachycardia.
Ventricular ectopic beats are defined as single or multiple ventricular ectopic beats mixed with or without supraventricular ectopic beats.
For idioventricular rhythms, complexes are only
of ventricular origin, with or without uniform morphology.
The rate is <100 bpm, with at least 1 complex of
100
µVpp.
Asystole is defined as a maximum of 1 complex >100 µVpp and all complexes <200 µVpp.
Classification Process
The recordings were divided into 5-second
segments, classified by 3 pediatric electrophysiologists (F.C., J.C.P.,
D.L.A.) and by the AED analysis algorithm. The reviewers used
the following assumptions: (1) The patient is "unresponsive." (2)
The age of the patient is unknown. (3) The patient may or may not have
a pulse. (4) The AED cannot deliver a synchronized shock. Each reviewer
independently reviewed the segments, identified the rhythm class, and
made a shock or no-shock recommendation. The recommendations of the 3
reviewers were compiled, and disagreements were resolved for rhythms
that were not unanimously placed into a defined category. The results
reported in the present study reflect the final consensus of the
reviewers after discussion of the merit of each potential
recommendation.
The AED patient analysis system characterizes the ECG in terms of 4 rhythmic characteristics: the rate of ECG complex occurrence, morphological stability of the ECG complexes, evidence of rapidly conducted electrical signals, and signal amplitude. These characteristics are respectively referred to as rate, stability, conduction, and amplitude. Stability and conduction are measured on a scale of 0 to 1. Higher conduction and stability scores indicate more rapid conduction and less variability in the morphology of the ECG complexes. To optimize the robustness of the analytical system, redundant assessments of the ECG measurements are performed with the use of both temporal and transform-based analyses. Amplitude measurement is used only for identifying asystole. Rate, stability, and conduction measures are assessed concurrently to make a shock/no-shock determination. Each measure exerts an influence over the decision, but none is independently capable of triggering a shock recommendation. This synergistic use of the rhythm characteristics ensures that a high rate rhythm will not cause a shock recommendation if evidence of supraventricular origin is present and that a lower VF rate will receive a shock recommendation if it demonstrates poor stability and conduction properties.
Statistical Analysis
Data are expressed as median (range) or mean±SD.
With consensus from a panel of 3 electrophysiologists as the gold
standard, the sensitivity and specificity of the algorithm for
detecting shockable rhythms were calculated. Comparisons were made by
the Student t test and
2 test. A value of
P<0.05 was considered
significant.
| Results |
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Digitized Rhythms
Data were digitized from 57 children, and reviewers
classified a total of 82 rhythm strips.
All Rhythms
The children were divided into 3 groups according to
age:
1 year, >1 year to <8 years, and
8 years to
12 years. The
characteristics for the children are displayed in
Table 1
. A total of 696 rhythms were classified by the
reviewers and were subjected to algorithm analysis. There were
463 nonshockable, 131 shockable, and 102 intermediate rhythms.
Nonshockable rhythms constituted 67% of the total.
Table 2
summarizes the shockable rhythms, and
Table 3
summarizes the nonshockable rhythms. Sinus rhythm
was the most frequent nonshockable rhythm, at 37%. VF (n=73) was the
most common shockable rhythm, at 56%. The largest percentage of VF,
52%, was recorded in those aged <1 year. The low-amplitude VF and
shockable unspecified VT had the lowest percentages, 5% and <1%,
respectively.
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Sensitivity and Specificity of Rhythms
The sensitivity and specificity of the AED
analysis algorithm for the 3 age groups are shown in
Tables 2
and 3
. The overall sensitivity, specificity, 90%
one-sided lower confidence limits, and AHA performance goals
for each rhythm classification are displayed in
Table 4
. Specificity for nonshockable rhythms was 100%.
