(Circulation. 1999;100:826-831.)
© 1999 American Heart Association, Inc.
Clinical Investigation and Reports |
Fully Discharging Phases
A New Approach to Biphasic Waveforms for External Defibrillation
From the Department of Cardiology, Cleveland Clinic Foundation, Cleveland, Ohio, and SurVivaLink Corp, Minneapolis, Minn (J.E.B., K.F.O.).
Correspondence to Patrick J. Tchou, MD, Director, Clinical Cardiac Electrophysiology, Department of Cardiology/F15, Cleveland Clinic Foundation, 9500 Euclid Ave, Cleveland, OH 44195. E-mail tchoup{at}cesmtp.ccf.org
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Abstract |
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Background—Phase-2 voltage and maximum pulse width are dependent on phase-1 pulse characteristics in a single-capacitor biphasic waveform. The use of 2 separate output capacitors avoids these limitations and may allow waveforms with lower defibrillation thresholds. A previous report also suggested that the optimal tilt may be >70%. This study was designed to determine an optimal biphasic waveform by use of a combination of 2 separate and fully (95% tilt) discharging capacitors.
Methods and Results—We performed 2 external defibrillation studies in a pig ventricular fibrillation model. In group 1, 9 waveforms from a combination of 3 phase-1 capacitor values (30, 60, and 120 µF) and 3 phase-2 capacitor values (0=monophasic, 1/3, and 1.0 times the phase-1 capacitor) were tested. Biphasic waveforms with phase-2 capacitors of 1/3 times that of phase 1 provided the highest defibrillation efficacy (stored energy and voltage) compared with corresponding monophasic and biphasic waveforms with the same capacitors in both phases except for waveforms with a 30-µF phase-1 capacitor. In group 2, 10 biphasic waveforms from a combination of 2 phase-1 capacitor values (30 and 60 µF) and 5 phase-2 capacitor values (10, 20, 30, 40, and 50 µF) were tested. In this range, phase-2 capacitor size was more critical for the 30-µF phase-1 than for the 60-µF phase-1 capacitor. The optimal combinations of fully discharging capacitors for defibrillation were 60/20 and 60/30 µF.
Conclusions—Phase-2 capacitor size plays an important role in reducing defibrillation energy in biphasic waveforms when 2 separate and fully discharging capacitors are used.
Key Words: defibrillation • death, sudden • ventricles
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Introduction |
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The use of an exponential biphasic waveform has been shown to be superior to an exponential monophasic waveform in external1 as well as internal2 3 defibrillation. Use of a single capacitor for generating exponential biphasic waveforms poses several limitations, however. Increasing the phase-1 duration may not generate the optimal phase-2 leading-edge voltage or delivered charge, because phase-2 leading-edge voltage is dependent on phase-1 pulse width, phase-1 leading-edge voltage, capacitance, and impedance. Therefore, the optimal single-capacitor biphasic waveform may not generate the best combination of phase 1 and phase 2 compared with the optimal combination of 2 separate capacitors for the 2 phases.
In a previous report,4 we found that extending the phase-1 tilt of a biphasic waveform from 30% to 70% lowered the defibrillation threshold (DFT) energy when a 60-µF phase-1 capacitor was used. This finding suggested that the optical tilt may be >70%. Second, that previous report4 compared the use of a phase-2 capacitor (20 µF) that was smaller than the phase-1 capacitor with the use of same-size capacitors in both phases. The results indicated that the use of a smaller capacitor in phase 2, when charged to the same leading-edge voltage as phase 1, yielded the optimal DFT energies.
External defibrillation efficacy is influenced by multiple factors,
such as electrode pad size,5 6 shock
waveform,1 7 8 9 10 and sequential shocks.11
Because capacitor size may also play a role in determining external
defibrillation efficacy,12 optimizing capacitor sizes may
contribute to maximizing defibrillation efficacy. Theoretical models,
assuming a 50-
impedance, have suggested that capacitors in the 30-
to 60-µF range may provide the optimal capacitor for phase 1 of a
biphasic waveform.13 14 Use of a smaller capacitor in a
fully discharging capacitor waveform may be particularly important,
because larger capacitors could result in exceedingly long pulse widths
when the defibrillation shock is applied through a higher-impedance
pathway. For example, for transthoracic impedance of 80 to
100 ,8 9 10 15 discharging a 120-µF capacitor to 95%
tilt would take >30 ms.
