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Circulation. 1996;94:2507-2514

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(Circulation. 1996;94:2507-2514.)
© 1996 American Heart Association, Inc.

Articles

Multicenter Comparison of Truncated Biphasic Shocks and Standard Damped Sine Wave Monophasic Shocks for Transthoracic Ventricular Defibrillation

Gust H. Bardy; Francis E. Marchlinski; Arjun D. Sharma; Seth J. Worley; Richard M. Luceri; Raymond Yee; Blair D. Halperin; Christopher L. Fellows; Thomas S. Ahern; Donald A. Chilson; Douglas L. Packer; David J. Wilber; Thomas A. Mattioni; Ramakota Reddy; Richard A. Kronmal; Ralph Lazzara; for the Transthoracic Investigators*

the Department of Medicine, University of Washington (Seattle).

Correspondence to Gust H. Bardy, MD, Box 356422, University of Washington, University of Washington Medical Center, Seattle, WA 98195-6422. E-mail gbardy@u.washington.edu.


*    Abstract
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 *Abstract
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 down arrowResults
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Background The most important factor for improving out-of-hospital ventricular fibrillation survival rates is early defibrillation. This can be achieved if small, lightweight, inexpensive automatic external defibrillators are widely disseminated. Because automatic external defibrillator size and cost are directly affected by defibrillation waveform shape and because of the favorable experience with truncated biphasic waveforms in implantable cardioverter-defibrillators, we compared the efficacy of a truncated biphasic waveform with that of a standard damped sine monophasic waveform for transthoracic defibrillation.

Methods and Results The principal goal of this multicenter, prospective, randomized, blinded study was to compare the first-shock transthoracic defibrillation efficacy of a 130-J truncated biphasic waveform with that of a standard 200-J monophasic damped sine wave pulse using anterior thoracic pads in the course of implantable cardioverter-defibrillator testing. Pad-pad ECGs were also examined after transthoracic defibrillation. After the elimination of data for 24 patients who did not meet all protocol criteria, the results from 294 patients were analyzed. The 130-J truncated biphasic pulse and the 200-J damped sine wave monophasic pulse resulted in first-shock efficacy rates of 86% and 86%, respectively (P=.97). ST-segment levels measured 10 seconds after the shock in 151 patients in sinus rhythm were -0.26±1.58 and -1.86±1.93 mm for the 130- and 200-J shocks, respectively (P<.0001).

Conclusions We found that 130-J biphasic truncated transthoracic shocks defibrillate as well as the 200-J monophasic damped sine wave shocks that are traditionally used in standard transthoracic defibrillators and result in fewer ECG abnormalities after the shock.


Key Words: defibrillation • electrical stimulation • fibrillation


*    Introduction
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 up arrowAbstract
 *Introduction
down arrowMethods
 down arrowResults
 down arrowDiscussion
 down arrowAppendix: Investigators and...
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In the United States, only a small percentage of the {approx}300 000 annual victims of cardiac arrest survive.1 2 3 4 5 Because of such poor resuscitation results, individual physicians as well as the AHA, the American College of Emergency Physicians, and the European Resuscitation Council have advocated the widespread dissemination of AEDs as a means of improving out-of-hospital resuscitation rates for victims VF.6 7 8 9 10 The AHA has taken the position that every effort should be made to ensure public access to AEDs.11 12

The current use of a high-energy transthoracic shock for ventricular defibrillation actually makes the goal of widespread AED dissemination difficult because the shock waveform and energy dictate AED size, weight, and cost. This is a consequence of using defibrillation waveforms that have remained essentially unchanged since DC transthoracic defibrillators were first developed in the 1960s. If defibrillation energy requirements could be reduced, AED capacitor and battery size could be smaller, and the large, less-reliable high-voltage mechanical switches could be replaced with smaller, more-dependable solid state switches. In addition, the development of more-efficient waveforms would allow the inductor to be eliminated. Each of these factors indirectly facilitates AED dissemination by leading to a decrease in AED size and weight, factors that in turn affect AED portability, maintenance requirements, and cost.

