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JACC Vol. 32, No. 7
December 1998:2065–71
Encircling Overlapping Multipulse Shock Waveforms for
Transthoracic Defibrillation
LUIS A. PAGAN-CARLO, MD, JOHN J. ALLAN, MD, KIRK T. SPENCER, MD,
CLAY L. BIRKETT, BSEE, RICK MYERS, BSEE, RICHARD E. KERBER, MD, FACC
Iowa City, Iowa
Objectives. This study was performed to determine the efficacy
of new encircling overlapping multipulse, multipathway waveforms for transthoracic defibrillation.
Background. Alternative waveforms for transthoracic defibrillation may improve shock success.
Methods. First, we determined the shock success achieved by
three different waveforms at varying energies (18 –150 J) in 21
mongrel dogs after short-duration ventricular fibrillation. The
waveforms tested included the traditional damped sinusoidal
waveform, a single pathway biphasic waveform, and a new encircling overlapping multipulse waveform delivered from six electrode pads oriented circumferentially. Second, in 11 swine we
compared the efficacy of encircling overlapping multipulse shocks
given from six electrode pads and three capacitors versus encircling overlapping shocks given from a device utilizing three
electrodes and one capacitor.
Results. In the first experiment, the encircling overlapping
waveform performed significantly better than biphasic and
damped sinusoidal waveforms at lower energies. The shock success rate of the overlapping waveform (six pads) ranged from 67 6
4% (at 18 – 49 J energy) to 99 6 3% at >
2150 J; at comparable
energies biphasic waveform shock success ranged from 26 6 5%
(p < 0.01 vs. encircling overlapping waveforms) to 99 6 5% (p 5
NS). Damped sinusoidal waveform shock success ranged from 4 6
1% (p < 0.01 vs. encircling overlapping waveform) to 73 6 9%
(p 5 NS). In the second experiment the three electrode pads, one
capacitor encircling waveform achieved shock success rates comparable with the six-pad, three-capacitor waveform; at 18 –49 J,
success rates were 45 6 15% versus 57 6 12%, respectively (p 5
NS). At 100 J, success rates for both were 100%.
Conclusions. We conclude that encircling overlapping multipulse multipathway waveforms facilitate transthoracic defibrillation at low energies. These waveforms can be generated from a
device that requires only three electrodes and one capacitor.
(J Am Coll Cardiol 1998;32:2065–71)
©1998 by the American College of Cardiology
For the past three decades the damped sinusoidal waveform
(DS) has been the most widely used waveform to deliver
current in transthoracic defibrillation. However, with the advent of internal defibrillators, alternative waveforms for defibrillation were introduced (1– 4). Specifically, biphasic, sequential and multipulse shocks have been reported to be
superior for internal and transthoracic defibrillation in animals
and humans (1–9).
Multipulse, multipathway shocks could be superior for the
termination of ventricular fibrillation (VF) for a number of
reasons. An adequate intracardiac current flow might be
achieved in a greater portion of the ventricular myocardium
(9 –12). Depolarization of cardiac fibers and myocytes, which
are directionally sensitive to electrical field stimulation (13–
15), might be facilitated. Improved access might be gained to
the cell populations in the ventricle that are in various electrical states of polarization, hyperpolarization and recovery,
thereby depolarizing more of them and terminating the reentrant pathways of ventricular fibrillation.
We hypothesized that a multipulse, multipathway waveform, configured to generate a series of electrical vectors that
would rapidly and completely encircle the chest in an overlapping fashion, would optimize defibrillation by best accomplishing all the above considerations. The purpose of this study was
to compare the efficacy for transthoracic defibrillation of
encircling overlapping multipulse, multipathway waveforms
versus biphasic and damped sinusoidal single pathway waveforms.
From the Department of Internal Medicine, University of Iowa Hospital,
Iowa City, Iowa. This work was supported in part by a grant from the
Hewlett-Packard Corp. (Andover, Massachusetts), by a grant from the Laerdal
Foundation for Acute Medicine (Stavanger, Norway), by an Institutional National Research Service Award (HL07121) (LP-C), and an award from the
National Heart Lung and Blood Institute (NHLBI) (HL53284) (REK).