Sensitivity for the shockable rhythms was highest for VF, at 96%. The
overall accuracy was 97%. The AED analysis algorithm exceeded
the AHA performance goals for each rhythm classification except
for rapid VT. Intermediate rhythms (for which the benefits of
defibrillation are limited or uncertain and there are no
performance goals) had a sensitivity of 45% and specificity of
97%. There was no significant difference in sensitivity or specificity
between the 3 age groups.
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The sensitivity results for the rapid VT group were
further explored by examining the AED algorithm classification
parameters: stability, conduction, and rate. The mean
conduction scores for the 41 shock and 17 no-shock designations were
0.55±0.12 and 0.91±0.06, respectively
(P<0.001). The stability
scores were 0.48±0.28 and 0.94±0.03, respectively
(P<0.001). The rates were
288±74 and 261±28, respectively
(P=0.16).
Figure 1
illustrates 2 rhythms that were classified as rapid
VT by the reviewers.
Figure 1A
shows a high conduction and stability score (0.94
and 0.96, respectively) and was given a no-shock designation by the
analysis algorithm.
Figure 1B
shows a low conduction and stability score (0.52
and 0.18, respectively) and was given a shock
designation.
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Adult Versus Pediatric Rhythm
Characteristics
The pediatric ECG database was compared with a
previously collected database of adult
rhythms.9 The rhythm
characteristics (rate, stability, and conduction), as determined by the
algorithm, were compared between the databases.
Figure 2
demonstrates the rhythm characteristics for the
shockable rhythm groups, VF and rapid VT. The pediatric VF (n=73) had a
mean rate of 323±95 bpm. This was significantly higher than the adult
VF (n=300) rate of 289±71 bpm
(P<0.001). Similarly, VT rates
were significantly higher for pediatric subjects (n=58, 281±65 bpm)
than for adult subjects (n=100, 221±59 bpm)
(P<0.001). Conduction scores
were higher (P<0.001) for the
pediatric database in both shockable rhythm groups. Stability scores
were not statistically different between the 2 databases.
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The pediatric nonshockable rhythm groups, ie, sinus rhythm,
SVA, and ventricular ectopic beats, were also compared with
the adult database. These groups had the following overall mean rates
for the pediatric (n=384) and adult (n=500) databases: 129±57 and
87±46 bpm, respectively
(P<0.001). The average rate
for these rhythm groups in the pediatric database was fastest in the
youngest age group, at 136 bpm, and slowest for the oldest age group,
at 107 bpm. Of these 384 nonshockable rhythms, the rate was >180 bpm
in 70 (18.2%) rhythms, with a maximum of 300 bpm. In the comparable
adult database, there were 500 rhythms. The maximal rate was 250 bpm,
and 7% were >180 bpm.
Figure 3
shows the distribution of the rates of the
nonshockable rhythms in these groups.
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| Discussion |
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Prior Studies
There are minimal data evaluating arrhythmia
analysis or AED use in pediatric patients. Hazinski et
al6 presented data
demonstrating high sensitivity and specificity in 21 hospitalized
infants and children aged <8 years. Atkins et
al7 reported a sensitivity of
88% and specificity of 100% when AEDs were used in older children
during out-of-hospital cardiac arrest and resuscitation. Atkins et al
further reported that there were 3 of 25 instances in which VF was
initially not recognized, but the second analysis was correct,
and shocks were delivered. In addition, 43% survival was observed in
patients who received an electric countershock as opposed to 11%
survival in those who had a nonshockable life-threatening rhythm, such
as asystole. This emphasizes the importance of early recognition of VF
followed by defibrillation in the young.
In adults, studies using prerecorded and field-tested data- bases9 10 11 12 13 14 15 16 have reported sensitivity for VF (81% to 100%) similar to that obtained in the present study. Most studies have not separated VT into a separate category, but in the study that did, there was sensitivity of 65% for sustained VT recorded in the electrophysiology laboratory.16 To set standards for specifying and reporting the performance of the arrhythmia analysis algorithm, the AHA published recommendations in 1995.8 The present study is the first to incorporate those recommendations regarding the performance of the arrhythmia analysis algorithm into AED testing for pediatric patients.