Truncated biphasic waveforms are ideally suited to transforming AED technology and, therefore, out-of-hospital resuscitation efforts. These defibrillation waveforms have had a dramatic impact in the field of ICDs and have improved defibrillation efficiency.13 14 15 16 17 18 Parallel advances in AED technology could have a significant impact on public health. Consequently, in an effort to advance the field of cardiac resuscitation, it was the purpose of this study to examine the efficacy of low-energy truncated biphasic waveforms referenced to a standard high-energy damped sine monophasic waveform for transthoracic defibrillation. To safely evaluate each waveform in a controlled fashion, the transthoracic ventricular defibrillation test protocol was conducted in the course of ICD testing.


*    Methods
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 up arrowAbstract
 up arrowIntroduction
 *Methods
down arrowResults
 down arrowDiscussion
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Patient Population
Informed consent was obtained from each patient before enrollment in this prospective, randomized, blinded multicenter trial comparing low-energy truncated biphasic pulses and high-energy damped sine wave monophasic pulses for transthoracic ventricular defibrillation. The study was performed during transvenous ICD surgery or testing in patients who previously survived a sustained ventricular tachyarrhythmia cardiac arrest. Patients were eligible for study if they were undergoing original transvenous ICD surgery, ICD replacement surgery, or ICD testing. Patients were ineligible for inclusion if they had an epicardial ICD or an ICD system that used an inferolateral left thoracic subcutaneous patch electrode, because these internal electrode systems could interfere with thoracic current distribution during a transthoracic shock. Patients with a body weight of <80 lb or who were receiving an investigational drug were also excluded. A total of 316 patients was recruited from 14 North American centers from November 23, 1994, through October 31, 1995.

Transthoracic Defibrillation Electrode System
Adhesive transthoracic defibrillation pad electrodes with an active surface area of 100 cm2 (model K-defib, Katecho) were applied to the high anterior thorax immediately inferior to the right clavicle in the midclavicular line and to the left lower anterolateral thorax such that the medial margin of the pad was at the midclavicular line and the lateral margin of the pad was at the midaxillary line and centered over the inferior margin of the rib cage. To avoid short circuits, care was taken to ensure that no defibrillation pad lay on or immediately adjacent to any monitoring electrodes. The defibrillation pad electrodes were then connected to a switch system that allowed delivery of either truncated biphasic pulses or standard damped sine wave monophasic pulses.

Study Goals, Sample Size, and Randomization Procedure
The primary hypothesis of this study was that a 130-J truncated biphasic pulse would have the same defibrillation success rate as a standard 200-J damped sine wave monophasic pulse. The 130-J truncated biphasic pulse was selected for study because it had been shown in a detailed animal and a human pilot study of biphasic defibrillation19 20 to be equally effective as a standard 200-J damped sine wave monophasic pulse. The study population size that was needed to address the hypothesis was based on the assumption that the probability of defibrillation for the control waveform (200-J damped sine wave) would be >90% and that the lower limit of the confidence interval of the probability of defibrillation for the investigational waveform would be >=80%. With these assumptions, we calculated a minimum sample size of 157 defibrillation attempts for each waveform to reach a power of 80% and a significance level of 0.05 to detect a >10% difference between the 130-J truncated biphasic and the 200-J damped sine waveforms.

A secondary aim of the study was to demonstrate a margin of safety for the 130-J pulse by evaluating an even lower biphasic shock energy, 115 J, {approx}10% less than the biphasic shock strength of primary interest. AEDs in widespread use may expect long intervals between maintenance. In worst-case scenarios, nominally 130-J AEDs could vary in their output by as much as 10%. Therefore, 115-J was selected to ensure that low-energy biphasic pulses not only are nominally effective but also exhibit an operational margin of safety in practical situations. Another secondary aim of the study was to determine whether there was an advantage in using a 360-J high-energy damped sine wave for ventricular defibrillation given previous findings that suggest that such high-energy shocks have significant clinical disadvantages compared with lower-energy pulses of comparable defibrillation effectiveness.21 22