The purpose of this study was to assess the optimal capacitor sizes (phase 1 and phase 2) for a fully discharging (95% tilt) biphasic waveform in the above-discussed range of 30, 60, and 120 µF that could be implemented in a standard external defibrillation device.
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Methods |
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The use of experimental animals in this study was approved by the Animal Research Committee of the Cleveland Clinic Foundation and conformed to the recommendations of the American Heart Association on Research Animal Use.
Study Preparation and Surgical Procedure
Anesthetized pigs were used in this study. Animal
preparation and surgical procedures have been described in detail in
previous publications.4 16 Briefly, the swine were
intubated with a cuffed endotracheal tube and ventilated with room air
supplemented with oxygen through a Drager SAV respirator (North
American Drager), which was adjusted as needed to maintain normal
arterial blood gases. A transvenous unipolar defibrillation
lead (model 497, Intermedics Inc) was inserted into the right
ventricular (RV) apex through the right external jugular
vein under fluoroscopy. Three adhesive pad electrodes, each with a
surface area of 75 cm2, were applied to the right
upper pectoral, the left upper pectoral, and the cardiac apical area on
shaved skin.
Defibrillation Protocol
The 3 adhesive pad electrodes and the RV apex defibrillation
lead were connected to an external defibrillator custom-made by
SurVivaLink Corp. The function of this external defibrillator was
described in detail in our previous publication.4
Ventricular fibrillation (VF) was induced by delivery of
60-Hz AC (15 V) for 3 seconds through the RV apex defibrillation lead
and the left pectoral pad electrode. After VF sustained for 10 seconds,
1 of the test waveforms was delivered between the right upper pectoral
pad electrode and the apical pad electrode at the end-respiration
phase. The apical pad electrode was the anode for phase 1 of the
biphasic waveforms. If defibrillation failed for the test shock
waveform, a rescue shock (450 to 900 V) was delivered between the RV
defibrillation lead and the left upper pectoral pad electrode. A
recovery period of 
3 minutes was allowed between each episode of VF.
VF was not reinitiated until heart rate and blood pressure returned to
the preshock values.
Defibrillation Waveforms
Two groups of experiments were performed for this report. In
group 1, 3 phase-1 capacitor sizes (30, 60, and 120 µF) and 3 phase-2
capacitor sizes (0=monophasic, 1/3 of, and equal to phase-1
capacitor) were tested for a total of 9 different test shock waveforms.
These 9 waveforms consisted of 3 exponential monophasic and 6
exponential biphasic waveforms, as illustrated in Figure 1A
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In group 2, a more detailed assessment of optimal phase-2 capacitor was
performed using phase-1 capacitor sizes of 30 and 60 µF. Because of
concerns regarding the potential length of phase-1 discharge when
120-µF capacitors were used and the experimental limitation of the
number of waveforms that can be tested in a single experiment, the
120-µF phase-1 capacitor was not evaluated. Five phase-2 capacitor
sizes (10, 20, 30, 40, and 50 µF) were used in combination with the
30- and 60-µF phase-1 capacitor. Ten different exponential biphasic
waveforms were tested in this group to evaluate defibrillation
efficacy, as shown in Figure 1B
.
Evaluation of Defibrillation Efficacy
Defibrillation efficacy of each waveform was estimated by
V50, defined as the leading-edge voltage of the
waveform associated with a 50% likelihood of successful
defibrillation. V50 was measured by a previously
described Bayesian estimation technique.4 17 Ten
defibrillation tests were performed for each waveform to evaluate
V50. The first shock phase-1 leading-edge voltage
was 1650 V in all waveforms. Sequential step changes in voltage were
350, 200, 150, 150, 100, 100, 50, 50, and 0 V. These steps in voltage
change were either positive or negative, depending on failure or
success in defibrillation of the preceding shock, respectively.
E50 was the energy stored in the capacitors
calculated from these V50 values.
In each animal, the effect of the first shock (1650 V) was tested first in a randomized order for all waveforms. Then, second shock voltages for the waveforms were determined, based on the result of the first, and were applied in random order as well. Thereafter, by the same procedure, the third through 10th shocks were delivered.
Statistical Analysis
Data in all DFT parameters were expressed as mean
values±SD. Repeated-measures 1-way ANOVA was used to compare DFT
parameters in group 1 and group 2. Pairwise comparisons of
the waveforms were made for each parameter, with the least
significant difference test used in group 1 and group 2. The null
hypothesis was rejected for P<0.05.
