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(Circulation. 2000;101:1282.)
© 2000 American Heart Association, Inc.
Clinical Investigation and Reports |
Transthoracic Cardioversion of Atrial Fibrillation
Comparison of Rectilinear Biphasic Versus Damped Sine Wave Monophasic Shocks
From The New York Hospital-Cornell Medical Center, New York, NY (S.M., K.M.S., S.M.M., D.J.S., M.A.S., B.B.L.); Zoll Medical Corporation, Burlington, Mass (S.A.); University of Pittsburgh Medical Center, Pittsburgh, Pa (D.S., D.C.); and Cleveland Clinic Foundation, Cleveland, Ohio (P.J.T.).
Correspondence to Bruce B. Lerman, MD, Division of Cardiology, The New York Hospital-Cornell Medical Center, 525 East 68th Street, Starr 4, New York, NY 10021. E-mail blerman{at}mail.med.cornell.edu
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Abstract |
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Background—Clinical studies have shown that biphasic shocks are more effective than monophasic shocks for ventricular defibrillation. The purpose of this study was to compare the efficacy of a rectilinear biphasic waveform with a standard damped sine wave monophasic waveform for the transthoracic cardioversion of atrial fibrillation.
Methods and Results—In this prospective, randomized, multicenter
trial, patients undergoing transthoracic cardioversion of
atrial fibrillation were randomized to receive either damped sine wave
monophasic or rectilinear biphasic shocks. Patients randomized to the
monophasic protocol (n=77) received sequential shocks of 100, 200, 300,
and 360 J. Patients randomized to the biphasic protocol (n=88) received
sequential shocks of 70, 120, 150, and 170 J. First-shock efficacy with
the 70-J biphasic waveform (60 of 88 patients, 68%) was significantly
greater than that with the 100-J monophasic waveform (16 of 77
patients, 21%, P<0.0001), and it was achieved with
50% less delivered current (11±1 versus 22±4 A,
P<0.0001). Similarly, the cumulative efficacy with the
biphasic waveform (83 of 88 patients, 94%) was significantly greater
than that with the monophasic waveform (61 of 77 patients, 79%;
P=0.005). The following 3 variables were
independently associated with successful cardioversion: use of a
biphasic waveform (relative risk, 4.2; 95% confidence intervals, 1.3
to 13.9; P=0.02), transthoracic impedance
(relative risk, 0.64 per 10-
increase in impedance; 95% confidence
intervals, 0.46 to 0.90; P=0.005), and duration of
atrial fibrillation (relative risk, 0.97 per 30 days of atrial
fibrillation; 95% confidence intervals, 0.96 to 0.99;
P=0.02).
Conclusions—For transthoracic cardioversion of atrial fibrillation, rectilinear biphasic shocks have greater efficacy (and require less energy) than damped sine wave monophasic shocks.
Key Words: cardioversion • atrial fibrillation • shock
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Introduction |
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Atrial fibrillation is the most commonly encountered sustained clinical arrhythmia.1 Since its introduction 3 decades ago, transthoracic electrical cardioversion has remained the most effective method for terminating atrial fibrillation.2 However, currently available defibrillators use a damped sine wave monophasic waveform, which may be effective in <80% of patients.3 4 5 6
Multiple studies have shown that biphasic waveforms have greater efficacy than monophasic waveforms for endocardial defibrillation.7 8 9 10 11 12 More recent studies of transthoracic ventricular defibrillation have also shown that, compared with monophasic shocks, biphasic shocks are equally effective and use less delivered energy,13 14 which may result in less post-shock myocardial dysfunction.13 15 16 17 To determine whether a biphasic waveform provides increased efficacy with less energy during the transthoracic cardioversion of atrial fibrillation, we prospectively compared the efficacy of a standard damped sine wave monophasic waveform with a rectilinear biphasic waveform in patients undergoing transthoracic cardioversion of atrial fibrillation.