Each transthoracic test shock was selected via weighted randomization without replacement for each patient. To maximize the value of a limited population of patients and to address the central hypothesis of this study with sufficient power, shock delivery was weighted 2:1 to the delivery of 130-J biphasic and 200-J damped sine wave over 115-J biphasic and 360-J damped sine wave pulses. The secondary aims for the 115-J truncated biphasic and the 360-J monophasic damped sine waveforms were tested to detect a difference of >15% between these pulses and the 130-J biphasic or the 200-J damped sine wave evaluated with 80% power and a significance level of P=.05.

Shock Waveforms
The 115- and 130-J biphasic waveforms were generated with a 95-µF capacitor using a custom external defibrillation system (Heartstream). The waveform parameters of total duration, relative phase duration, and tilt were varied by the device to accommodate variations in transthoracic impedances.19 Biphasic waveforms for a standard 50-{Omega} load are represented in Fig 1Down. The damped sine transthoracic defibrillation pulses were the standard monophasic 200- and 360-J pulses available with a Hewlett-Packard Codemaster XL defibrillator. Damped sine pulses were generated by delivering the charge stored on a 32-µF capacitor through a 50-mH inductor with a 10-{Omega} resistance. The standard damped sine wave pulses are shown in Fig 1Down as delivered to a 50-{Omega} load. The nominal stored defibrillator values are given in detail in Table 1Down for all four waveforms tested. Delivered voltage waveforms were recorded for subsequent calculation of pulse duration and delivered current, resistance, and energy.



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  		Figure 1. Representative recordings of 95-µF truncated biphasic pulses at 115 and 130 J and of 32-µF, 50-mH damped sine wave pulses at 200 and 360 J. All pulses are delivered into a 50-{Omega} load.


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  		Table 1. Nominal Stored Defibrillator Values

Defibrillation Testing
VF, induced with alternating current, rapid pacing, or low-energy shocks on the T wave,23 was allowed to persist for {approx}10 seconds before the delivery of a transvenous test shock. If the transvenous shock was unsuccessful, one of the four transthoracic test shocks was delivered as soon as it could be confirmed that the episode of VF would not stop spontaneously. Consequently, VF episodes for transthoracic test shocks persisted for {approx}13 to 15 seconds and were preceded by a failed transvenous ICD test shock. If the first transthoracic defibrillation pulse failed, a second transthoracic rescue pulse using a standard 200-J monophasic damped sine wave, or a second transvenous rescue shock was delivered. Only the first transthoracic defibrillation attempts for each VF episode were included in the analysis. The rest time between VF inductions was a minimum of 3 minutes. After each termination of VF and before the next induction, the surface ECG and blood pressure were examined to ensure that the patient was stable before VF was reinitiated. Each transthoracic waveform was tested no more than once per patient. Patients could receive all four transthoracic test shocks depending on transvenous ICD testing procedures and transvenous defibrillation failure rates.

VF was defined as a polymorphic ventricular tachyarrhythmia associated with complete hemodynamic collapse, confirmed through blood pressure monitoring. Defibrillation of VF was defined as successful if the patient was defibrillated on the first shock by restoring a supraventricular, paced, or baseline rhythm within 16 RR intervals after shock. Bradycardia and an idioventricular rhythm after the delivery of a VF rescue shock were also defined as a successful termination of VF. All investigators responsible for determination of shock success or failure were unaware of the waveform delivered or of the study results until completion of the study and tabulation of the final results.

Defibrillation pad ECGs (approximately limb lead II) were recorded before and after delivery of each transthoracic shock and were examined 10 seconds after transthoracic rescue shock delivery for those patients in whom sinus rhythm was present after the delivery of the transthoracic shock. The ECG ST-segment measure was made 80 msec after the J-point with the use of electronic calipers and referenced to the preceding TP segment on the digitized ECG.

