(Circulation. 1997;95:1677-1682.)
© 1997 American Heart Association, Inc.
Articles |
Key Words: diagnosis cardiopulmonary resuscitation AHA Medical/Scientific Statements defibrillation myocardial infarction
| Introduction |
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A logical extension of the AED concept is "public access defibrillation" or widespread distribution and use of AEDs by nonmedical, minimally trained personnel (eg, security guards, spouses of cardiac patients).2 Public access defibrillation poses unique challenges. AEDs must be simple to operate, because in many cases the operator is a first-time user with minimal training. The device must accurately diagnose lethal arrhythmias under unfavorable conditions that may degrade performance. It could be misused, either inadvertently (eg, the patient is conscious and breathing) or deliberately. Safety must be emphasized, and the risk of injury to patient and rescuer minimized. An existing standard for AED construction and performance recognizes the challenges inherent in the various potential uses of AEDs.3
| Purpose |
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The accuracy of the arrhythmia analysis algorithms incorporated into AEDs
New or alternative defibrillation techniques, especially waveforms
The safety of AEDs when used by minimally trained lay rescuers (public access defibrillation).
This is a consensus document, reflecting the views of the members of the American Heart Association Task Force on Automatic External Defibrillation, its Subcommittee on AED Safety and Efficacy, and the AED Manufacturers' Panel. This document is intended to supplement existing documents concerning AEDs, such as ANSI/Association for the Advancement of Medical Instrumentation (AAMI) DF39,3 the AHA Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiac Care,1 and the AHA Textbook of Advanced Cardiac Life Support.4 All AEDs, whether public access or not, should meet similar algorithm performance specifications.
| Demonstrating Accuracy of the Arrhythmia Analysis Algorithm |
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Shockable rhythms: lethal rhythms that terminate in the patient's death unless defibrillation is delivered very quickly. These rhythms include coarse ventricular fibrillation (VF) and rapid ventricular tachycardia (VT) and are always (VF) or almost always (rapid VT) associated with a pulseless, unresponsive patient.
Nonshockable rhythms: benign (or even normal) rhythms that must not be shocked, especially in patients with a pulse, because no benefit will follow and deterioration in rhythm may result. Nonshockable rhythms include normal sinus rhythm, supraventricular tachycardias, sinus bradycardia, atrial fibrillation and flutter, heart block, idioventricular rhythms, premature ventricular contractions, and other rhythms accompanied by a palpable pulse and/or occurring in a conscious patient. To maximize safety in the event of misapplication of the device/electrodes, asystole is included in this group. The AHA Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiac Care discourage shocks for asystole.1
Intermediate rhythms: Rhythms for which the benefits of defibrillation are limited or uncertain. These include fine VF (associated with pulselessness and low survival rates) and VT that does not meet all criteria for inclusion in the shockable VT rhythm category.
Various cardiac rhythms are categorized in Table 1
.
Reporting Algorithm Performance
The task force divided arrhythmias into three categories:
shockable, nonshockable, and intermediate (Table 2
).
Patients with shockable rhythms (VF, rapid VT)
potentially receive the greatest benefit (survival) from defibrillation
at essentially no risk. High sensitivity for AED analysis
(Table 3
) is required for this group. Patients with
nonshockable rhythms derive no benefit from defibrillation and are at
maximum risk. For reasons of safety, asystole is included in this
group. High specificity is required. Patients with
intermediate rhythms are unlikely to derive benefit or be at risk from
defibrillation, making performance requirements inappropriate.
Reporting arrhythmia analysis algorithm specificity or sensitivity is
sufficient for this group.
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Table 2
shows desired performance goals for each rhythm category. These
goals reflect a consensus among the participants on ANSI/AAMI (DF39)
standards.3
Performance during developmental testing is an indication of what to
expect during validation. During developmental testing of automatic
rhythm analysis systems, the performance goal should be met or
exceeded. This maximizes chances of equaling or exceeding the goal
during validation testing, which should be performed with at least the
minimum sample size per category given in Table 2
. (A sample consists
of data required to make a single shock/no-shock decision.) The sizes
selected in Table 2
reflect a balance between reasonable confidence in
performance and realistic limits on data available to demonstrate it.