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Methods |
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Study Population
This study was a prospective, randomized, multicenter trial that compared the efficacy of a rectilinear biphasic waveform with a damped sine wave monophasic waveform for the transthoracic cardioversion of atrial fibrillation. During the study period (between August 1998 and February 1999), 174 patients were enrolled from 7 centers. Patients were eligible for the study if they were undergoing electrical cardioversion of atrial fibrillation. Patients with atrial fibrillation for
48 hours were anticoagulated with warfarin for 3
weeks and achieved an international normalized ratio 2.0. Patients
who had not been anticoagulated for 3 weeks underwent a
transesophageal echocardiogram while receiving
therapeutic heparin (or warfarin) immediately before the cardioversion
that documented the absence of a left atrial thrombus.18
All patients were then required to be anticoagulated for 3 to 4 weeks
after cardioversion. In addition, all patients were required to have
had an echocardiogram within 3 months of the cardioversion for
assessment of the left atrial size and left ventricular
(LV) ejection fraction. Patients were ineligible if they were <18
years of age, were pregnant, or were undergoing cardioversion of an
atrial arrhythmia other than atrial fibrillation. The
Institutional Review Board at each participating institution approved
the investigational protocol, and informed written consent was obtained
from all patients.
Study Design
The primary hypothesis of the study was that rectilinear
biphasic shocks would have a greater cardioversion efficacy than
monophasic shocks. Assuming an 80% cumulative efficacy with the damped
sine wave monophasic waveform, 154 patients were required to detect a
>15% difference in efficacy between the monophasic and biphasic
waveforms and to have a power of 80% and a significance level of
0.05.
Shock Waveforms
Figure 1
depicts a
representative 100-J damped sine wave monophasic
waveform and a 70-J rectilinear (constant-current first phase) biphasic
waveform delivered across a 50- load. The monophasic waveforms were
generated by a Zoll PD-2000 defibrillator, which delivered the stored
charge on a 45-µF capacitor through a 20-MHz inductor and an internal
resistance of 14 . Biphasic waveforms were generated from a 100-µF
capacitor using the Zoll PD-2100 defibrillator. The rectilinear
biphasic waveform consisted of a constant-current 6-ms first phase,
followed by a truncated, exponential 4-ms second phase. The time
between the trailing edge of the first phase and the leading edge of
the second phase was 100 µs. For a selected energy, a
constant-current first phase was produced by automatically adjusting
the internal resistance of the defibrillator circuit on the basis of
the patient’s transthoracic impedance, which was
automatically determined at the onset of shock delivery.
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The defibrillator’s internal switching resistors, along with impedance
compensation, provided a constant first-phase current as well as the
step ripple. The amplitude of the first and second phases varied with
the selected energy. The initial amplitude of the second phase was
approximately equal to the final amplitude of the first phase. When 170
J was selected for the biphasic waveform and the measured patient
impedance was 85 , the first phase of the biphasic waveform
tilted. However, all other waveform parameters, including
phase duration, interphase delay, and integrated impedance
measurement-sensing pulse, were unchanged. The effect of
transthoracic impedance on delivered current for the
monophasic and biphasic waveforms is outlined in Table 1.
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Shock Electrodes
Wet polymer gel pads19 (Zoll
Cardiology Specialty Pad) for transthoracic
cardioversion were applied to the right parasternal area and to the
left scapula posteriorly.20 The anterior electrode was
circular and had a diameter of 10 cm, which corresponded to an active
surface area of 78 cm2. The posterior electrode
was rectangular and had a diagonal length of 14.5 cm, which
corresponded to an active surface area of 113
cm2. The pads were connected to a
multiple-defibrillation interface unit, which in turn was connected to
the monophasic and biphasic defibrillators. This setup was used to
ensure that both defibrillators delivered a shock into the same pad and
same location. Furthermore, the interface unit transferred the voltage
and current delivered to the patient to a laptop personal computer for
data collection purposes.
Protocol for Cardioversion
Patients were randomized at each center, using a simple
block-randomization scheme, to either the monophasic or biphasic
waveform protocol. Patients randomized to the monophasic protocol
received sequential shocks of 100, 200, 300, and 360 J, if necessary.
If the 360-J shock failed to cardiovert the patient, a final 170-J
biphasic shock was delivered. Patients randomized to the biphasic
protocol received sequential shocks of 70, 120, 150, and 170 J, if
necessary. If the 170-J shock failed to cardiovert the patient, a final
360-J monophasic shock was delivered. Successful cardioversion was
defined as the conversion of atrial fibrillation to sinus rhythm for
30 s after the shock.