Data and Safety Monitoring
The data and safety monitor was responsible for data quality and patient safety throughout this study. The data were analyzed each successive quarter of the study to ensure completeness of the data and that the observed probabilities of defibrillation were clinically acceptable. A waveform at a specified energy was defined to be acceptable if the 95% confidence interval of the observed probability of defibrillation included 80%. Group-sequential one-sided confidence intervals were calculated based on the equivalent group sequential test for the primary hypothesis at each of these interim reviews.24 Investigators remained blinded to efficacy data until study completion.

Statistical Analysis
For the primary hypothesis, the efficacies of 130-J truncated biphasic and 200-J damped sine wave monophasic shocks were compared with the use of a {chi}2 test. The Pearson {chi}2 test was used to compare the efficacy of all four waveforms. To be conservative in accounting for multiple binary comparisons, the Hochberg step-up procedure was then used to determine statistical significance.25

Defibrillation shock characteristics and ECG ST-segment data were analyzed using one-way ANOVA to assess the way in which the mean value was affected by classifications of the data. Comparison of all pairs with Tukey-Kramer HSD was used to determine significant differences at P=.05.26


*    Results
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 up arrowAbstract
 up arrowIntroduction
 up arrowMethods
 *Results
down arrowDiscussion
 down arrowAppendix: Investigators and...
 down arrowReferences
 
Patient Clinical Characteristics
Of the 316 patients enrolled in the study, 294 were eligible for analysis. Of the 22 patients excluded, 16 inadvertently underwent testing with different transthoracic defibrillation pads than those selected for study. These pads had a different surface area (80 cm2), pulsing impedance, and skin interface properties and therefore altered the characteristics of the shocks delivered. Other reasons for exclusion included inadvertent concomitant testing of an investigational ICD (3 patients), inadvertent concomitant testing of an investigational drug (1), inadvertent inclusion of a patient with epicardial defibrillation patches (1), and improper connection of the defibrillation cable (1). Thus, 294 patients were eligible for analysis. In addition, on analysis of the 294 study patients, the results of transthoracic shocks were excluded for 10 episodes (in 10 patients) because the shocks were delivered to monomorphic VT rather than to true VF. These 10 patients had other, true episodes of VF as well. Two patients were mistakenly enrolled twice, on different days; only the first enrollment was included. A total of 513 transthoracic test shocks were analyzed. The average number of test shocks delivered per patient was 1.7.

Of the 294 eligible patients, 229 (78%) were male. The mean weight was 178±35 lb (range, 92 to 317 lb). The mean age was 65±12 years (range, 17 to 87 years). Coronary artery disease was the primary structural heart disease in 246 (84%), 40 (13%) had an idiopathic dilated cardiomyopathy, 9 (3%) had primary electrical disease, 5 (2%) had hypertrophic cardiomyopathy, 5 (2%) had long QT syndrome, and 2 (1%) had right ventricular dysplasia. A few patients had more than one cardiac disease process. The mean left ventricular ejection fraction was 0.34±0.15 (range, 0.09 to 0.80). The index arrhythmia leading to device implantation was VT in 153 patients (52%), both VF and VT in 61 patients (21%), and VF in 80 patients (27%). Antiarrhythmic drugs included amiodarone in 81 patients, ß-blocker in 64, digoxin in 115, diltiazem in 8, disopyramide in 1, flecainide in 1, lidocaine in 2, mexiletine in 5, procainamide in 9, propafenone in 2, quinidine in 4, sotalol in 22, tocainide in 1, and verapamil in 2.

Defibrillation Shock Characteristics and Efficacy Data
The stored and delivered defibrillation data are shown in Fig 2Down and Tables 1 and 2UpDown. Percent defibrillation efficacy and 95% confidence intervals are summarized in Table 3Down. For the 130-J biphasic (n=167) and the 200-J damped sine wave pulse (n=166), the percent defibrillation efficacy was 86% (confidence interval, 81% to 92%) and 86% (confidence interval, 81% to 91%), respectively (P=.97 by {chi}2 analysis). The percent defibrillation efficacy for the 115-J biphasic shock (n=97) was 89% (confidence interval, 82% to 95%). The percent defibrillation efficacy for the 360-J damped sine wave shock (n=83) was 96% (confidence interval, 92% to 100%). Neither the 115-J biphasic nor the 360-J damped sine wave pulse differed statistically from the two principal waveforms under study using the Hochberg procedure.