Manuscript received January 5, 1998; revised manuscript received July 29,
1998, accepted August 20, 1998.
Address for correspondence: Dr. Kerber, Department of Internal Medicine,
University of Iowa Hospital, 200 Hawkins Drive, Iowa City, Iowa 52242. e-mail:
[email protected].
Methods
©1998 by the American College of Cardiology
Published by Elsevier Science Inc.
This investigation was approved by the University of Iowa
Animal Care and Use Committee. Studies were performed on
21 adult mongrel dogs with body weights ranging from 20 to
30 kg (Study 1) and on 11 adult swine of 16 –21 kg body weight
(Study 2). We performed the second study on swine rather
than dogs to show that an overlapping waveform’s efficacy was
not limited to a single chest configuration, but was effective on
0735-1097/98/$19.00
PII S0735-1097(98)00486-0
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ENCIRCLING OVERLAPPING DEFIBRILLATION
JACC Vol. 32, No. 7
December 1998:2065–71
Abbreviations and Acronyms
DS 5 damped sinusoidal waveform
EO 5 encircling overlapping multipulse waveform
VF 5 ventricular fibrillation
a different species with a different chest configuration. Anesthesia was induced with IV Fentanyl/Droperidol and sodium
pentobarbital; after endotracheal intubation, ventilation was
maintained with a Harvard respirator. Anesthesia was maintained by periodic supplemental IV administration of sodium
pentobarbital. Arterial pressure and heart rate were monitored
throughout the experiment. Arterial blood gases were checked
periodically, and ventilation was adjusted to maintain physiologic parameters.
To induce ventricular fibrillation the internal jugular vein
was isolated and a bipolar pacing electrode was introduced into
the right ventricle; 60 Hz alternating current (5–10 V) was
delivered down the electrode for 5 s. Confirmation of VF was
obtained by ECG and blood pressure recordings. All defibrillating shocks were delivered after 30 s of ventricular fibrillation, at end-exhalation.
Electrodes and pathways. Study 1 (dogs). Six self-adhesive
pad-paired electrodes (surface area 25 cm2 each; HewlettPackard Corporation, McMinnville, Oregon), constructed of a
conductive-adhesive polymer with a nonconductive backing
were used. Firm electrode to chest wall pressure was assured
by placing the electrodes under an elastic Velcro chest wrap.
The electrodes were placed on a closely shaven chest wall
encircling the chest in a single axial plane, equally spaced.
Three current pathways resulted from this configuration (Fig.
1). Once placed, the pads were never moved.
Study 2 (swine). Three pads were placed in a sagittal plane,
equally spaced; two of the electrodes were on the lateral chest
and the third was placed on the sternum. Three different
current pathways were created by this configuration (Fig. 2).
Defibrillators. Study 1. The defibrillation shocks were delivered using three modified Hewlett-Packard defibrillators
(Hewlett-Packard Corp.), each with a 242-mF capacitor. The
defibrillators were modified to deliver truncated exponential
shocks; the timing and polarity of the shocks were controlled
by a Hewlett Packard Arbitrary Waveform generator (HP
33120 A) attached to each defibrillator. The waveform generators were programmed using a personal computer connected
by an IEEE-488 bus. A computer program allowed selection of
the number of pulses, polarity, duration and timing. The
voltage of the shocks was set using a control on each defibrillator.
We planned the experiments so that each waveform tested
would deliver the approximate equivalent of four energies: 25,
50, 100, 150 J. The voltage of each capacitor was adjusted to
approximate the desired energy (e.g., 25 to 150 J), but we were
limited to preset voltage increments and could not achieve the
exact voltages desired. Thus, we have grouped the calculated
Figure 1. Study 1. Electrode positions on the thorax (1– 6), and
electrical current vectors (a– k) for the encircling overlapping waveform. The thorax is represented as a circle with the heart in the middle.
