(Circulation. 2000;101:1324.)
© 2000 American Heart Association, Inc.
Basic Science Reports |
Ventricular Defibrillation With Triphasic Waveforms
From the Cardiac Rhythm Management Laboratory, Division of Cardiovascular Diseases, Department of Medicine, Department of Physiology, and Department of Biomedical Engineering, University of Alabama at Birmingham (J.H., D.L.R., W.M.S., R.E.I.), Birmingham, Ala, and Guidant Corporation (B.H.K.), Cardiac Rhythm Management Group, St Paul, Minn.
Correspondence to Raymond E. Ideker, MD, PhD, Cardiac Rhythm Management Laboratory, Volker Hall B140, 1670 University Blvd, Birmingham, AL 35294-0019. E-mail rei{at}crml.uab.edu
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Abstract |
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Background—It has been reported that triphasic defibrillation waveforms cause less myocardial injury than biphasic waveforms. This study compared the defibrillation thresholds (DFTs) of triphasic and biphasic waveforms.
Methods and Results—DFTs were determined for a transvenous lead system and a 300-µF-capacitor defibrillator. In 8 pigs (group 1), DFTs were determined for 5 triphasic waveforms with tilts of 80%, 83%, and 86% and for 1 biphasic waveform. DFTs were determined in another 8 pigs (group 2) for 2 triphasic and 4 biphasic waveforms with tilts of 43%, 49%, and 56%. In both groups, a biphasic waveform from a 140-µF-capacitor defibrillator was also evaluated, and both shock polarities were tested for each waveform. In group 1, with the 300-µF-capacitor defibrillator, the leading-edge voltage and energy stored at DFT were significantly lower for triphasic waveforms with phase-duration ratios of 50/33/17 and an anode at the right ventricular electrode for phase 1 than for biphasic waveforms (P<0.001). In group 2, the stored energy of triphasic waveforms with 56% and 49% tilt was significantly lower than that of biphasic waveforms with the same tilts for anodal but not cathodal phase 1 at the right ventricular electrode. Electrode polarity significantly affected the DFT of triphasic waveforms for both studies.
Conclusions—Some 80% tilt triphasic waveforms defibrillate more efficiently than biphasic waveforms with a 300-µF-capacitor defibrillator. The triphasic waveforms for both groups were not superior to 140-µF-capacitor biphasic waveforms. The efficacy of triphasic waveforms depends on phase durations and electrode polarity.
Key Words: defibrillation • waveforms • ventricles
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Introduction |
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Biphasic waveforms may defibrillate better than monophasic waveforms because they have a lower defibrillation threshold (DFT)1 2 and cause less myocardial damage.3 4 5 6 7 However, the optimal second-phase strength for minimization of the DFT is much larger than that for minimization of myocardial damage. Jones8 suggested that the benefits of both effects could be harnessed through the use of a triphasic waveform, in which the second phase is the larger strength, to lower the DFT, and the third phase is the lower strength, to minimize damage. They postulated that the first phase acts as a "conditioning prepulse," the second phase as a "defibrillating" phase, and the third phase as a "healing postpulse." However, subsequent studies in animals and humans9 10 11 12 13 failed to demonstrate the superiority of triphasic waveforms.
These disappointing results may have been caused by suboptimal
electrode polarity and phase durations. An anode at the right
ventricular (RV) electrode resulted in a lower DFT than a
cathode for some monophasic waveforms.14 15 16 17 Biphasic
waveforms in which the second phase was 
2 ms longer than the first
phase also had a lower DFT when the RV electrode was an anode. In
contrast, alterations in polarity of biphasic shocks in which the
second phase was equal to or shorter than that of the first phase had
no effect on the DFTs.17 18 19 20 21 22 23 Addition of a 1-ms second
phase to monophasic waveforms of 3 to 8 ms duration abolished the
effect of electrode polarity on the DFT, even though the second phase
accounted for as little as 2.4% of the total delivered
energy.17
These results suggest that the polarity of both phases can be important for defibrillation and that for both phases, an anode at the RV electrode can be better than a cathode. It is, of course, impossible to set both phases of a biphasic waveform to the same polarity and maintain the low DFT caused by polarity reversal between the 2 phases. However, this can be achieved with a triphasic waveform. We tested the hypotheses that a triphasic waveform with the first and third phases with the RV electrode as anode defibrillated more efficiently than some biphasic waveforms and more efficiently than triphasic waveforms with the reversed polarity. We also tested the hypothesis that defibrillation with a triphasic waveform of optimal phase ratio and polarity would be more efficient than a biphasic waveform.