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  		Figure 2. Stored and delivered energy, stored and delivered voltage, delivered current, and 10 second postshock ECG ST-segment levels for 115- and 130-J truncated biphasic and 200- and 360-J damped sine wave monophasic pulses. P values are for one-way ANOVA.


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  		Table 2. Delivered Waveform Variable


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  		Table 3. Defibrillation Efficacy and Shock Effect

ECG Changes After Shock
Normal sinus rhythm ECGs were available for analysis after shock in 151 patients. At 10 seconds after defibrillation, the ST segment was found to be -0.32±1.10, -0.26±1.58, -1.86±1.93, and -3.25±3.35 mm (10 mm/mV) for the 115-, 130-, 200-, and 360-J shocks, respectively. The high-energy damped sine monophasic waveforms changed ST-segment levels significantly more than did the low-energy truncated biphasic waveforms at the P=.05 level (with comparison of all pairs Tukey-Kramer HSD analysis). The 360-J damped sine wave shock also resulted in a statistically greater ST change than the 200-J shock (Table 2Up and Figs 2 and 3UpDown).



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  		Figure 3. Baseline and postshock ECGs 10 seconds after transthoracic shock delivery in a patient who received all four transthoracic shocks. Note the marked ST-segment depression after delivery of the monophasic damped sine wave shocks with progressive effects after the 360-J shock. P-wave and QRS amplitudes also are depressed after the 360-J shock.


*    Discussion
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 up arrowAbstract
 up arrowIntroduction
 up arrowMethods
 up arrowResults
 *Discussion
down arrowAppendix: Investigators and...
 down arrowReferences
 
Sudden cardiac death resulting from VF remains a major public health problem, with most victims dying before reaching a hospital.27 Survival for VF victims is much more likely if defibrillation therapy is delivered within 6 minutes of collapse.28 29 30 31 When defibrillation occurs within 1 to 2 minutes, as usually occurs in supervised cardiac rehabilitation centers, a total of 90 of 101 victims (89%) have been successfully resuscitated.32 33 34 35 This contrasts sharply with the typical survival rates of <8% in most communities.2 3 4 5 Consequently, widespread dissemination of AEDs to facilitate early defibrillation could substantially improve VF resuscitation rates.6 7 8 9 10 11 12 28 29 30 31 Any reduction in AED size, weight, maintenance requirements, and cost, factors that are related to defibrillation waveform, would, in turn, facilitate AED dissemination.

Impact of Defibrillation Waveform on AED Size, Weight, and Cost
A typical AED that uses a damped sine wave pulse is approximately the size of a briefcase, weighs 15 to 30 lb, and costs $5000 to $9000. Damped sine wave pulses require nearly twice the voltage to defibrillate and waste considerable energy compared with truncated biphasic pulses. The inductor, a component of such transthoracic defibrillators that has never been proved to be necessary, is a substantial burden.36 To deliver a 360-J damped sine wave shock to a 50-{Omega} load, 435 J (5200 V) must be generated in the device. The inductor required to provide the damped sine wave shape accounts, by itself, for 150 mL of volume and 300 g of mass. Furthermore, inefficiencies of the inductor force the capacitor volume to increase to make up for the wasted energy. When a typical inductor (with a resistance of 10 {Omega}) is put in series with a patient with a resistance of 50 {Omega}, 17% of the energy stored in the capacitor is lost within the inductor. This means that 240 J must be stored to deliver a standard 200-J shock to the patient (and 435 J for a 360-J shock), increasing capacitor volume by 17%. Such inefficiencies also contribute to the need for larger batteries and transformers, additional insulation, and large, high-voltage mechanical switching devices.