These are minimum sample sizes and may be exceeded. Data may be
acquired from prehospital or in-hospital events. The size and geometry
of the electrodes used to acquire the data should be reported.
For each category, the observed test results must equal or exceed the performance goal. For each rhythm category, the exact single-sided 90% lower confidence limit should be calculated, based on test results. This process will give a 90% probability that the actual performance is greater than the lower confidence limit calculated.
Table 2
provides an example of calculation of lower confidence limit
for observed performance equal to performance goals for each rhythm
with specified performance goals.
Differences in ECG data acquisition preclude the development of a
common (single) database against which every AED arrhythmia analysis
algorithm could be tested. Therefore, the task force recommends that
AED manufacturers report the performance of arrhythmia analysis
algorithms of their own devices to the Food and Drug Administration
(FDA), using the format in Table 2
. Appropriate electronic and/or hard
copy documentation should be available for inspection on
request.
Validating Performance
The data used for algorithm development must be different
from the data used for testing and validation. Validation of
performance should be obtained in both the presence and absence of
artifacts likely to be encountered in field use.
The signal characteristics of the data acquisition system used to gather the validation data set should be specified (bandwidth, phase characteristics, dynamic range).
The waveforms may include a discharge deflection and a postdischarge recovery period, making the timing of playback into a separate device critical, so that the device being tested is not required to analyze these discharge artifacts introduced during recording. If additional non-ECG signals are used (eg, respirometer, impedance detector), their acquisition characteristics should also be specified.
Algorithms may examine different rhythms recorded from the same patient. However, there can be only one sample of each specific rhythm from each patient.
Because many ECG rhythm segments may be classified differently by different physicians, the task force recommends that classification of segments as shockable, nonshockable, or intermediate require agreement among at least three qualified expert reviewers of cardiac arrest rhythms. Rhythm segments on which reviewers fail to reach 100% agreement can be classified, but the expert disagreement should be reported. The reviewers should use ECG criteria on which they have previously agreed. They should reach a consensus on the distinction between fine VF (an intermediate rhythm that should be shocked) and asystole (which should not be shocked) by employing the same criteria used by the AED being tested.
Effects of Artifacts
In real world situations in which AEDs are used, it is
inevitable that artifacts will corrupt ECG data to varying degrees,
potentially degrading specificity and sensitivity. Manufacturers should
determine the effects of various artifacts, with emphasis on diagnosis
of shockable and nonshockable rhythms. The effect of artifacts on
diagnosis of intermediate rhythms is of less concern.
The most commonly encountered artifacts are motion artifacts, which are typically generated by cardiopulmonary resuscitation, agonal breathing or seizures, handling of the patient, and transport by stretcher and vehicle. Pacemaker stimuli can also interfere with algorithm performance. Static electric fields (commonly present in dry environments) exacerbate these artifacts.
Because there is no standard database of ECG signals or noise signals for testing AED algorithms, manufacturers should determine how to test their devices for reasonable performance in the presence of noise and specify in detail how this testing was done.
| Alternative Waveforms for Defibrillation |
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At present there are no published data on prehospital transthoracic defibrillation using alternative waveforms. The absolute success rate of any waveform for termination of VF will be lower in the prehospital setting (in which VF is often present for a prolonged period before shocks are administered) than in the rapid-shock environment of the electrophysiology laboratory or the coronary/intensive care unit. However, there is no a priori reason to suspect that the relative advantage of alternative waveforms over monophasic waveforms will not be maintained in prehospital use if such an advantage is demonstrated in hospital. In fact, in vitro studies by Jones et al11 and a study of intact dogs by Walcott et al12 have suggested that the superiority of biphasic waveforms may actually be increased over monophasic waveforms when shocks are delivered after longer durations of VF.
It is the consensus of the task force (with the exception of one manufacturer) that if alternative waveforms for transthoracic defibrillation are convincingly demonstrated to be equivalent or superior to standard waveforms in the electrophysiology laboratory or other hospital or prehospital settings, they should be provisionally approved for use in AEDs, pending acquisition of prehospital data. Performance of waveforms tested in the electrophysiology laboratory or other in-hospital or prehospital settings and incorporated into AEDs should be monitored as part of a postmarket surveillance program designed to carefully observe total system performance of these devices in their intended settings.