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Results |
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Protocol 1
In all of the results below, anodal or cathodal refers to the polarity of the RV electrode during the first phase of the waveform. Both the waveform and electrode polarity had a significant effect (P<0.001 for leading-edge voltage and stored energy, respectively) on the DFT (Figures 1
and 2). The leading-edge voltage of
the anodal triphasic waveform with the phase-duration distribution
50/33/17 (Figure 1F) was significantly lower than that of anodal
biphasic waveforms with the phase-duration ratio of 60/40 with either
the 300- or 140-µF capacitors (Figures 1A and 1G). Although the point
estimates of the means of the leading-edge voltage and energy DFTs for
the anodal triphasic waveforms with phase-duration distributions of
60/30/10, 60/20/20, and 55/36/9 were lower than for the biphasic
waveforms with a phase-duration ratio of 60/40, this difference did not
reach statistical significance. Similarly, there was no significant
difference in the total stored energy between the anodal triphasic
300-µF waveform with a phase-duration distribution of 50/33/17 and
the 60/40 biphasic waveform with the 140-µF capacitor.
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Leading-edge
voltages for waveform that significantly differed from 60/40 biphasic
waveform with a 300-µF capacitor of the same polarity;
leading-edge voltages that significantly differed from 60/40
biphasic waveform with a 140-µF capacitor.
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For 300-µF cathodal waveforms, the leading-edge voltage and total
energy DFTs were significantly higher for the 60/10/30 triphasic
waveform than for the 60/40 biphasic waveform. The more conservative
Student-Neuman-Keuls test indicated that the stored energy requirement
of the 60/10/30 cathodal triphasic waveform was significantly higher
than the other waveforms (Figure 2
). The 60/10/30 cathodal triphasic
waveform also had significantly higher leading-edge voltage
requirements than all other waveforms except the 55/36/9 cathodal
triphasic waveform and 60/40 cathodal biphasic waveform with the
140-µF capacitor.
The leading-edge voltage and stored energy of the DFTs of the triphasic waveforms of duration ratios 60/10/30 and 50/33/17 were significantly lower when the RV electrode was an anode than a cathode. There was no statistically significant difference in leading-edge voltage or total energy DFT for the 2 shock polarities for the other waveforms.
Protocol 2
Both waveform and electrode polarity had a significant effect on
the stored energy of the DFT (Figures 3 and 4).
Factorial ANOVA analysis indicated that the stored energy
requirements of the anodal triphasic waveforms with 56% and 49% tilts
(Figures 4A and 4D) were significantly lower than those of
anodal biphasic waveforms with the same tilts (Figures 4C and 4F).
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Leading-edge voltage DFTs were significantly lower for the anodal
triphasic waveforms with 56% tilt and 49% tilt (Figures 3A and 3D) than for the biphasic waveforms with the same tilts (Figures 3C and 3F) by Student’s t test (P<0.05).
However, the differences were not statistically significant when the
general factorial ANOVA test was used, in which the triphasic waveform
was used as the reference category for comparison with the biphasic
waveforms. Both triphasic waveforms with anodal first phase
defibrillated with a lower leading-edge voltage and energy than the
cathodal first phase (P<0.05).
The leading-edge voltage DFTs for most 300-µF waveforms were significantly lower than those of the biphasic 140-µF waveforms. However, there were no significant differences among the stored energies between the 300- and 140-µF waveforms.
There was no significant difference in impedance for any waveform or
for any waveform phase by ANOVA. The mean impedance was 45±6 
.
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Discussion |
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Our principal finding is that some but not all 300-µF triphasic waveforms have a lower DFT than some comparable biphasic waveforms, with the efficacy of the triphasic waveform depending on the phase durations. This study also indicates that electrode polarity has an important effect on the DFT of triphasic waveforms; when the initial and final phases at the RV electrode were anodal, the DFT appeared lower than when they were cathodal, with this difference reaching statistical significance in some comparisons. Lowest mean DFTs were observed, however, for cathodal biphasic waveforms, with both 140- and 300-µF capacitors in protocol 2, although the differences compared with the lowest anodal triphasic waveform DFT did not reach statistical significance.