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Results |
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Group 1
A complete data set was obtained for 10 pigs (36±2 kg). All parameters at DFT are shown in Table 1
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Figure 2 shows the
E50 of stored energy and
V50 of phase-1 leading-edge voltage for all
waveforms. For waveforms using a 30-µF phase-1 capacitor, there was a
trend toward lower E50 in the biphasic waveforms,
but this difference did not reach statistical significance. However,
there was a significant drop in leading-edge voltage for the biphasic
waveforms, with the lowest voltage seen in the 30/30-µF waveform. In
the waveforms using a 60-µF phase-1 capacitor, the optimal
E50 and V50 were seen with
use of the 20-µF phase-2 capacitor (P=0.008 and
P=0.013, respectively). In the waveforms using a 120-µF
phase-1 capacitor, the optimal E50 and
V50 were observed with a phase-2 capacitor of 40
µF. Interestingly, using a phase-2 capacitor equal to phase 1
(120/120 µF) resulted in E50 and
V50 that were significantly higher even compared
with the corresponding monophasic (120-µF) waveform. Thus, the
biphasic waveform with a smaller phase-2 capacitor than phase-1
capacitor reduced the E50 of stored energy when
60- and 120-µF capacitors were used in phase 1. For a phase-1 30-µF
capacitor, the 30/30-µF waveform appears to be optimal when
considering V50, even though the
E50 estimates did not reach statistical
differences. These results suggest that a phase-2 capacitor of 20 to 40
µF may be optimal for a wide range of phase-1 capacitors.
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Group 2
A complete data set was obtained for 10 pigs (33±4 kg). All
parameters at DFT are shown in Table 2.
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Figure 3 shows E50
and V50 of the experimental results. In both
phase-1 capacitor waveforms, the E50 and
V50 values showed a minimum at phase-2 capacitor
values of 20 or 30 µF. For a 30-µF phase-1 capacitor, the
E50 and V50 rose markedly
once phase-2 capacitor size exceeded 30 µF. For a 60-µF phase-1
capacitor, however, this rise was much less prominent. This difference
may be related to the ratio of phase-1 to phase-2 capacitors and their
effects on pulse width of the phases.
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Discussion |
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The results of this study show that DFT voltage and/or energy can be significantly lowered with the use of biphasic fully discharging capacitor waveforms compared with their corresponding monophasic waveforms. It is interesting to compare the effects of the phase-2 capacitor on the DFT. When a 30-µF phase-1 capacitor was used, the addition of a phase-2 capacitor (10 or 30 µF) lowered both DFT energy and voltage by
40%. The lowest DFT was associated with the 30-µF
phase-2 capacitor. With the 60- and 120-µF phase-1 capacitors, adding
the corresponding 20- and 40-µF phase-2 capacitors had similar
effects in lowering the DFT. However, when the corresponding phase-2
capacitors were made equal to the phase-1 capacitors, DFTs increased
again, and for the 120-µF capacitors, biphasic thresholds were even
higher than monophasic thresholds. These results would suggest that the
optimal phase-2 capacitor for 30- to 120-µF phase-1 capacitors is in
the range of 20 to 40 µF regardless of the capacitor used in phase 1.
The results of these experiments showed that the capacitor combinations
of 60/20 and 120/40 µF were clearly superior to the corresponding
ones using the same capacitors for both phase 1 and phase 2 (60/60 and
120/120 µF). Perhaps because 30 µF was in the range of the optimal
phase-2 capacitor, the difference between the 30/10- and the 30/30-µF
waveforms was small.
The results shown in Table 1 reveal that the mean impedances in
our experimental model were in the 40- to 50- range. However,
typical transthoracic impedances for human defibrillation
are higher, 60 to 80 , with values in the 80- to 100- range seen
occasionally.8 9 10 The use of a 120-µF capacitor may be
problematic in this impedance range. Based on the pulse
widths shown in Table 1, one can extrapolate that the use of the
same 120-µF waveforms in humans can extend the pulse widths to the
20- to 30-ms range. Such long pulse widths are likely to be not as
effective for defibrillation, because their duration will be well
beyond the chronaxie of the defibrillation strength-duration
curves.14 Thus, group 2 data shown in Table 2 were
limited to a more detailed analysis of the 30- and 60-µF
waveforms.