Vectors a, c, e, g, i and k result from an electrical pulse generated by
a single-capacitor discharge and flowing between a single pair of
electrodes (e.g., electrodes 1– 4, pathway 1). Electrical pulses 4, 5 and
6 flow between the same electrode as pulses 1, 2 and 3 but are reversed
in polarity. Vectors b, d, f, h and j result from the overlapping
discharge of two capacitors yielding pulses flowing between two pairs
of electrodes simultaneously (e.g., 1– 4 and 2–5), with the mean
electrical vector lying between the two pathways defined by the two
electrode pairs. Each electrical vector is of equal duration. The
damped sinusoidal and biphasic shocks were always given utilizing
pathway 2 (pads 2–5).
energies into energy ranges (18 – 49, 50 –99, 100 –149,
150 –200 J) rather than specific energies. The component pulse
widths of each waveform were chosen before the study began
and were not adjusted to achieve desired energies.
Damped sinusoidal waveform shocks were given from a
standard Hewlett-Packard “Codemaster XE” defibrillator. For
the damped sinusoidal waveform shocks, current and impedance data were inadvertently not recorded; we have provided
data from previous studies in our laboratory using the same
defibrillator and similar sized dogs (16).
Study 2. A single modified defibrillator with a 242-mF
capacitor was used to generate the encircling overlapping
multipulse waveform (EO) shocks given from three electrode
pads, while the three defibrillators (three capacitors) used in
Study 1 were also used here to generate the EO shocks given
from six electrode pads.
The formulas for the calculation of delivered energy from
the voltages are given in the Appendix.
Shock waveforms. Study 1. Three shock waveforms were
evaluated. Biphasic waveform shocks and damped sinusoidal
shocks were delivered along a single pathway, pathway 2
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PAGAN-CARLO ET AL.
ENCIRCLING OVERLAPPING DEFIBRILLATION
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Figure 2. Study 2. Electrode positions on the thorax (1– 6),
electrical current vectors (a– g) and timing of the multiple
overlapping electrical pulses. EO6 5 encircling overlapping
six-electrode shocks. EO3 5 encircling overlapping threeelectrode shocks. Note that this EO6 waveform has only seven
vectors as opposed to the 11 vectors of the EO6 waveform in
Study 1.
(Fig. 1). This pathway was chosen based on preliminary
experiments that showed that the three pathways had similar
impedances but pathway 2 tended to have the lowest impedance. Encircling overlapping shocks were delivered over three
pathways (Fig. 1).
The EO and biphasic waveform in our experiments were all
variations of a truncated exponential waveform, with about
25% total tilt. The biphasic waveform consisted of an initial
5-ms positive pulse followed by a 1-ms negative pulse, separated by 0.1-ms delay, delivered along the same pathway. In all
multipulse multipathway shocks three capacitors were used
and the leading edge voltage of each pulse was identical. In the
biphasic single pathway shocks one capacitor was used, and the
leading edge voltage of the negative component was equal to
the trailing edge voltage of the positive component.
The EO waveform in this study was constructed by discharging one capacitor followed by a second capacitor discharge before the end of the first pulse. The sequential
discharges resulted in a current vector that encircled the chest
in a counterclockwise manner resulting in 11 electrical vectors.
At any one time, one or two capacitors were discharging either
simultaneously or in sequence. Figure 1 shows the electrode
placement and resultant electrical vectors: vector a was the
result of capacitor 1 discharging, which generated a pulse
traveling between electrodes 1 and 4; vector b was the result of
capacitors 1 and 2 discharging simultaneously (capacitor 2
discharge begins 0.64 ms after capacitor 1 discharge begins),
which generated simultaneous pulses between electrodes 1– 4
and 2–5, respectively; vector c was the result of capacitor 2
discharging (capacitor 1 off), which generated a pulse between
2 and 5 only; vector d was the result from capacitor 2 and 3
discharging simultaneously, which generated simultaneous
pulses between electrodes 2–5 and 3– 6, etc. This sequence was
repeated until the encircling current pulses were completed.