Pediatric Database Creation and Algorithm
Development
Development of a pediatric database for algorithm
testing is essential to ensure adequate safety and efficacy of AEDs in
a pediatric population. Our rhythm database collected from children was
clearly different from the adult database. Children had higher heart
rates for shockable VF and rapid VT. Importantly, the nonshockable
rhythm group contained rhythms with faster maximal and overall rates.
The maximal rate in the pediatric supraventricular
tachycardia group was 300 versus 250 bpm in the adult
database. Garson et al17
reported that the overall mean rate of pediatric
supraventricular tachycardia was 240 bpm, and
for infants aged <4 months, it was 268 bpm.
The pediatric population had higher conduction scores for
the VF and rapid VT group than did the adult population. This is
consistent with large 12-lead ECG recording studies in
normal children.18 Heart
rate decreases and QRS duration increases during childhood. The lower
sensitivity found in the rapid VT group can potentially be explained by
these differences between adults and children. The AED algorithm
assigned no-shock designations to the most stable and well-conducting
episodes of rapid VT, as seen in
Figure 1
. These rhythms are rare in children and may not be
clinically relevant in pediatric victims of cardiac
arrest.19
The present data illustrate the importance of using multiple parameters in a rhythm-detection algorithm for shock designation in children. The analysis algorithm of the Agilent Heartstream FR2 AED uses the 4 rhythm characteristics (rate, conduction, stability, and amplitude) as covariables in determining whether a particular rhythm is shockable. The large number of nonshockable pediatric rhythms with rates >180 bpm indicates that simplistic algorithms with shock criteria merely based on rate would be unsafe if used in children. The ability of the algorithm to evaluate conduction and stability in addition to rate reduces the potential for inappropriate shock recommendations for rhythms that are simply faster. This emphasizes the importance of testing each AED manufacturers algorithm in a distinct pediatric database. That database should contain multiple age groups, especially infants aged <1 year. Shockable and nonshockable rhythms should be represented in each age group. Those nonshockable rhythms should include rhythms with rates >250 bpm, with at least 10% having rates >180 bpm.
Study Limitations
The major limitation of the present study is the
lack of wide-bandwidth recordings of spontaneous or out-of
hospital VF. However, obtaining wide-bandwidth recordings of
spontaneous shockable ventricular arrhythmias from
children is extremely difficult. These events occur so infrequently
that thousands of hours of recording are required to capture a
single event. Each of the surgical patients, a high-risk group for
ventricular arrhythmias, had recording
continued for 12 hours after surgery. However, no shockable rhythms
were recorded in the postoperative period.
Field testing is essential ultimately for assessing performance of the AED arrhythmia algorithm. Weaver et al20 demonstrated that an algorithm derived from a prerecorded rhythm database performed poorly in field testing. Despite the fact that the algorithm used in the present study has been field-tested in adults and did very well,14 a postmarket surveillance study is required to assess its performance in children. The present study examined the performance of the arrhythmia analysis system and did not address the issue of appropriate energy dosage for children.
Conclusions
A pediatric database of shockable and nonshockable
rhythms that was significantly different from an adult database was
created. This pediatric database was used to test the Agilent
Heartstream FR2 AED analysis algorithm. Excellent sensitivity
and specificity in all age groups for VF and nonshockable rhythms were
demonstrated. The high sensitivity to VF and the high specificity to
nonshockable rhythms indicate that the analysis algorithm is
both safe and effective for pediatric rhythms. Importantly, the
analysis algorithm is unlikely to inappropriately shock a
pediatric rhythm. These results indicate that the use of 1 algorithm
for both adults and children is
feasible.
| Acknowledgments |
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| Footnotes |
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Received November 8, 2000; revision received February 28, 2001; accepted March 8, 2001.
| References |
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