As illustrated in Figure 3, the pattern of changes in the DFT
voltage and energy associated with changing phase-2 capacitors was
different when the 30-µF and the 60-µF phase-1 capacitors were
compared. For the 30-µF phase-1 capacitor, there was a clear minimum
V50 associated with the use of a 30-µF
capacitor for phase 2. This minimum was reflected in the
E50 values as well. When the size of the
phase-2 capacitor exceeded that of phase 1, the DFT values rose
markedly. However, in the 60-µF phase-1 capacitor, the variation in
the phase-2 capacitor did not have as clear a minimum as that shown for
the 30-µF phase-1 data, perhaps partly because a phase-2 capacitor
size that exceeded that of phase 1 was lacking. A minimum for both
E50 and V50, however, was
seen at a phase-2 capacitor value of 20 to 30 µF. Like the
observations in internal defibrillation,18 19 the results
of this study showed that exponential biphasic waveforms with a very
small capacitor (20 to 40 µF) in phase 2 were effective for external
defibrillation. Furthermore, the 60-µF waveform had lower
V50 values than the 30-µF waveforms. We would
postulate that the 60-µF biphasic waveform with a phase-2 capacitor
of 20 to 30 µF may be the optimal combination among the ones
reported here for human external defibrillation. The 120/40-µF
waveform had a lower-voltage DFT at similar-energy DFT but would
probably generate excessively long pulses for the higher human
impedances.
Fully Discharging Capacitor Waveform
Because the tail of the defibrillation pulse may act to
refibrillate the myocardium, an appropriate truncation of
waveforms has been postulated to reduce DFT energy.13
However, previous publications20 21 have reported somewhat
conflicting results of truncation in monophasic capacitor discharge
waveforms. The effect of truncating phase 1 in biphasic waveforms has
not been studied in detail for defibrillation with a dual-capacitor
system. Our recent external defibrillation study4 showed
that DFT energy decreased with phase-1 tilt, increasing from 30% to
70% when a separate phase-2 capacitor was used. This finding suggested
that the optimal phase-1 tilt for dual-capacitor biphasic waveforms may
be >70%. Despite previous experimental observations that truncation
is important in monophasic waveforms, the data reported here raise the
interesting possibility that such truncation may not play as
significant a role in biphasic waveforms. The results with the 120-µF
phase-1 waveform are an example of this possibility. Despite use of a
full-tilt phase 1 that has relatively high E50
and V50 when tested as a monophasic waveform, the
addition of a small phase-2 capacitor markedly decreased the DFT to
values that were even better than those for the 30- and 60-µF
capacitors.
Changing Capacitance at Phase Reversal
A recent external defibrillation study1 showed the
superiority of single-capacitor biphasic waveforms over exponential
monophasic waveforms. However, there are a few limitations in such a
single-capacitor biphasic waveform. Maximizing the phase-1 pulse width
may not generate the optimal phase-2 leading-edge voltage or charge
transfer, because phase-2 leading-edge voltage is dependent on phase-1
pulse width. In addition, several recent defibrillation
studies19 22 23 have shown that a biphasic waveform with a
smaller phase-2 capacitor can achieve lower DFT energy when a phase-1
capacitor of 60 µF is used. Therefore, optimal biphasic waveforms may
be best generated with 2 separate capacitors, 1 for phase 1 and 1 for
phase 2.
As shown in Figure 2, the addition of an appropriate phase-2
capacitor can improve the E50 of the biphasic
waveform over that obtained with the corresponding monophasic waveform.
However, the relative improvement of the E50
appears to be greater when larger capacitors are used in phase 1.
Specifically, the biphasic waveforms of 30/10, 60/20, and 120/40 µF
reduced E50 by 38%, 67%, and 81%,
respectively, compared with their respective monophasic waveforms.
Thus, the beneficial effects of adding a phase-2 capacitor to the
monophasic waveform appears to depend on the capacitor size in phase 1.