The result was a current vector that traveled from pad 1 to 6 in
a counterclockwise manner, creating all the 11 vectors (e– k)
shown in Figure 1. Pulses 1 and 6 are shorter than pulses 2–5;
the duration of each electrical vector is equal, approximately
0.64 ms. The total shock duration was approximately 7 ms.
Study 2. The EO waveforms in Study 2 were constructed as
shown in Figure 2. Using three electrodes and a single
capacitor, or six electrodes and three capacitors, seven vector
(a– g) encircling overlapping multipulse shocks were created.
In the three-electrode configuration current flowed from one
anode (1) to either one cathode (2) or simultaneously to two
cathodes as the multipulse shock continued. The total duration
of the shock was 7 ms, from both the three-electrode and
six-electrode system. The three-capacitor, six-electrode EO
waveform in this study was similar to that used in Study 1,
except that some of the intermediate vectors were deleted, so
that each pulse was 2 ms in duration, and a total of seven rather
than 11 directional vectors were created during the multipulse
shock. For comparison, damped sinusoidal and biphasic waveforms were also tested. The biphasic waveform used in this
study was comprised of an initial 5-ms positive pulse followed
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PAGAN-CARLO ET AL.
ENCIRCLING OVERLAPPING DEFIBRILLATION
by a 3-ms negative pulse. The damped sinusoidal waveform
was identical to that used in Study 1.
In addition to the damped sinusoidal, biphasic and shortduration EOs discussed above, we also evaluated other waveforms but have not included the data in this paper: monophasic
(trapezoidal) waveforms, encircling nonoverlapping waveforms and long-duration encircling overlapping waveforms. A
total of 17 different waveform configurations of varying durations, overlap and symmetry were evaluated in Study 1. In
Study 2, in addition to the four waveforms reported, singlecapacitor three-electrode-encircling overlapping waveforms of
6- and 10-ms duration were also studied. Data from these
additional waveforms are not included in this report. To keep
the experiments a reasonable length, each animal typically
received shocks from only three to four waveforms, and the
studies lasted 4 –5 h. Hemodynamic deterioration was uncommon; if it did occur the study was terminated.
Experimental protocol. Preparation of the contact skin between electrodes was accomplished by delivering three preliminary shocks using a standard damped sinusoidal shock defibrillator, at an energy of 50 J, along each pathway before any
data collection. This was done to minimize the effects of
repeated shocks on transthoracic impedance (TTI), since the
greatest TTI decline occurs after the initial shocks (17,18).
Following 30 s of VF, one of the experimental shocks was
delivered. If it failed to terminate VF, a damped sinusoidal
shock of at least 200 J was delivered to “rescue” the dog; data
from such “rescue” shocks were not used to calculate waveform efficacy. The animal was allowed to return to preshock
arterial pressure before the next fibrillation/defibrillation episode; the minimum time between VF episodes was 3 min. This
sequence was repeated four times at each energy. The energies
fell within the following ranges, 18 – 49, 50 –99, 100 –149,
150 –200 J. The results of the four shocks at each energy were
averaged to yield one data point. The percent success of each
shock in terminating VF was calculated. In both experiments 1
and 2, the sequence of waveforms studied in each animal was
random, and the energy sequence within each waveform was
random. Once a waveform and energy were chosen, all four
shocks were delivered at that energy.
Statistical analysis. Study 1. Proportion success data obtained from 21 dogs in a factorial arrangement of three
waveforms and four energy level classes were fit by a mixed
model analysis of variance (ANOVA), with the dog as the
random variance source, and waveform, energy level class and
their interaction as the treatment sources of variation. The
model accounted for 53.7% of the variation in proportion
success. Multiple comparisons involved Tukey’s adjustment
(19). All results are presented as mean 6 SE.
Study 2. Friedman’s test was used to compare percent
shock success among the four waveforms at each energy level.
Post-hoc testing involving Friedman mean rank scores was
performed for pairwise comparison between waveforms. The p
values from the post-hoc test have been adjusted by the total
number of comparisons. Adjusted p , 0.05 was considered
statistically significant. All results are presented as mean 6 SE.