In the case of a truncated biphasic pulse, 1600 to 1750 V appears to be sufficient for defibrillation. The lower voltage and, consequently, lower energy needs translate to much physically smaller capacitors (despite higher capacitance), smaller batteries, decreased insulation, and smaller cable and transformer size. These changes allow the use of smaller, more-reliable, and less costly solid state switches rather than mechanical ones. Taken together with the removal of the inductor, these factors contribute to a 75% reduction in size and 60% reduction in weight. AED costs fall in turn because lower voltage and energy requirements allow the use of mass produced, yet reliable integrated circuits and small, inexpensive, long-life batteries that do not require daily performance checks. All of these changes can influence the use and availability of AEDs in the general community.

Clinical Advantages of Truncated Biphasic Pulses
There are other advantages that accrue from the use of truncated biphasic waveforms. Weaver et al21 showed in 1982 that the use of a modest defibrillation energy, 175 J, defibrillated equally well as a high-energy, 320 J, shock. In this out-of-hospital comparison with 249 cardiac arrest victims with VF, the 175-J shock defibrillated with fewer complications, such as postshock asystole, bradycardia, and atrioventricular block, than the 320-J shock. Both waveforms were damped sine wave monophasic pulses. Unnecessarily high-energy defibrillation shocks have since been shown to result in more postshock cardiac dysfunction as a result of a direct membrane-depressant effect and electroporation.22 37 38 39 40 41 42 43 This membrane effect may explain why high-energy damped sine wave pulses can result in a higher incidence of myocardial depression and bradyarrhythmias.21 22 44 45

In an earlier, smaller clinical study on transthoracic biphasic defibrillation, ST-segment shifts after shock were shown to be significantly less with a 130-J biphasic shock than with a 200-J damped sine wave shock (0.81 versus 2.10 mm, P<.0001), possibly indicating less cardiac injury.46 The present study confirms these findings and demonstrates a more marked ECG ST-segment depression after the 360-J damped sine wave monophasic shock. Such abnormalities are likely to be magnified in patients receiving multiple shocks.22 It is commonly recommended that 200, 200, and 360 J be delivered in rapid sequence in patients with refractory VF. Even if the patient can be defibrillated, the cardiac dysfunction that follows such a series of higher-energy shocks may diminish the long-term survival prospects of the typical cardiac arrest victim, who is likely to have marginal heart function. It is not uncommon to see ventricular defibrillation followed by asystole in the course of resuscitation.21 30 47 48 The lower the shock strength needed to defibrillate, the lower is the likelihood of postshock asystole and cardiac dysfunction.

It is also worth noting that regardless of the shock strength, biphasic truncated pulses have been shown to limit mechanical dysfunction after shock compared with monophasic pulses. Biphasic waveforms may help heal the cell membrane after shock.37 This effect may be especially important in patients with cardiac dysfunction.

Truncated biphasic waveforms also interfere less with pharmacological resuscitation efforts. In a recent animal study by Ujhelyi et al,49 lidocaine increased monophasic waveform ventricular defibrillation energy requirements by 92%, whereas a biphasic waveform decreased defibrillation energy requirements by 5.7%. Similar findings have been made with amiodarone.50 These observations imply that patients requiring antiarrhythmic drugs might be managed better with biphasic defibrillation.

The longer a patient is in VF, the more difficult defibrillation becomes as cellular hypoxia and acidosis supervene.51 52 However, for reasons that remain unclear, the efficacy of truncated biphasic shocks is less affected by the duration of VF. Animal studies have shown that the longer the VF duration, the greater is the relative superiority of truncated biphasic shocks over monophasic shocks.53 54 This time-dependent advantage of truncated biphasic waveforms favors their use in cardiac arrest patients, who are very likely to have been in VF for many minutes before help arrives.

