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(Circulation. 1995;91:1768-1774.)
© 1995 American Heart Association, Inc.
Articles |
Truncated Biphasic Pulses for Transthoracic Defibrillation
From the Department of Medicine, University of Washington, Seattle.
Correspondence to Gust H. Bardy, MD, Mail Stop RG-22, University Hospital, Seattle, WA 98195.
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Abstract |
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Background Early defibrillation is the single most important factor for improving out-of-hospital ventricular fibrillation resuscitation rates. To achieve the earlier response times required for survival, typically <6 minutes from time of collapse, it will be necessary to equip a far wider network of first responders (firefighters, police, and other individuals with responsibility for public safety) with small, lightweight, and inexpensive automatic external defibrillators (AEDs). An important step in reducing the size and cost of AEDs will be to improve defibrillation efficacy. Because biphasic waveform defibrillation has had a favorable impact on implantable cardioverter-defibrillators (ICDs), there are reasons to believe that biphasic waveforms would also improve transthoracic defibrillators. Our purpose, therefore, was to examine the efficacy of two different low-energy biphasic truncated waveforms referenced to a standard damped sine waveform for transthoracic defibrillation in humans.
Methods and Results We prospectively and randomly compared the transthoracic defibrillation efficacy of two different truncated biphasic waveforms, 115 J (70 µF) and 130 J (105 µF), with that of a standard 200-J (36-µF, 28-mH) damped sine wave pulse using right anterior and left lateral thoracic pads (R2 Medical Systems) in 30 cardiac arrest survivors during transvenous ICD surgery. The right anterior patch electrode was used as the cathode and the left lateral thoracic pad as the anode. Transthoracic ventricular defibrillation rescue shocks were tested after a failed transvenous defibrillation shock delivered in the course of ICD testing. Each of the three different rescue shocks was tested in random order in each patient. All shocks were delivered at end expiration. The investigators responsible for determining transthoracic shock efficacy were blinded throughout the study to the transthoracic rescue waveform used. A total of 33 patients were considered for study, but three patients failed to satisfy all entry criteria or did not have a sufficient number of ventricular fibrillation inductions to allow for testing of all three waveforms. Percent efficacy for the three waveforms was then compared in the 30 patients who satisfied entry criteria and completed the protocol. The study population had a mean age of 61±11 years, with 22 (73%) being men. The mean left ventricular ejection fraction was 0.39±0.14. Coronary artery disease was present in 22 (73%). The 115-J (70-µF) biphasic pulse, the 130-J (105-µF) biphasic pulse, and the 200-J (36-µF, 28-mH) damped sine wave pulse were equally effective, resulting in a 97% first-shock ventricular defibrillation efficacy rate. Each waveform failed to defibrillate once, with each waveform failing in a different patient.
Conclusions The results of this study suggest that biphasic truncated transthoracic shocks of low energy (115 and 130 J) are as effective as 200-J damped sine wave shocks used in standard transthoracic defibrillators. This finding may contribute significantly to the miniaturization and cost reduction of transthoracic defibrillators, which could enable the development of a new generation of AEDs appropriate for an expanded group of out-of-hospital first responders and, eventually, layperson use.
Key Words: death, sudden • fibrillation • tachycardia
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Introduction |
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TopAbstractIntroductionMethods Results Discussion References |
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The American Heart Association, the American College of Emergency Physicians, and the European Resuscitation Council have advocated the widespread dissemination of automatic external defibrillators (AEDs) as a means of improving out-of-hospital ventricular fibrillation (VF) resuscitation rates.1 2 3 4 In communities with excellent medic systems having out-of-hospital response times of <4 minutes, 26% of sudden cardiac death (SCD) victims are successfully resuscitated.5 In four studies of cardiac arrest in supervised cardiac rehabilitation environments, where response was nearly instantaneous, a total of 90 of 101 victims (89%) were successfully resuscitated.6 7 8 9 In most communities and under most circumstances, however, the resuscitation rate is <8%,10 11 12 largely a consequence of delays from the time of collapse to defibrillation attempts.13 14 15 To improve response times nationwide, an essential element will be the institution of a far wider network of defibrillator-equipped first responders, including firefighters, police, and other individuals with responsibility for public safety.16 A defibrillator appropriate for this widespread distribution will need to be small, lightweight, and inexpensive.