JACC Vol. 32, No. 7
December 1998:2065–71
Table 1. Study 1: Percent Success of Waveforms
Energy (J)
Waveform Groups
% Success
(Mean 6 SE)
18 – 49
DS
Bi
EO6
DS
Bi
EO6
DS
Bi
EO6
DS
Bi
EO6
461
26 6 5*
67 6 4*†
27 6 8
54 6 5
89 6 4*†
53 6 8
96 6 4*
98 6 3*
73 6 9
99 6 4
99 6 3
50 –99
100 –149
150 –200
DS: damped sinusoidal waveform; Bi: biphasic waveform; EO6: encircling
overlapping waveform, six electrodes, three capacitors. *p , 0.01 vs. DS. †p ,
0.01 vs. Bi.
Results
Study 1. In Table 1 the percent success rate of the three
waveforms are presented. The encircling overlapping multipulse waveform shocks achieved significantly (p , 0.01) higher
success rates than both biphasic and damped sinusoidal waveform shocks at energy ranges of 18 – 49 and 50 –99 J; EO
success rates were 67 6 4% at 18 – 49 J and 89 6 4% at
50 –99 J. At 100 –149 J the encircling overlapping shocks were
superior to damped sinusoidal shocks at 18 – 49 and 100 –149 J.
These results are highlighted in Figure 3.
Study 2. Table 2 presents the percent shock success of the
four waveforms tested: encircling overlapping shocks from six
electrode pads (three capacitors), encircling overlapping
shocks from three electrode pads (one capacitor), biphasic
shocks and damped sinusoidal shocks. The EO six-electrode,
three-capacitor shock success ranged from 57 6 12% at
18 –49 J to 100% at .150 J. The EO three-electrode, oneFigure 3. Study 1. Graph showing percent shock success of encircling
overlapping 6-electrode (EO6) shocks, biphasic (Bi) shocks and
damped sinusoidal (DS) shocks. The EO6 waveform shocks achieved
higher success rates than both Bi and DS waveforms at energies of
18 –99 J, and higher than DS at energies up to 149 J.
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PAGAN-CARLO ET AL.
ENCIRCLING OVERLAPPING DEFIBRILLATION
Table 2. Study 2: Percent Success of Waveforms
Energy (J)
Waveform Groups
% Success (Mean 6 SE)
18 – 49
DS
Bi
EO6
EO3
DS
Bi
EO6
EO3
DS
Bi
EO6
EO3
DS
Bi
EO6
EO3
060
767
57 6 12*†
45 6 15*
14 6 8
61 6 12*
91 6 7*
80 6 10*
65 6 9
95 6 5*
100 6 0*
100 6 0*
89 6 7
100 6 0
100 6 0
100 6 0
50 –99
100 –149
150 –200
DS 5 damped sinusoidal waveform; BI 5 biphasic waveform; EO6 5
encircing overlapping waveform, six electrodes, three capacitors; EO3 5 encircling overlapping waveform, three electrodes, one capacitor. *p , 0.05 vs. DS.
†p , 0.05 vs. Bi.
capacitor success rates ranged from 45 6 15% to 100%, and
were not significantly different from the six-electrode, threecapacitor success rates at any energy. Biphasic shock success
ranged from 7 6 7% to 100%, and was less than the EO
six-electrode, three-capacitor waveform at the lowest energy
range. Damped sinusoidal shock success ranged from 0% to
89 6 7%, and was lower than both the encircling overlapping
waveforms at all energy ranges except .150 J, and lower than
the biphasic waveform success at 50 –99 J and 100 –145 J. These
results are highlighted in Figure 4.
Discussion
The main conclusions of these studies are that: 1) at low
energies, encircling overlapping multipulse, multipathway
Figure 4. Study 2. Percent shock success of encircling overlapping
six-electrode (EO6) and three-electrode (EO3) shocks, as well as
biphasic (Bi) and damped sinusoidal (DS) shocks. EO6 and EO3 shock
success rates were equal at all energy levels. *p , 0.05 vs. DS 1 p ,
0.05 vs. Bi n 5 11 swine.