Finally, biphasic waveform transthoracic defibrillation may offer advantages for patients with antibradycardia pacemakers. Monophasic defibrillation has been shown to lead to postshock failure to capture, presumably due to a shock-related rise in pacing threshold that sometimes persists for as long as 10 minutes.55 56 Biphasic waveform defibrillation, on the other hand, has not interfered with antibradycardia pacing and may be safer in pacemaker-dependent patients.57

Previous Transthoracic Defibrillation Trials Examining Waveform Shape
There has been surprisingly little clinical work examining transthoracic defibrillation efficacy, especially regarding the use of alternative waveforms. There are only two clinical transthoracic defibrillation studies available for review.20 58 Greene et al58 examined the relative efficacy of a standard 200-J monophasic damped sine wave transthoracic pulse compared with that of a 200-J biphasic quasidamped sine wave pulse (Gurvich waveform) for both cardioversion (ie, synchronized shock) of induced monomorphic VT and defibrillation (ie, asynchronous shock) of induced VF during electrophysiology studies. Although success rates for cardioversion of VT cannot be assumed to equal to success rates for defibrillation of VF, this study did demonstrate the overall superiority of the Gurvich waveform compared with to the standard damped sine wave pulse. In 171 patients, the 200-J biphasic damped sine wave pulse resulted in a higher combined first-shock cardioversion/defibrillation rate of 97.6% versus 85.2% (P=.0054). Although the results for the Gurvich waveform were favorable, this type of biphasic waveform would not facilitate defibrillator size, weight, or cost reduction. Not only is the inductor still a necessary device component for this waveform, but also it and the capacitor required to generate the Gurvich waveform are larger than normal.

In an earlier, smaller single-center study of ventricular defibrillation efficacy using truncated biphasic waveforms, 115-J (70-µF) and 130-J (105-µF) truncated biphasic pulses were compared with a standard 200-J (36-µF, 28-mH) damped sine wave pulse in 30 cardiac arrest survivors during transvenous ICD surgery.20 All three waveforms were equally effective, resulting in a 97% first-shock ventricular defibrillation efficacy rate. The findings in this earlier study are consistent with those found in the present multicenter study and with previous animal work.19 59 60 61 62 63 64

Study Limitations
The study is limited in its comparison to the out-of-hospital setting in that only induced VF was tested. In the out-of-hospital setting, spontaneous VF of prolonged duration is often associated with significant myocardial ischemia, something that did not occur in this study. However, as mentioned above, animal studies show that the relative performance of biphasic waveform defibrillation improves with the length of duration of VF.53 54 Thus, there is no reason to believe that biphasic defibrillation pulses will underperform monophasic pulses in the out-of-hospital setting.

Our interpretation of the ECG ST-segment changes may or may not have significance in the clinical setting of a full cardiac arrest. It is reasonable to infer, however, that these ST-segment changes are an undesirable outcome of high-energy monophasic shocks and likely reflect at least transient injury to the cardiac cells. This finding may be important in the out-of-hospital setting, where multiple shocks are often used and the negative inotropic consequences of multiple high-energy shocks may ultimately affect the success of the resuscitation effort.

High-Energy Output and Practice Recommendations
Although the statistical analysis showed no efficacy differences, the first-shock defibrillation rate for the 360-J monophasic damped sine wave shock appeared to be higher. If one assumes that the 360-J shock would indeed prove to be more effective for first-shock defibrillation in a larger study, it still cannot be recommended as first-line therapy. Because of the potential deleterious consequences of high-energy shocks, the standard protocol for defibrillation is to deliver 360 J only after two 200-J shocks fail. Using the data observed in this in-hospital study and the fact that defibrillation is probabilistic, a theoretical combined efficacy rate with a 200 J/200 J/360 J sequence would be 99%. The use of the same assumptions for the truncated biphasic waveform makes a 130 J/130 J/130 J therapy sequence yield a combined defibrillation efficacy rate of 99% as well. Thus, it is not clear that a 360-J shock would necessarily lead to better clinical results in the out-of-hospital setting, especially in light of the disadvantages associated with high shock strengths.

Summary
The findings from this prospective, multicenter study show that truncated biphasic pulses defibrillate as effectively as monophasic damped sine wave pulses while using substantially less energy and voltage. These reduced voltage and energy requirements directly translate into smaller, lighter, more-reliable, and lower-cost AEDs. In addition, the use of low-energy biphasic pulses results in significantly less postshock ST abnormalities compared with high-energy damped sine wave monophasic pulses.