Statistical Analysis
All continuous variables are expressed as mean±SD.
Comparisons of dichotomous and continuous variables between the
monophasic and biphasic waveform groups were calculated using the
2 and Student’s t tests,
respectively. The first-shock efficacy of the 70-J biphasic and 100-J
monophasic shocks and the cumulative efficacy of biphasic and
monophasic shocks were compared using Fisher’s exact test. To
determine the variables independently associated with successful
cardioversion, multivariate forward stepwise logistic
regression was performed using the following clinical variables:
patient weight, duration of atrial fibrillation, left atrial size, LV
ejection fraction, transthoracic impedance, and shock
waveform used. We calculated 95% confidence intervals for each
relative risk. For all comparisons, P<0.05 was considered
statistically significant.
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Results |
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Patient Characteristics
Cardioversion was performed in 174 patients. Nine patients were excluded from analysis. Reasons for exclusion included (1) failure of the investigator to follow the prespecified step-up shock protocol (n=7), (2) pretreatment with ibutilide (n=1), and (3) inability to access cardioversion shock data due to a computer malfunction (n=1). The remaining 165 patients constituted the study population.
The population was 66±12 years of age (range, 30 to 92 years), had a
mean weight of 91±23 kg (range, 46 to 168 kg), and was predominantly
male (70%). The size of the left atrium was 4.7±0.9 cm (range, 2.7 to
9.7 cm), and the LV ejection fraction was 50±14% (range, 15% to
75%). Structural heart disease (including hypertension) was
present in 69% of patients. Of the 165 patients who underwent
cardioversion, 88 (53%) were randomized to the biphasic group and 77
(47%) to the monophasic group. The 2 groups were similar with respect
to age, sex, weight, left atrial size, LV ejection fraction, underlying
cardiac disease, New York Heart Association class, duration of atrial
fibrillation, and use of cardioactive drugs, including antiarrhythmic
medications (Table 2).
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Cardioversion Data
The first-shock cardioversion data for the 2 groups are summarized
in Table 3. First-shock efficacy with the
70-J rectilinear biphasic waveform (60 of 88 patients, 68%) was
significantly greater than that with the 100-J damped sine wave
monophasic waveform (16 of 77 patients, 21%; P<0.0001)
(Figure 2). In addition, increased
efficacy with the 70-J biphasic shocks was achieved with 50% less
delivered current (11±1 versus 22±4 A, P<0.0001). No
significant difference existed in the transthoracic
impedance between the biphasic (76±17 ) and monophasic (78±16 ,
P=NS) waveform groups.
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The cumulative cardioversion efficacy of the 100, 200, 300, and 360 J
monophasic shocks was 21%, 44%, 68%, and 79%, respectively. In
contrast, the cumulative cardioversion efficacy of the 70, 120, 150,
and 170 J biphasic shocks was 68%, 85%, 91%, and 94%, respectively.
The cumulative efficacy with the rectilinear biphasic waveform (83 of
88 patients, 94%) was significantly greater than that with the damped
sine wave monophasic waveform (61 of 77 patients, 79%;
P=0.005) (Figure 2
). Of note, the mean peak current
delivered to patients with the 100-J monophasic shocks was equal to
that delivered with 170-J rectilinear biphasic shocks (Table 1).
Furthermore, 170-J biphasic shocks delivered 
50% less current than
360-J monophasic shocks.
Sixteen patients in the monophasic group could not be cardioverted with a maximal monophasic shock of 360 J. Eight of these patients (50%) were successfully cardioverted with a 170-J biphasic shock. Five patients in the biphasic group could not be cardioverted with a maximal biphasic shock of 170 J. These patients received a 360-J monophasic shock, which was also unsuccessful in each patient. There were no complications associated with either the monophasic or biphasic shocks.
The cumulative cardioversion efficacy was significantly affected by the
patient’s baseline transthoracic impedance (Figure 3). For patients with an impedance
70 , the cumulative efficacy of the biphasic waveform (29 of 29
patients, 100%) was equivalent to that of the monophasic waveform (27
of 28 patients, 96%; P=NS). In contrast, in patients with
an impedance >70 , the cumulative efficacy of the biphasic waveform
(53 of 58 patients, 91%) was significantly greater than that of the
monophasic waveform (30 of 44 patients, 68%; P=0.004).