Reasons for the Efficacy of Triphasic Waveform
Defibrillation
One mechanism underlying the lower DFT of biphasic than monophasic
waveforms may be the improved ability of biphasic waveforms to excite
myocardial cells that must be excited during their relative refractory
period during fibrillation for successful termination of
fibrillation.24 25 The amplitude of the second phase of
these biphasic waveforms should range from 50% to 200% of the first
phase.26 A biphasic waveform with a second-phase amplitude
that was 5% to 20% of the first phase reduced shock-induced
dysfunction3 and decreased the incidence of
atrioventricular block.4 Thus, the optimum
strength of the second phase that minimizes DFT is much larger than the
optimum strength that minimizes damage.
To try to gain the benefits of both effects, Jones8 suggested a triphasic defibrillation waveform, in which the second phase was of the optimum strength for lowering the DFT and the third phase was smaller and optimized to minimize myocardial damage by the shock. They demonstrated in chick embryo cells that triphasic waveforms had a greater safety factor than biphasic waveforms.8 The safety factor was defined as the ratio of the minimum shock voltage gradient producing a 4-second arrest to the minimum shock voltage gradient capturing the tissue.
The present study is consistent with Jones’ prediction in
that the triphasic waveform with the lower DFT had a third-phase
amplitude that was 18% of the first-phase amplitude (Figures 1F and 2F).
However, another triphasic waveform with an 18% third-phase amplitude
did not significantly reduce the DFT compared with a biphasic waveform
(Figures 1A, 1E, 2A, and 2E). Further
studies are needed to confirm whether these triphasic waveforms reduce
shock-associated dysfunction.
Previous studies have suggested that the lower DFT with polarity reversal depends on either the total biphasic shock duration18 or second-phase duration.17 The present study also shows that the effect of shock polarity on the DFT depends on total or individual phase durations of triphasic waveforms. The triphasic waveforms with a lower DFT in the present study may combine the optimal total or individual phase durations and optimal electrode polarity. Additional studies are needed to further define how phase duration affects the DFT of triphasic waveforms with polarity reversal.
Comparison With Previous Triphasic Studies
The few previous studies reported that triphasic waveforms
defibrillate with a lower DFT than monophasic waveforms but with an
equal or higher DFT than biphasic waveforms.9 10 11 12 13 This
study demonstrates that 300-µF triphasic waveforms, with a
phase-duration ratio of 50/33/17 and a tilt of 86%, offer a slight
advantage over biphasic waveforms in defibrillation efficacy.
Consistent with previous reports, the other triphasic waveforms
tested in the present study were no more effective than the
biphasic waveforms and in some cases were less effective.
The durations of the 2 phases of a biphasic waveform markedly affect defibrillation efficacy.2 27 28 For monophasic shocks, when the RV electrode is an anode, the DFT is significantly lower than when it is a cathode.23 In some cases, polarity appears to affect the efficacy of biphasic waveforms, whereas in other cases it does not.17 20
This study demonstrates that the efficacy of defibrillation for triphasic waveforms is sensitive to both phase duration and electrode polarity. An anodal initial phase at the RV electrode for the triphasic waveform had a lower DFT than a cathodal initial phase. Chapman et al10 used the cathodal RV electrode configuration for their study and did not show any benefit for defibrillation with a triphasic waveform. For the triphasic waveforms used by Stellbrink et al13 and Chapman et al,10 phase 1 and phase 3 were each 25% and phase 2 was 50% of the total duration. These are not optimal phase-duration ratios according to the findings from the present study and our previous biphasic waveform study.17
Study Limitations
This study only tested 300-µF-capacitor triphasic waveforms; it
is unknown whether the findings would also apply to triphasic waveforms
delivered from other-size capacitors. We chose the 300-µF capacitor
because it has been shown that large-capacitor waveforms reduce DFT
peak voltage without increasing DFT energy.29 30 Reducing
peak voltage may decrease detrimental shock effects and may allow lower
voltage ratings of the electronic components in the defibrillator,
which would allow the size of the components to be reduced.
