Automated External Defibrillators: Design ...

VOL. 5, No. 2 Printed in U.SA.

NEW HORIZONS

Copyright © 1997 by the Society of Critieal Care Medieine

Automated External Defibrillators: Design Considerations Mark W. Kroll, PhD; James E. Brewer, MB

transthoracic model incorporates elements into a cell response model that extends it to external defibrillation. External waveform design principles demonstrate reductions in capacitance, voltage, duration, and delivered energy. Therefore, design principles based on cardiac electrophysiology may provide a means to significantly reduce the energy required for safe and efficacious external defibrillation. Footnotes, formulae, and figures augment this presentation in order to clarify the defibrillation waveform theory. (New Horiz 1997; 5:128-136) KEY WORDS: defibrillator; implantable; transthoracic; monophasic; biphasic; waveform

Biphasic defibrillation waveforms are now the standard of care in clinical use for defibrillation with implantable cardioverterdefibrillators (ICDs), due to the superior performance demonstrated over that of comparable monophasic waveforms. To better understand these significantly different outcomes, ICD research has developed cardiac cell response models to defibrillation. Waveform design criteria have been derived from these first principles and have been applied to monophasic and biphasic waveforms to optimize their parameters. These principles-based design criteria have produced significant improvements over the current art of waveforms. Monophasic defibrillation waveforms remain the standard of care in clinical use for transthoracic defibrillation. Waveform design has not yet been influenced by the important gains made in ICD research. The limitations of present transthoracic waveforms may be due in part to a lack of application of these design principles to determine optimal waveform characteristics. To overcome these limitations, design principles based on cell response have recently been developed for external defibrillation waveforms. The

INTRODUCTION The foeus of this presentation at the Wolf Creek IV eonferenee is design eonsiderations for. the automated external defibrillator (AED). The impaet of monophasie and biphasie waveform design on the effieacy of the AED is explored by the author. Major discussants in the subsequent panel include G. Guy Knickerboeker, PhD; Mark C. Vasu, MD; Byron L. Gilman, MS; Gad Amith, EMT-P; Carlton Morgan, MSEE, PE; Max Harry Weil, MD, PhD, FCCM; and Mark W. Kroll, PhD. A substantial amount of research in fibrillation and the therapy of defibrillation has been done sinee the introduction ofthe first commercially available defibrillators (1-5). Much of the most recent research has concentrated on understanding the effeets that a defibrillation shock pulse has on fibrillation to terminate such a eondition (6-8). Over time, researchers (9, 10) found monophasie and biphasic waveforms that worked well(particularly in implantable cardioverter-defibrillators [ICDs]), but much of our present understanding is empirically based. Several eompeting hypotheses have been proposed to explain how a shoek pulse terminates fibrillation (11-14). These hypotheses are

TABLE OF ABBREVIATIONS

AED BLS

reD 1 2

automated external defibrillator basic lifesupport implantable cardioverter-defibrillator phase 1 of a biphasic waveform phase 2 of a biphasic waveform

From Paeesetter, !ne., aSt. Jude Medieal Company, Sylmar, CA; and SurVivaLink®, Minneapolis, MN. Address requests for reprints to: Mark W. Kroll, PhD, Paeesetter, Ine., ASt. Jude Medieal Company, 15900 Valley View Court, PO Box 9221, Sylmar, CA 91392-9221. . 1063-7389/97/0502-128$03.00/0

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AUTOMATED EXTERNAL DEFIBRILLATORS: DESIGN CONSIDERATIONS

based on the interpretation of experimental data, and no single hypothesis has adequately explained the observed mechanism of defibrillation. It would appear that these propos'ed hypotheses will require a careful synthesis to provide a complete understanding.

drew their hypotheses, and so they were, therefore, unable to explain why the different waveforms had such a wide varlety of effectiveness. In the study, many waveform parameters were changed from one waveform to the next without a clear understanding as to how the parameters related to one another. In fact, Jones (15) cites this particular research as an example of why it seems so difficult to determine mechanisms underlying improving defibrillation therapy through threshold reduction. In the past 5 yrs, new research in ICD therapy has developed and demonstrated defibrillation models that provide waveform design rules from first principles. These defibrillation models and their associated design rules for the development of defibrillation waveforms were first developed by Kroll (18) and Irnich (19) for monophasic waveforms using effective and rheobase current concepts. Subsequently, Kroll (20), Walcott and associates (21), Cleland (22), and others developed the passive cardiac cell membrane response model for monophasic and biphasic waveforms, herein called the "cell response model" (Table 2). A significant increase in our understanding of waveform design has occurred and substantial improvements have been made by using these newly developed design principles. Block and Breithardt (23) have recently written a comprehensive survey of the new principles-based theories and their impact on optimizing internal defibrillation. For example, these defibrillation models are largely responsible for our newly acquired understanding that capacitor size is an important design parameter (25-27), and that capacitor sizes presently used in a majority of ICDs are larger than needed for optimal defibrillation (28). The well known Weiss-Lapicque strength-duration relation is determined by stimulating excitable tissue with a constant current pulse. For a pulse duration t and stimulation current I, the strengthduration relation may be expressed by equation 1 (Fig. 1), where Ir is the rheobase and t c is the chronaxie. The Weiss-Lapicque strength-duration relation was analyzed to optimize the performance of a monophasic shock pulse derived from the capacitive discharge of an ICD (18). At the defibrillation threshold, I r may be interpreted as the minimum. current ,. needed to defibrillate with an infinite duration pulse, and we defined an ICD's effective current I e to be the maximum rheobase current that a given pulse can satisfy. Effective current is to the rheobase what ICD shock energy is to the heart's defibrillation

DISCUSSION Dr. Kroll. Many rules-of-thumb were developed to design waveforms, and have achieved dogmatic rather than scientific acceptance. An example is the rule-of-thumb that the duration ofphase 2 (2) should always be equal to or less than the duration of phase 1 (