Compared with patients with an impedance 70 (60±8 ), patients
with an impedance >70 (86±12 ) were heavier (98±24 versus
78±12 kg, P<0.0001) and had a larger left atrium (4.8±1.0
versus 4.5±0.8 cm, P=0.04). However, no significant
differences existed in weight, duration of atrial fibrillation, left
atrial size, LV ejection fraction, and transthoracic
impedance between patients treated with the monophasic and biphasic
waveforms in either the 70 or >70 groups.
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Use of a biphasic waveform (P=0.005), patient weight
(P=0.002), and baseline transthoracic impedance
(P=0.005) were univariate predictors of
successful cardioversion (Table 4). In
addition, a trend toward a lower success rate was observed in patients
with a longer duration of atrial fibrillation (P=0.08).
Multivariate logistic regression identified 3
variables independently associated with cardioversion success; they
were shock waveform used for cardioversion (P=0.02),
baseline transthoracic impedance (P=0.01), and
duration of atrial fibrillation (P=0.02). The adjusted
relative risks (95% confidence intervals) for cumulative efficacy were
4.2 (1.3, 13.9) for use of a biphasic shock waveform, 0.64 (0.46, 0.90)
for each 10- increase in transthoracic impedance, and
0.98 (0.97, 0.99) for each additional 30-day period of atrial
fibrillation (Table 5).
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Discussion |
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The principal finding of this study is that rectilinear biphasic shocks are significantly more effective than damped sine wave monophasic shocks for the transthoracic cardioversion of atrial fibrillation. Use of a rectilinear biphasic waveform was the most significant independent predictor of successful cardioversion. In addition to increased efficacy, rectilinear biphasic shocks could cardiovert with markedly less delivered current. The advantage of biphasic shocks was most pronounced in patients with a transthoracic impedance >70
. In the 3 decades since the introduction of transthoracic electrical cardioversion, 2 alternative nonpharmacological approaches have been introduced for patients who fail the conventional cardioversion of atrial fibrillation: dual external defibrillators using an orthogonal electrode array, resulting in a 720-J defibrillator discharge,3 and internal catheter-based cardioversion.4 The limitations of these alternatives include potential muscular damage from high-energy shocks3 and the attendant inconvenience and risks of the invasive internal approach. More recently, it has been suggested that the cardioversion efficacy of damped sine wave monophasic shocks can be improved with ibutilide pretreatment.21 However, this approach is limited by the increased cost associated with the use of ibutilide and the cost and inconvenience of 3 to 4 hours of continuous electrocardiographic monitoring after cardioversion to exclude ibutilide-induced torsade de pointes. In addition, the approach may not be applicable to patients with severely depressed LV systolic function because of an increased risk of torsade de pointes in these patients. Rectilinear biphasic cardioversion has no such limitation. Our study suggests that the routine use of rectilinear biphasic shocks for the transthoracic electrical cardioversion of atrial fibrillation should significantly reduce the need to resort to these less desirable alternatives.
Most currently available defibrillators use a damped sine wave monophasic waveform, which is not impedance-compensated. Although the operator is required to select a particular energy setting, it is known that transthoracic current is a more precise descriptor of defibrillation threshold.22 23 Because at a given energy setting the current delivered is dependent on transthoracic impedance, excessive current may be delivered to patients with low transthoracic impedance (increased toxicity), and insufficient current may be delivered to patients with high impedance (decreased efficacy).24 In fact, in this study, higher baseline transthoracic impedance was an independent predictor of shock failure.
In contrast to the damped sine wave monophasic waveform, the
rectilinear biphasic waveform is advantageous in high-impedance
patients because it is relatively insensitive to changes in
transthoracic impedance. This is because of impedance
compensation, which ensures a constant current in the first phase. For
example, the peak current delivered to a patient with a 125-
impedance from a monophasic shock is, on average, only 55% of that
delivered to a patient with a 50- impedance. In contrast, the peak
current delivered by a biphasic shock to a patient with a 125-
impedance is, on average, 68% of that delivered to a patient with a
50- impedance, thereby reducing the adverse effect of increased
transthoracic impedance on delivered
current.24 25 Consistent with this relationship
was the increased efficacy of the rectilinear biphasic shocks observed
in patients with an impedance >70 . On the basis of the mean human
transthoracic impedance of 70 ,24 25 the
increased efficacy of rectilinear biphasic shocks is pertinent to
50% of patients undergoing cardioversion.