Clinical Significance
The best triphasic waveform had a slightly lower DFT than some
comparable biphasic waveforms. This benefit is probably not
sufficiently great to merit alteration of the hardware construction of
the defibrillator. However, most defibrillators that can deliver a
biphasic waveform already contain the switches and other hardware
necessary for delivery of a triphasic waveform. Therefore, slight
alteration of the software in the defibrillator to switch the waveform
a second time to deliver the third phase is the main change required
for delivery of a triphasic shock. Thus, if a triphasic waveform is
shown to have similar benefit in patients, its clinical use may be
justified.
It has also been suggested that triphasic waveforms have less-detrimental effects than either monophasic or biphasic waveforms.8 However, this suggestion is based on findings in a culture of chick embryo cells. This possibility needs to be confirmed in intact hearts.
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Methods |
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This study consisted of protocol 1, in which high-tilt triphasic waveforms were evaluated, and protocol 2, in which low-tilt triphasic waveforms were evaluated. The same animal preparation and DFT measurement were used in both. The study was performed in accordance with the guidelines established in the "Position of the American Heart Association on Research Animal Use " adopted by the American Heart Association on November 11, 1984.
Animal Preparation
Sixteen pigs, 8 for each protocol, weighing 31 to 59 kg were
positioned in dorsal recumbency. The methods of anesthesia,
muscle relaxation, ventilation, and monitoring have been given
elsewhere.17
Electrodes and Waveforms
Under fluoroscopic guidance, via right external jugular access,
a 0094 Endotak lead (CPI) with a 4.7-cm-long RV electrode and a
6.9-cm-long superior vena cava electrode was positioned with the tip at
the RV apex. A titanium can with a surface area of 92
cm2 was placed in the left pectoral region and
made electrically common with the superior vena cava electrode.
Protocol 1
DFTs were determined for truncated exponential triphasic
waveforms with 80% tilt delivered from a 300-µF capacitor (Figures 1 and 2). DFTs were also determined for 2 biphasic
waveforms of 80% tilt, 1 from a 300-µF capacitor (Figure 1A)
and the other from a 140-µF capacitor (Figure 1G). First-phase
duration was 60% of the total biphasic waveform duration. For 3
triphasic waveforms (Figures 1B through 1D), the first-phase
duration was the same as the first-phase duration of the 300-µF
biphasic waveform (Figure 1A), 60% of total duration. The sum
of the duration of the second and third phases was 40% of the total
duration; hence, as the duration of the third phase was increased, the
duration of the second phase was reduced.
Two other triphasic waveforms were tested in which the durations of the
first 2 phases were identical to those of the biphasic waveform and in
which the third phase increased the total duration of the triphasic
waveform 10% and 20% compared with that of the biphasic waveform
(Figures 1E and 1F). The tilts for these 2 waveforms were 83%
and 86%, and the phase-duration ratios were 55/36/9 (percentage of
total shock duration in phase 1/phase 2/phase 3) and 50/33/17,
respectively. The 300-µF-capacitor waveforms were generated with an
arbitrary waveform generator controlled by a Macintosh computer. For
all triphasic and biphasic shocks, the leading edge of the subsequent
phase began 0.25 ms after and was the same current as the trailing edge
of the preceding phase, with a slight variation caused by a maximum
variation of impedance between phases of 4.3%, thereby simulating a
single-capacitor waveform. The 140-µF-capacitor waveform was
generated with a model 2815 Ventak external cardioverter defibrillator
(CPI).