Miniaturization of AEDs will favor their widespread dissemination by resulting in a decrease in production costs, and therefore price to the user, and by making them more portable for easier carrying. A major obstacle to the miniaturization of AEDs, however, is the present need for high-energy storage and delivery, which directly drives the volume, weight, and cost of the equipment. This is a consequence of the defibrillation waveforms used in today's AEDs, which remain essentially identical to those used since the advent of DC transthoracic defibrillators in the early 1960s.
Truncated biphasic waveform defibrillation has had a very favorable impact on implantable cardioverter/defibrillator (ICD) energy requirements.17 18 19 20 21 22 It is reasonable to believe, therefore, that transthoracic biphasic waveform defibrillation will also result in a marked reduction of energy storage and delivery requirements for AEDs. In turn, this will result in a reduction in the major size-determining elements of an AED: energy storage capacitors, batteries, and high-voltage switches. The need for wave-shaping inductors, for example, as commonly used in damped sine-type defibrillators, would be eliminated. Our purpose, therefore, was to examine the efficacy of two different low-energy biphasic truncated waveforms referenced to a standard damped sine waveform for transthoracic defibrillation in humans.
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Methods |
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Patient Population
After informed verbal and written consent was obtained, a comparison of two different low-energy truncated biphasic transthoracic pulses and a damped sine wave transthoracic pulse was undertaken in a prospective, randomized, and blinded fashion. The comparison was undertaken at the time of implantation of a transvenous ICD in patients who had previously survived an episode of VF and/or syncopal ventricular tachycardia (VT). Thirty-three patients were enrolled for study, and 30 patients completed the protocol.
Transthoracic Defibrillation Electrode System
The
transthoracic defibrillation electrode system consisted of
adhesive pad electrodes 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
electrode was at the midclavicular line and the inferior margin of the
electrode was at the inferior margin of the rib cage. The pad
electrodes were made by R2 Medical Systems (model 610), and each had an
active surface area of 80 cm2. The high right anterior pad
was used as the cathode. The pad electrodes were then connected to a
switch system that allowed delivery of one of the two truncated
biphasic pulses or the standard damped sine wave pulse. Transthoracic
shocks were randomly allocated in equal proportions to one of the three
different treatment arms in the normal course of ICD surgery. Only the
first three episodes of induced VF that failed to be terminated by
transvenous ICD test pulses were used for evaluation of transthoracic
defibrillation waveform efficacy.
Defibrillation Waveforms
The biphasic waveforms tested in
this study were selected for
their potential applicability to a miniaturized AED.23 The
nominal stored defibrillator values are detailed in Table 1
.
The biphasic waveforms were generated by a custom
defibrillation system (Heartstream, Inc). Biphasic pulse 1 was designed
to minimize energy storage requirements while remaining below the upper
voltage limit of solid-state switches and avoiding the need for
mechanical switches. This resulted in a pulse capable of delivering
approximately 115 J as a nominal energy. Biphasic pulse 2 was designed
to minimize defibrillation voltage requirements yet avoid excessive
capacitance sizes. This resulted in a pulse capable of delivering
approximately 130 J as a nominal energy. Biphasic pulses 1 and 2 are
shown in Fig 1A and 1B.
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The damped sine
wave pulse was the standard 200-J transthoracic
Edmark defibrillation pulse available in the Physio-Control Lifepak 6s
defibrillator. This pulse was generated by delivering the charge stored
on a 36-µF capacitor to a 28-mH inductor having a 12-
resistance.
The standard damped sine wave transthoracic pulse is shown in Fig
1C
.
Defibrillation Efficacy Testing
The test protocol for the
transthoracic shocks was conducted
during ICD surgery. In the course of determining the defibrillation
threshold for various transvenous lead systems and pulsing techniques,
the transvenous test shock would, from time to time, fail to
defibrillate. This provided the opportunity to test the transthoracic
defibrillator shocks under controlled conditions. Each episode of VF
was induced with AC or low-energy (0.6-J) shocks on the T wave and was
allowed to persist for 10 seconds before delivery of a transvenous test
shock.24 If the transvenous shock was unsuccessful,
the transthoracic test shock was delivered within 4 seconds
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 no longer than 14 seconds but always more than 10 seconds
and were always preceded by a failed transvenous test shock of <34
J.