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waveforms are superior to single-pathway biphasic and
damped sinusoidal waveforms for transthoracic defibrillation.
At higher energies, this superiority was not maintained; and 2)
effective encircling overlapping waveforms can be generated
from a three-electrode single-capacitor defibrillator, a potentially clinically applicable device.
Why should an overlapping encircling multipulse shock
waveform facilitate defibrillation? Myocardial fiber orientation in relation to a defibrillating or stimulating intracardiac
current has been shown to be important in achieving depolarization of the cell. Individual myocytes are easier to stimulate
and depolarize when the cell is parallel to the stimulating field
than when it is orthogonal to it (13–15). Roberts et al. (20)
have shown that cardiac fiber orientation influences myocardial conduction velocity and tissue resistivity. Thus, the orientation of the delivered transthoracic current may play a role in
achieving defibrillation. A single-pathway electrical pulse may
depolarize those myocytes oriented parallel to the electrical
field (the optimum orientation) but not perpendicular to the
field. In contrast, the rapidly shifting electrical vector achieved
by an overlapping multipulse waveform (Figs. 1 and 2) may
facilitate depolarization of a larger population of myocytes of
varying orientations. These explanations implicitly assume that
the electrical field is uniform throughout the ventricles for each
pathway, and that the depolarizing effect of a particular
electrical field is the same for all cells that are oriented at a
particular angle to that field.
We have also previously shown that overlapping multipulse,
multipathway shocks achieve higher intracardiac current flow
during the overlap phase (8).
Applying these concepts, we have previously shown that
sequential overlapping dual-pulse shock waveforms delivered
over two orthogonal pathways facilitate transthoracic defibrillation compared with a monophasic truncated exponential
waveform, and that the direction of the net electrical vector
changes during such overlapping pulses (8). The present
investigation extends this concept to a completely encircling
multipulse waveform.
Potential clinical applications. In our second study, we
showed that encircling overlapping waveforms can be generated from a three-electrode, single-capacitor defibrillator;
shocks from this device were as effective as those from a more
complex six-electrode, three-capacitor device, and at low energies (but not higher energies) maintained their superiority
over damped sinusoidal and biphasic waveform shocks. This is
important in considering potential clinical applications of this
approach; a multipulse, multipathway waveform would have
little clinical use if it required a large, heavy defibrillator and
many electrodes. The three electrodes used in experiment 2
could be preincorporated into an elasticized belt with Velcro
closures similar to what we used in this study; such a belt could
be rapidly wrapped around a collapsed patient (or the patient
could be rolled onto the belt). Since only a single capacitor is
needed, the defibrillator size and weight could be reduced.
It has been shown in animals (21–25) and patients (2– 4)
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ENCIRCLING OVERLAPPING DEFIBRILLATION
that single-pathway biphasic waveforms are superior to the
standard damped sinusoidal monophasic waveforms for transthoracic defibrillation. The apparent ability of this waveform to
achieve equivalent defibrillation efficacy at lower energies has
permitted the introduction of smaller and lighter defibrillators
for prehospital use; such units are especially suitable for
“public access defibrillation” (26). The overlapping encircling
multipulse approach we have evaluated in these studies demonstrated superiority over single-pathway biphasic waveforms
at low energies. However, the three electrodes required will
necessarily increase the electrical complexity of the defibrillator to some degree. Whether a multipulse multipathway defibrillator for clinical use could be constructed of a size, weight
and cost similar to the new single-pathway biphasic defibrillators remains to be determined.
There have been several anecdotal reports of successful
human transthoracic defibrillation with multipulse/multipathway
shocks (5,6). In those reports the shocks were delivered
through two separate lead systems, and were temporally
separated by several seconds. Although encouraging, those
results cannot be directly extrapolated to our approach since
our encircling shocks were continuous.
Limitations. There are several limitations of our studies.