*    Appendix: Investigators and Participating Institutions
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 up arrowAbstract
 up arrowIntroduction
 up arrowMethods
 up arrowResults
 up arrowDiscussion
 *Appendix: Investigators and...
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Principal investigator, Thomas S. Ahern, MD, Foad Moazez, MD, William Resh, MD, and Jill Lysgaard, RN (Sunrise Medical Center, Las Vegas, Nev); principal investigator, Gust H. Bardy, MD, Peter J. Kudenchuk, MD, G. Lee Dolack, MD, Jeanne E. Poole, MD, Fran Munckenbeck, MD, Brad O. Hofer, MD, Marye J. Gleva MD. Greg K. Jones, MD, Ramakota Reddy, MD, Jill Anderson, RN, and Charlie Troutman, RN (University of Washington Medical Center [Seattle]); principal investigator, Donald A. Chilson, MD, Harold R. Goldberg, MD, Michael Kwasman, MD, and Joni Baxter, RN (Deaconess Medical Center, Spokane, Wash); principal investigator, Christopher L. Fellows, MD, and Liz Brown, RN (Virginia Mason Medical Center, Seattle, Wash); principal investigator, Blair D. Halperin, MD, John H. McAnulty, MD, Jack Kron, MD, and Ron Oliver (Oregon Health Sciences University [Portland]); principal investigator, Richard M. Luceri, MD, Philip Zilo, MD, Daniel N. Weiss, MD, Adella Jonas, RN, and Reuben Hernandez (Holy Cross Hospital, Ft Lauderdale, Fla); principal investigator, Francis E. Marchlinski, MD, Charles D. Gottlieb, MD, David J. Callans, MD, David S. Schwartzman, MD, Steven J. Nierenberg, MD, Mark W. Preminger, MD, and George Winton, RN (Philadelphia Heart Institute, Presbyterian Medical Center, Philadelphia, Pa); principal investigator, Thomas A. Mattioni, MD, David W. Riggio, MD, and Sue Welch, RN (Arizona Heart Institute, Healthwest Medical Center, Phoenix, Ariz); principal investigator, Douglas L. Packer, MD, Stephen C. Hammill, MD, Michael J. Osborn, MD, Marshall S. Stanton, MD, Win K. Shen, MD, Thomas M. Munger, MD, and Suellen Grice, RN (Mayo Foundation, St Mary's Hospital, Rochester, Minn); principal investigator, Arjun D. Sharma, MD, Gearoid O'Neill, MD, Douglas Rothrock, MD, and Anne Skadsen, RN (Sutter Memorial Hospital and Mercy General Hospital, Sacramento, Calif); principal investigator, David J. Wilber, MD, John Kall, MD, Doug Kopp, MD, and Marian Stasi, RN (University of Chicago [Illinois]); principal investigator, Seth J. Worley, MD, Douglas C. Gohn, MD, Michael J. Sarik, DO, John Minnich, and Joann Tuzi, RN (Lancaster General Hospital, Lancaster, Pa); principal investigator, Raymond Yee, MD, George J. Klein, MD, and Marilyn Braney, RN (University Hospital, London, Ontario, Canada); biostatistician, Richard A. Kronmal, PhD (University of Washington [Seattle]); and data and safety monitor, Ralph Lazzara, MD (University of Oklahoma Health Sciences Center [Oklahoma City]).


*    Selected Abbreviations and Acronyms
 
AED = automatic external defibrillator
AHA = American Heart Association
ICD = implantable cardioverter defibrillator
VF = ventricular fibrillation


*    Acknowledgments
 
This work was supported in part by a grant from Heartstream (Seattle, Wash). Dr Bardy performs consulting work for Heartstream.


*    Footnotes
 
*A list of participants and institutions is given in the "Appendix."

Received January 25, 1996; revision received May 31, 1996; accepted June 17, 1996.


*    References
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 *References
 

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