The order of transthoracic defibrillation test shocks was randomly allocated. If the first transthoracic defibrillation pulse failed, a second transthoracic rescue pulse using a standard damped sine wave of 200 J was delivered. Repetitive VF inductions were delayed by a minimum rest period of 3 minutes. Between each induction and termination of VF, care was taken to ensure that ECG QRS duration, ST-T segments, and arterial pressure had returned to baseline values before VF was reinitiated.
Defibrillation with one of the three transthoracic test shocks was defined as successful if VF terminated within 3 seconds after shock delivery without any additional intervention. The implanting physician responsible for determining success or failure of the transthoracic rescue pulse was blinded to transthoracic waveform morphology. All investigators responsible for determination of shock success or failure were unaware of the study results until completion of the study and tabulation of the final results.
Statistical Analysis
The population size of this study was
based on the assumption
that the probability of defibrillation for the control waveform (damped
sine wave) would be 90% and that the lower limit of the CI of the
probability of defibrillation for either investigational waveform would
be at least 75%. With these assumptions, we estimated a minimum sample
size of 30 patients for a power of 80% likelihood to observe a <25%
difference at a P<.05 level for the three waveforms
evaluated. Statistical comparisons of defibrillation values were made
for the three waveforms by ANOVA with JMP 3.0.2 software
(SAS Institute).
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Results |
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TopAbstractIntroductionMethods Results Discussion References |
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Patient Clinical Characteristics
Three of 33 patients failed to satisfy criteria for inclusion in the transthoracic defibrillation study. One had received chronic amiodarone therapy and was inducible only into monomorphic VT. The second had previously had epicardial patch electrodes applied and was disqualified for evaluation in this study because of the likely alteration of the electrical field by the epicardial electrodes. The third patient was clinically unstable and required only one transthoracic rescue pulse and therefore failed to have the necessary number of VF episodes needed to test all three waveforms.
Of the 30 patients who completed the study, 22 (73%) were men. The mean weight was 179±37 pounds; range, 135 to 287 pounds. The mean age was 61±11 years; range, 39 to 79 years. Coronary artery disease was the primary structural heart disease in 17, 5 had both coronary disease and dilated cardiomyopathy, 5 had dilated cardiomyopathy only, 1 had primary electrical disease, 1 had hypertrophic cardiomyopathy, and 1 had long-QT syndrome. The mean left ventricular ejection fraction was 0.39±0.14; range, 0.15 to 0.65. The index arrhythmia leading to device implantation was VF in 17 patients, both VT and VF in 5 patients, and VT in 8 patients.
Defibrillation Efficacy
The defibrillation data are shown in
Fig 2 and in
Tables 1 and 2. The stored energy for biphasic
pulse 1
was 126 J to deliver the energy setting of 115 J to a standard 50-
load. The delivered energy for biphasic pulse 1 was 113±2 J; range,
110 to 116 J. The voltage stored on the capacitor was 1900 V. The
measured leading-edge voltage was 1857±14 V; range, 1816 to 1886 V.
The measured leading-edge current was 25.1±5.7 A; range, 14.9 to 39.5
A. The measured mean resistance was 78±18 ; range, 46 to 127
.
Total pulse width was 8.3±0.4 ms; range, 8.0 to 9.9 ms.
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The
stored energy for biphasic pulse 2 was 141 J to deliver the energy
setting of 130 J to a standard 50- load. The delivered energy for
biphasic pulse 2 was 126±3 J; range, 118 to 130 J. The voltage stored
on the capacitor was 1640 V. The measured leading-edge voltage was
1611±13 V; range, 1583 to 1637 V. The measured leading-edge current
was 21.9±5.1 A; range, 13.6 to 34.4 A. The measured mean resistance
was 78±18 ; range, 46 to 120 . Total pulse width was
12.0±0.0
ms; range, 11.9 to 12.1 ms.