Protocol 2
As reported below, protocol 1 found the lowest triphasic DFT was
achieved with a 50/33/17, 86% tilt waveform with the RV electrode as
anode for phase 1; therefore, in protocol 2, two 50/33/17 low-tilt
triphasic waveforms were compared with low-tilt biphasic waveforms
(Figure 3). On the basis of the assumption that the optimal
first phase of a triphasic waveform is the same as the optimal
monophasic waveform with the same waveform time constant, the
theoretical optimal first-phase duration of a triphasic waveform was
calculated from a resistor-capacitormathematical
model27 (Figure 3A). The first-phase duration was
then used to calculate the total phase duration for the phase ratio of
50/33/17. The 56% waveform tilt was obtained from the equation
tilt=1-e-d/RC, where total phase
duration d was the value calculated above and the impedance
R was the mean from protocol 1. For 2 biphasic waveforms
(Figures 3B and 3C), the duration of phase 1 was the same as the
phase 1 duration of the triphasic waveform. The duration of phase 2 of
1 biphasic waveform (Figure 3B) also was the same as phase 2 of
the triphasic waveform. The phase-duration ratio for this biphasic
waveform was 60/40, and the tilt was 49%. The phase 2 duration of the
other biphasic waveform (Figure 3C) was the sum of phases 2 and
3 of the triphasic waveform. For this biphasic waveform, the
phase-duration ratio was 50/50, and the tilt was 56%.
Some data suggest that a lower DFT may be achieved with a waveform in
which phase 1 duration is shorter than that indicated as optimal by the
mathematical models.31 Therefore, we tested another
triphasic waveform (Figure 3D) in which phase 1 duration was 0.8
ms shorter than but the phase-duration ratio was the same as the
triphasic waveform shown in Figure 3A. The tilt of this waveform
was 49%. Two corresponding biphasic waveforms (Figures 3E and 3F) were tested in which the phase durations were established by the
same rules for the biphasic waveforms in Figures 2B and 2C. The tilts of these biphasic waveforms (Figures 3E and 3F) were 43% and 49%, respectively. A biphasic waveform generated
from a 140-µF capacitor (Figure 3G) with a tilt of 73% was
also tested. All waveforms were generated by the arbitrary waveform
generator.
In both protocols, both polarity configurations were compared. Randomization was performed by drawing 2 chits, 1 for waveform and 1 for polarity. After DFTs were determined with the randomly chosen polarity for that waveform, DFTs for the opposite electrode polarity were determined for that waveform.
Fibrillation and Defibrillation Procedures
Ventricular fibrillation was induced with 30-V,
60-Hz alternating current through the defibrillation catheter.
Fibrillation was allowed to continue for 10 seconds before a single
defibrillation test shock was given. If a test shock failed,
fibrillation was immediately terminated with a biphasic rescue shock at
the minimum reliable defibrillation energy from these electrodes
(typically 400 to 500 V). The leading-edge current of the first test
shock was the mean DFT from the previous animals. For the first animal,
the initial shock was 8 A. Depending on the success or failure of the
shock, leading-edge current was decreased or increased by 1 A,
respectively. This up-down algorithm was continued until the third
reversal of success to failure or failure to success. The DFT was the
mean of the 4 shock strengths delivered around the 3
reversals.17 At least 4 minutes was allowed to elapse
after every fibrillation-defibrillation episode until blood pressure
and heart rate returned to normal.
At the end of each study, euthanasia was performed by electrically inducing ventricular fibrillation. The heart was then removed and weighed.
Data Acquisition and Statistical Analysis
Shock current, voltage, impedance, and energy were determined as
described previously.17 Results are expressed as the
mean±1 SD. The effect of waveform and electrode polarity on DFT
leading-edge voltage and total stored energy for triphasic waveforms
was assessed by 2-factor multivariate ANOVA with
waveforms and polarity as factors (SPSS Inc). The interaction term
indicated whether the waveform effect on thresholds was different for
anodal compared with cathodal shocks. Because the waveform effect was
significant, the effect of the waveform on DFT leading-edge voltage and
stored energy was analyzed separately for each electrode
polarity by 1-factor general ANOVA with waveform as the factor. When
differences were found, individual differences were tested, with either
the 300- or 140-µF biphasic waveforms as the reference category. The
differences in DFTs between electrode polarities for individual
waveforms were evaluated with the paired t test. For all
analyses, P<0.05 was considered statistically
significant.
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Acknowledgments |
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This study was supported in part by National Institutes of Health Research grant HL-42760; CPI-Guidant Corp, St. Paul, Minn; and Physio-Control Corp, Seattle, Wash.
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Footnotes |
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The Methods section of this article can be found at http://www.circulationaha.org
Received April 8, 1999; revision received September 30, 1999; accepted October 8, 1999.
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