First, only 30 s of VF was allowed before a shock was delivered.
In the usual clinical scenario, VF occurs for much longer
periods of time before the first shock; the efficacy of the
waveforms demonstrated in this experiment may not be as high
in the clinical setting after prolonged periods of fibrillation.
Second, our defibrillator did not allow us to adjust the tilt of
the waveforms generated to maximize their effects. Several in
vivo studies, as well as mathematical models, have shown that
tilt can play a significant role in maximizing defibrillation
(27–29). Third, even though we showed that the overlapping
encircling waveform is effective in two different species with
different chest configurations, there are still differences compared with humans; the canine and swine hearts are more
medially located within the thorax than in the human heart.
Whether this difference would alter the effectiveness of encircling waveforms for defibrillation remains to be determined.
In summary, encircling overlapping multipulse multipathway shock waveforms were superior to single-pathway waveforms at low energies, but not at higher energies. Further
evaluation of these waveforms are appropriate.
We gratefully acknowledge the review and criticisms of Drs. Janice Jones and
Michael Kallok, the statistical assistance of Carl K. Brown and Miriam B.
Zimmerman, the technical assistance of Robin A. Smith, BA and the secretarial
assistance of Diane Phillips.
Appendix
The defibrillation shocks were measured using an ISC-16 digitizing
board and EGAA software provided by R.C. Electronics. The data
acquisition board was configured for 610 V, single ended. The sample
rate was 50 kHz per channel.
The following terms were used in the calculation of delivered
JACC Vol. 32, No. 7
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energy: Edel 5 Energy delivered to load; Esto 5 Energy stored on
capacitor; Vi 5 Initial voltage on capacitor at start of discharge; Vf 5
Final voltage on capacitor at end of discharge; VL 5 Leading edge
voltage, voltage across the chest; IL 5 Leading edge current, current
through the chest; RL 5 Pathway impedance (chest impedance),
calculated from VL/IL; RI 5 Internal impedance (Capacitor ESR,
wiring resistance, etc.); C 5 Energy storage capacitor
Energy stored on a capacitor is Esto 5 1/2CV2. Energy removed
from the capacitor during a discharge can then be found from 1/2C(VI2
2 Vf2). Then, energy delivered to the load is: Edel 5 1/2C(VI2 2
Vf2)zRL (RL 1 RI).
Study 1. In study 1, the voltage on the capacitor (VI and Vf) was
not measured, but can be calculated from the following: VI 5 VLz(RL
1 RI)/RL, where RL 5 VI/IL. Vf can then be calculated from Vf 2
Vie2t(RC), where t is the width of the discharge pulse (seconds) and
R 5 RI 1 RI. Combining the above equations leads to the following
formula for calculating delivered energy:
Energy 2 1/2C~1.07zVL!2~~RL 1 RI!/RL!z~1 2 e22t(RC)).
The term t (time) in the above equation is the time a given pathway is
on. For example, using a biphasic waveform with a positive phase of
5 ms, and a negative phase of 3 ms, t would be 8 ms.
The energy obtained using the above formula was multiplied by 3
if an encircling waveform was used. The manufacturer of the defibrillator (Hewlett-Packard Corp.) tested the pulse characteristics of the
three defibrillators before the beginning of the study to determine the
pulse characteristics of the three were the same.
The term 1.07 is a correction factor to account for the underestimation of the actual voltage measurement. Recalibration of the
equipment at the end of the study demonstrated our digitizing board
underestimated the voltage and current measurement by approximately 7%. The correction term 1.07 in the formula corrects for this
error.
Study 2. A single modified defibrillator with a 242 mF capacitor
was used to generate the EO shocks from three electrode pads, while
the three defibrillators (three capacitors) used in Study 1 were also
used here to generate the EO shocks given from six electrode pads.
The test system was modified to allow a direct measurement of the
voltage on the energy storage capacitor. Then, delivered energy was
calculated using the following formula:
Energy 5 1/2C~VI2 2 Vf2!zRL/~RL 1 RI!.
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