The stored energy for the damped sine wave
pulse energy setting of 200
J (delivered to a 50- load) was actually 248 J. The voltage stored
on the capacitor was 3710 V. The delivered energy for the damped sine
wave pulse was 212±6 J, P<.0001 (ANOVA of the three
waveforms); range, 198 to 222 J. The measured delivered peak voltage
was 2497±175 V, P<.0001; range, 2067 to 2842 V. The
measured delivered peak current was 33.8±5.2 A, P<.0001;
range, 23.7 to 44.9 A. The measured mean pulse resistance was 76±17
, P=.95; range, 46 to 120 . Total pulse width was
6.1±1.0 ms; range, 4.5 to 8.5 ms, P<.0001, where pulse
width for the damped sine wave pulse was defined as the time for
the voltage to decay to 20% of the peak value.
Percent Defibrillation Efficacy
For each transthoracic pulse,
the percent defibrillation efficacy
was 97%. Each waveform failed to defibrillate once, with each pulse
type failing in a different patient. No patient had more than one
failure of any of the transthoracic test pulses. The one failure of
biphasic pulse 1 occurred in patient 12 during the second VF episode
requiring transthoracic defibrillation. The one failure of biphasic
pulse 2 occurred in patient 23, also during the second VF episode
requiring transthoracic defibrillation. The one failure of the damped
sine wave pulse occurred in patient 26 during the first episode of VF
that required transthoracic defibrillation.
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Discussion |
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TopAbstractIntroductionMethods Results Discussion References |
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This study has demonstrated that truncated biphasic pulses can defibrillate as efficiently as damped sine wave pulses using substantially lower energy, voltage, and current than typically used in standard transthoracic defibrillators. The findings suggest that transthoracic defibrillators, and AEDs in particular, can be considerably more efficient and consequently significantly smaller, lighter, and less costly if truncated biphasic waveforms are used.
Clinical Impact on Out-of-Hospital Resuscitation
SCD remains
a major clinical and public health problem, with the
vast majority of SCDs due to VF.25 Many victims have never
had any prior symptom of heart disease, and most die before reaching a
hospital. Early defibrillation is the single most important lifesaving
therapy under these circumstances, with survival much more likely if
delivered within 6 minutes of
collapse.13 14 15 In four
studies of cardiac arrest in supervised cardiac rehabilitation
environments, where response was nearly instantaneous, a total of 90 of
101 victims (89%) were successfully resuscitated, compared with
typical survival rates of <8% in most
communities.6 7 8 9 10 11 12
It is for these reasons that the American Heart Association, the
American College of Emergency Physicians, and the European
Resuscitation Council have advocated the widespread dissemination of
AEDs to facilitate early defibrillation as a means of improving SCD
resuscitation rates.1 2 3 4
The invention of battery-powered portable defibrillators in the late 1960s made it possible to reduce response times by bringing the defibrillator to the victim in the field. However, portable defibrillators are expensive, and communities are typically limited in the number of these medical response units they can afford. Moreover, response times to an SCD victim vary significantly as a function of the location of the victim. Even in the optimum case, an SCD victim is often difficult to reach within the critical time period of 6 minutes. Home and work placement of AEDs would be an ideal way to improve response times; however, the size, weight, and cost of the current generation of AEDs has limited their appeal for widespread distribution. A typical AED is about the size of a briefcase, weighs 10 to 20 pounds, and costs several thousand dollars. The findings in this study could lead to the advent of an AED significantly smaller, lighter, and less costly. In turn, these changes could result in greater availability of AEDs and therefore better resuscitation rates for VF victims.
Transthoracic Defibrillation Comparative Trials Examining Waveform
Shape
Clinically, the damped sinusoidal waveform is used in the
majority
of defibrillators and has received the most study over the past 30
years that DC external defibrillators have been commercially available.
In some AEDs, a monophasic truncated exponential waveform is used, but
this waveform has had only a limited clinical
evaluation.26 Neither waveform has been extensively
evaluated in comparative clinical trials against other waveforms.