The task force recommends the following as a minimum standard for
demonstrating equivalence of an alternative waveform versus standard
waveforms: the upper boundary of the 90% confidence interval (with 5%
in each tail) of the difference between standard and alternative
waveform efficacy must be
10%, which permits a slight (5%) chance
of acceptance of a waveform that is >10% less effective than the
standard waveform. Similarly, the task force suggests that to
demonstrate superiority of an alternative waveform over standard
waveforms, the upper boundary of the 90% confidence interval of the
difference between standard and alternative waveforms must be <0%
(ie, alternative is greater than standard). If the standard waveform
efficacy equals 90%, and the true (or hypothesized) alternative
waveform efficacy is 95%, approximately 52 patients per group would be
required to demonstrate equivalence, and 471 patients per group would
be required to demonstrate superiority with a power of 0.9. These
sample sizes are based on statistical tests of equivalence of new
treatments described by Blackwelder.13
| Postmarket Surveillance |
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| Enhancing Safety |
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To overcome these potential problems, the task force recommends that AEDs be specifically designed to prevent injury in the event of misuse. Innovative features that enhance safety are encouraged, such as voice chips that deliver a series of prompts to a rescuer who is opening or activating an AED (eg, "Shake the victim. If he or she groans or moves, do not attach the electrodescall the emergency number."). Alternatively, after the arrhythmia analysis algorithm has been satisfied, an AED might administer an unpleasant but low-strength "wake-up" shock; if the algorithm diagnosis was incorrect and the patient was not in cardiac arrest but merely in a deep sleep or intoxicated, such a preliminary shock would stimulate the patient to move or respond, alerting the rescuer not to deliver a defibrillation-strength shock. These suggestions are intended as examples only; other innovations/approaches may be even more effective.
The task force also encourages the design of devices that enhance rapid and effective deployment in conjunction with local emergency medical services, integrating AEDs into the AHA chain of survival. This can be accomplished through advanced communication technology. For example, AEDs could be designed to automatically activate the local emergency medical services system when the device is removed from its holder or its cover is opened. Other approaches and innovations may be even more effective.
| Summary |
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| Acknowledgments |
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| Footnotes |
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A single reprint is available by calling 800-242-8721 (US only) or writing the American Heart Association, Public Information, 7272 Greenville Avenue, Dallas, TX 75231-4596. Ask for reprint No. 71-0104. To purchase additional reprints: up to 999 copies, call 800-611-6083 (US only) or fax 413-665-2671; 1000 or more copies, call 214-706-1466, fax 214-691-6342, or To make photocopies for personal or educational use, call the Copyright Clearance Center, 508-750-8400.
| Appendix 1 |
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Richard E. Kerber, MD
R. Pat McGoldrick
Arthur J. Moss, MD
Graham Nichol, MD
Joseph P. Ornato, MD
David G. Palmer, Esq
Barbara Riegel, DNSc
Sidney C. Smith, Jr, MD
AED Safety and Efficacy Subcommittee
Richard E. Kerber, MD, Chair
Lance B. Becker, MD
Joseph D. Bourland, EE, PhD
Richard O. Cummins, MD, MPH
Bram D. Zuckerman, MD
Mary B. Michos, RN, Fire Chief
Joseph P. Ornato, MD
Roger D. White, MD
AED Research Subcommittee
Joseph P. Ornato, MD, Cochair
Barbara Riegel, DNSc, Cochair
Alfred P. Hallstrom, PhD
Graham Nichol, MD
AED Manufacturers Panel
Carlton B. Morgan, Heartstream, Inc.
William L. Post, Hewlett-Packard Company
John E. Kuphal, Laerdal Medical Corporation
Donald E. Brodnick, Marquette Electronics, Inc.
Robert A. Niskanen, Physio-Control Corporation
Kenneth F. Olson, SurVivaLink Corporation
Gary A. Freeman, Zoll Medical Corporation
| References |
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