It is important to note that increased efficacy with rectilinear biphasic shocks was achieved with significantly lower delivered current than with monophasic shocks. Previous studies during ventricular defibrillation have demonstrated that biphasic shocks, which defibrillate with less delivered current, result in less post-shock myocardial dysfunction than monophasic shocks.13 15 16 17
Prior Studies
Lown et al2 initially reported a cardioversion
efficacy of 90%; however, their patients differed significantly
from the patients who undergo electrical cardioversion in the current
era in that their patients were younger, most had rheumatic mitral
valve disease, and many were receiving treatment with quinidine. In
contrast, our patients were older, had a mixed cardiac origin of
atrial fibrillation, and were receiving a variety of antiarrhythmic
drugs, including amiodarone.
More recently, the reported efficacy of monophasic shocks for the cardioversion of atrial fibrillation has varied widely, ranging from 38% to 96%.2 3 4 5 6 26 27 This variation largely reflects differences in the baseline characteristics of patients selected for cardioversion. As in our study, prior studies have demonstrated that a longer duration of atrial fibrillation and an increased transthoracic impedance predict unsuccessful cardioversion.5 6 24 25 In this study, the superiority of the rectilinear biphasic waveform over the damped sine wave monophasic waveform was demonstrated in 2 groups who were similar at baseline with respect to variables reported to affect shock success, including weight, transthoracic impedance, and duration of atrial fibrillation.
Limitations
A potential limitation of this study is that it is not possible to
extrapolate the benefit observed with the rectilinear biphasic waveform
to other types of biphasic waveforms. In addition, it is unknown
whether other types of monophasic waveforms, unrelated to a damped sine
wave, would be associated with a higher success rate.
Conclusions
Rectilinear biphasic shocks have a significantly greater efficacy
than damped sine wave monophasic shocks for the
transthoracic electrical cardioversion of atrial
fibrillation. Additionally, increased efficacy with rectilinear
biphasic shocks is achieved with significantly less delivered current
than with monophasic shocks. The combination of increased efficacy and
decreased current requirements suggest that rectilinear biphasic shocks
may be the preferred method for the transthoracic
electrical cardioversion of atrial fibrillation.
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Appendix |
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Investigators and Participating Institutions
Johns Hopkins University Medical Center, Baltimore, Md: Hugh Calkins, MD (principal investigator), Rozann Hansford, RN; University of Michigan Medical Center, Ann Arbor, Mich: Bradley P. Knight, MD (principal investigator), Fred Morady, MD; The New York Hospital-Cornell Medical Center, New York, NY: Bruce B. Lerman, MD (principal investigator), Kenneth M. Stein, MD, Steven M. Markowitz, MD, Suneet Mittal, MD, David J. Slotwiner, MD, Marc A. Scheiner, MD, Maliza Sarmiento, RN, MA, ANP, Mary Wong, RN, MSN, ANP; MHSA Washington Hospital Center, Washington, DC: Edward V. Platia, MD (principal investigator), Jean Fenton, RN, MSN, Dulce Manno, RN; University of Pittsburgh Medical Center, Pittsburgh, Pa: David Schwartzman, MD (principal investigator), Doris Cavlovich, RN, BSN; Cleveland Clinic Foundation, Cleveland, Ohio: Patrick J. Tchou, MD (principal investigator), Donald R. Holmes, RN, MSN; and Duke University Medical Center, Durham, NC: J. Marcus Wharton, MD (principal investigator), Catherine Grill, RN, BSN.
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Acknowledgments |
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This work was supported in part by grants from the National Institutes of Health (RO1 HL-56139) and the Zoll Medical Corporation.
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Footnotes |
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Mr Ayati is an employee of Zoll Medical Corporation, which makes the defibrillators and gel pads used in this study. The remaining authors, including Dr Lerman who is a consultant to the Zoll Medical Corporation, do not have a financial interest in these devices.
Received July 21, 1999; revision received September 29, 1999; accepted October 12, 1999.
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