The results of only one comparative clinical transthoracic defibrillation study are available for review and have been reported only in abstract form. Echt and colleagues27 compared the relative efficacy of a standard 200-J monophasic damped sine wave transthoracic pulse with that of a 200-J biphasic damped sine wave pulse (Gurvich waveform) for cardioversion and defibrillation of induced VT and VF during electrophysiology studies. They demonstrated a higher combined cardioversion/defibrillation rate of 98.3% versus 85.5%, P<.02, with the biphasic damped sine wave pulse. Although the results for the Gurvich waveform were favorable regarding percent cardioversion/defibrillation efficacy, this type of biphasic waveform would actually hamper rather than facilitate AED miniaturization because the inductor and capacitor required to generate the Gurvich waveform are larger than normal and, in fact, would increase the size and weight of an AED.
Only one other previously reported clinical study compared a damped sine wave pulse with another waveform, but this study was performed during epicardial defibrillation in an effort to explore the value of damped sine wave pulses for ICDs.28 In this study, a standard damped sine wave monophasic pulse was compared with a standard 120-µF 65% tilt monophasic truncated pulse. Delivered energy, delivered peak voltage, and delivered peak current requirements for defibrillation were equivalent for the two waveforms, whereas the use of the damped sine wave pulse required much higher stored capacitor voltages compared with the truncated monophasic pulse, 675 versus 356 V, P<.0001. Damped sine wave pulses therefore offered no advantage for ICDs and in fact would result in an increase in ICD size and weight because of the need to generate higher voltages.
Studies in animals have also shown no advantage of damped sine wave pulses over monophasic truncated pulses. Bourland et al29 compared damped sine wave current with monophasic square wave current using transthoracic defibrillators in dogs and ponies. In that study, the average energy and current required for defibrillation were essentially the same for either pulse. In like fashion, Hinds et al30 concluded in their study of transthoracic defibrillation in dogs that the monophasic truncated exponential waveform was equally effective to the damped sine wave for treating VF. Similarly, Wilson et al,31 studying greyhounds and comparing the defibrillation threshold of a monophasic trapezoidal waveform with that of a damped sine wave, found that transthoracic current and energy were not significantly different for the Lown, Edmark, and Belfast varieties of monophasic sine wave pulses compared with a 5-ms trapezoid. Thus, our present study, together with previous clinical and animal work, indicates that there probably is no physiologically preordained role for inductor-based damped sine wave pulses in transthoracic defibrillators.
Alternative Waveforms for Transthoracic Defibrillation,
Noncomparative Trials
The only reported clinical study examining
monophasic
truncated exponential pulses in human transthoracic
defibrillation was not done comparatively with other wave
shapes.26 In 108 out-of-hospital VF victims, a monophasic
truncated pulse was effective in terminating VF in 70% of the patients
but required 2.72 shocks on average to do so. Assuming that
refibrillation did not occur, the average shock efficacy rate was 26%,
far below the average for damped sine pulses. Although one would
initially think that the absence of an inductor was responsible for
this poor showing, a closer look at the transthoracic waveform used may
explain the results. This truncated pulse had a maximum duration of 40
ms, a duration suspected to be less effective at defibrillation and
potentially capable of refibrillation compared with pulses of shorter
duration.32 33 34 In an earlier
epicardial defibrillation
study, no such shortcoming was found when the transthoracic pulse used
was consistent with known effective truncated monophasic
waveforms.28
Impact on AED Size and Weight
For damped sine wave pulses to
provide delivered defibrillation
energy, voltage, and current comparable to those of truncated pulses,
especially in comparison with truncated biphasic pulses, it may be
necessary to store nearly twice the voltage on the capacitor to
accomplish the same task, while wasting considerable energy. Without
the inductor and with truncated biphasic pulses, a substantial
opportunity is now available to miniaturize, and therefore further
disseminate, AEDs. Reducing energy storage and delivery requirements
will dramatically alter volume and weight of transthoracic
defibrillators. Defibrillator size and weight requirements today are
driven primarily by the need to generate and store sufficient energy to
deliver up to 360 J to a patient with 50- resistance for each shock.
This requirement determines the size and weight of capacitor, battery,
and waveform-shaping elements in the devices. The inductor alone, for
example, when used to generate a damped sine wave in a standard AED,
contributes on average 150 cm3 in volume and 300 g in mass.