Although the monophasic 120-µF waveform had the highest
E50 of the 3 capacitors, the greater improvement
in E50 seen with the addition of phase 2 made the
120/40-µF waveform perform as well as the 60/20-, 30/10-, and
30/30-µF waveforms. Another interesting observation is the effect of
having the same capacitor/pulse width in both phases of the biphasic
waveform. Although the 120/120-µF waveform had a poor
E50 compared with the 120/40-µF waveform, the
difference was much less prominent when the 60/60-µF and the
60/20-µF waveforms were compared, and the difference was nonexistent
when the 30/30-µF and the 30/10-µF waveforms were compared. This
observation would suggest that the optimal capacitor for a phase-2
fully discharging waveform may be fairly constant, 20 to 40 µF,
regardless of the phase-1 capacitor used. Because phase-2 capacitors
were charged to the same voltage as phase-1 capacitors in these
experiments, this observation would imply that the actual need for
charge transfer in phase 2 may be less for higher-capacitor phase-1
waveforms where the leading-edge voltage is lower, even though the
pulse width is longer, at least within the impedance range seen in our
experimental preparation. Thus, the need for phase-2 charge transfer
may be related to the leading-edge voltage of phase 1 regardless of the
phase-1 capacitor.
Applicability of the 120-µF Capacitor to Human External
Defibrillation
The impedance for external shock in the human chest is 60 to 80
and may exceed 100 in some cases.8 9 10 15
Conversely, the shock impedance of our pig model was 40 to 50 in
this study. Although the 120/40-µF biphasic waveform performed quite
well, as evidenced by the data presented in Table 1 and
Figure 2, its applicability to higher-impedance pathways may be
problematic. The phase-1 pulse width of the 120-µF
waveforms tested here was 14 ms with 40- impedance. For clinical
circumstances, this phase-1 pulse width may become 20 to 30 ms because
of the higher impedance and may be even higher in high-impedance cases.
Such a long pulse width would most likely be inefficient for external
defibrillation. Thus, our group 2 experiments did not include this
capacitor.
Clinical Implications
To facilitate widespread dissemination and public use of such
external defibrillators, it is important that these devices be reliable
and appropriately priced. Thus, technological approaches that would
minimize the cost of construction may be an important consideration in
public access defibrillators.
Full-tilt waveforms have several potential advantages over traditional biphasic waveform designs. These advantages include higher reliability and simpler design. All of this translates into a potentially better defibrillator. In a single-capacitor biphasic waveform, a switching system is necessary to change polarity at a high voltage of phase reversal and for truncation of phase 2. Conventional biphasic waveforms have always been generated with an H-bridge–style construction. This method used electronic switches to reverse the capacitor during the middle of the discharge cycle. Such a reversal requires switches that are capable of controlling both high currents and high voltages. Switching these high currents necessitates the use of isolated-gate, bipolar transistors (IGBTs), which are fairly bulky and relatively expensive. Because the polarity of the capacitor must be totally reversed, 4 IGBTs are required. All 4 must be carefully sequenced to properly reverse the voltage. In addition, dump resistors that are frequently used to dispose of residual charges on a capacitor after truncation would not be necessary with a nearly full-tilt waveform.
A 2-capacitor construction to implement the waveforms reported here would simplify the switching requirements, because the polarity of the capacitors does not need to be reversed. Two separate capacitor banks would be used for the 2 phases. This waveform design uses the same leading-edge voltage in both phases, which allows the use of a single-charge transformer. Because virtually all of the energy is delivered to the patient, truncation circuitry is greatly simplified. This simplification means fewer parts and less energy switching, which improves the reliability of the system while reducing space and power requirements.
Conclusions
The major findings of this external defibrillation study are as
follows. (1) For biphasic waveforms using 2 separate and almost
completely discharging (95% tilt) capacitors, the phase-2 capacitor
size is an important factor in maximizing defibrillation efficacy. (2)
Biphasic waveforms using 2 separate and fully discharging capacitors
appear to function best with a phase-2 capacitor 20 to 40 µF for
phase-1 capacitors in the 30- to 120-µF range. (3) In an external
defibrillator for human use, the 60/20- or 60/30-µF may be the
optimal choice among the waveforms tested here, because the 120-µF
capacitor may generate exceedingly long pulse widths and the 30-µF
model would use higher voltages, possibly generating greater costs
without additional benefits. (4) The use of simpler components in a
high-tilt biphasic waveform as described here may improve the
reliability and the expense of manufacturing external
defibrillators.
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Acknowledgments |
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This work was supported by a grant from the SurVivaLink Corporation. We would like to thank Donald G. Hills for his technical assistance in the care of the animals before and during the experiments.
Received March 10, 1999; revision received April 28, 1999; accepted May 26, 1999.
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