Furthermore, because of the inefficiencies of the inductor itself, the
capacitor volume has to be increased to make up for the wasted energy.
For example, a typical inductor has a resistance of approximately 10
. When put in series with a patient having a 50- resistance to
transthoracic current, about 17% of the energy stored in the capacitor
is dissipated in the inductor. Thus, to deliver 200 J to the patient,
about 240 J must be stored. In addition, because the capacitor energy
density (J/cm3) is fairly constant, the inductor resistance
burdens the capacitor volume by an additional 17%. Finally, because of
higher energy and voltage requirements with an inductor, larger
batteries and transformers, more insulation, and higher voltage
switches are needed.
The voltage needed to deliver the truncated biphasic pulse in our present study was <1900 V with capacitors no larger than 105 µF. Energy storage requirements for the pulse were <150 J. The reduced voltage requirements translate into decreased insulation and smaller cable and transformer sizes and allow the use of smaller, more reliable, and lower-cost solid-state switches rather than mechanical ones. The reduced energy requirements translate to much smaller capacitors and smaller batteries, contributing to a substantial size and weight reduction for the entire device. All of these changes would favorably influence the use and availability of AEDs in the general community.
Other Advantages of Truncated Biphasic Pulses
In addition to
the improved defibrillation efficacy of biphasic
pulses, other advantages accrue when this waveform is used. It is well
known that cardiac cell mechanical function is depressed after a
high-voltage
shock.35 36 37 38 39
Both lowering the voltage and
using a biphasic truncated pulse have been shown to limit mechanical
dysfunction after shock.40 41 Earlier reports have
linked
delivery of high-energy damped sine wave pulses with a higher incidence
of myocardial depression and bradyarrhythmias.42 43
The
use of truncated biphasic pulses, therefore, may have clinical benefits
beyond defibrillation.
Limitations
There are two potential limitations to this
study. The first
limitation relates to the controlled circumstances of the trial, which
do not parallel most settings in which transthoracic defibrillation is
used. Transthoracic shocks were delivered after only 12 to 14 seconds
of VF in a clinically stable cardiac patient. The out-of-hospital
patient may be in congestive heart failure or may be experiencing an
acute myocardial infarction. Transthoracic defibrillation may also be
delivered many minutes after VF onset. These more realistic
circumstances in the out-of-hospital setting will probably reduce
efficacy rates from the unreachably high level of 97% shown in our
study. Nevertheless, the percent efficacy results for each of the three
reported waveforms, albeit high, were equivalent. Moreover, given the
potential for biphasic shocks to reduce postshock mechanical
dysfunction, biphasic pulses may prove to be more helpful in these
sicker out-of-hospital patients than suggested purely from the data on
percent defibrillation efficacy.
An additional possible limitation of this study is the fact that the transthoracic shocks were delivered after a failed transvenous shock, which could have influenced the success or failure of the transthoracic rescue shock. The evidence to date in this regard suggests that subthreshold defibrillation shocks are more likely to adversely affect the subsequent rescue shock than improve its efficacy.44 Consequently, the delivery of a transvenous shock before a transthoracic shock should have proven a handicap rather than an advantage. In light of the high success rates with all three waveforms studied, it is unlikely that the antecedent transvenous shock was relevant to the percent efficacy findings for the truncated transthoracic biphasic or damped sine wave pulses used in this study.
Conclusions
The results of this study suggest that truncated
biphasic pulses
are as effective as standard damped sine wave pulses for transthoracic
defibrillation while using substantially less energy and voltage.
Applying these relatively low-energy transthoracic shocks has the
potential to dramatically decrease the size and weight of AEDs. The
practical advantages of a smaller AED are obvious: greater portability,
decreased costs, and much broader distribution. Truncated biphasic
transthoracic pulses may lead to improved resuscitation in cardiac
arrest victims if, by virtue of allowing for smaller, lighter, and less
expensive AEDs, they become more broadly distributed and thereby
decrease response times.
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Acknowledgments |
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This work was supported in part by a grant from Heartstream, Inc, and the Tachycardia Research Foundation, Seattle, Wash. The authors thank Joan McDaniel for secretarial assistance and Jan Bower for nursing assistance.
Received October 10, 1994; accepted October 23, 1994.
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