Author + information
- Received February 11, 1999
- Revision received June 18, 1999
- Accepted June 30, 1999
- Published online November 1, 1999.
- Suneet Mittal, MDa,
- Shervin Ayati, MSEE∗,
- Kenneth M. Stein, MD, FACCa,
- Bradley P. Knight, MD†,
- Fred Morady, MD, FACC†,
- David Schwartzman, MD, FACC‡,
- Doris Cavlovich, RN, BSN‡,
- Edward V. Platia, MD, FACC§,
- Hugh Calkins, MD, FACC∥,
- Patrick J. Tchou, MD, FACC∥,
- John M. Miller, MD, FACC#,
- J.Marcus Wharton, MD∗∗,
- Ruey J. Sung, MD, FACC††,
- David J. Slotwiner, MDa,
- Steven M. Markowitz, MD, FACCa,
- Bruce B. Lerman, MD, FACCa,* (, )
- for the ZOLL Investigators‡‡
- ↵*Reprint requests and correspondence: Dr. Bruce B. Lerman, Division of Cardiology, The New York Hospital–Cornell Medical Center, 525 East 68th Street, Starr 4, New York, New York 10021
We compared the efficacy of a novel rectilinear biphasic waveform, consisting of a constant current first phase, with a damped sine wave monophasic waveform during transthoracic defibrillation.
Multiple studies have shown that for endocardial defibrillation, biphasic waveforms have a greater efficacy than monophasic waveforms. More recently, a 130-J truncated exponential biphasic waveform was shown to have equivalent efficacy to a 200-J damped sine wave monophasic waveform for transthoracic ventricular defibrillation. However, the optimal type of biphasic waveform is unknown.
In this prospective, randomized, multicenter trial, 184 patients who underwent ventricular defibrillation were randomized to receive a 200-J damped sine wave monophasic or 120-J rectilinear biphasic shock.
First-shock efficacy of the biphasic waveform was significantly greater than that of the monophasic waveform (99% vs. 93%, p = 0.05) and was achieved with nearly 60% less delivered current (14 ± 1 vs. 33 ± 7 A, p < 0.0001). Although the efficacy of the biphasic and monophasic waveforms was comparable in patients with an impedance <70 Ω (100% [biphasic] vs. 95% [monophasic], p = NS), the biphasic waveform was significantly more effective in patients with an impedance ≥70 Ω (99% [biphasic] vs. 86% [monophasic], p = 0.02).
This study demonstrates a superior efficacy of rectilinear biphasic shocks as compared with monophasic shocks for transthoracic ventricular defibrillation, particularly in patients with a high transthoracic impedance. More important, biphasic shocks defibrillated with nearly 60% less current. The combination of increased efficacy and decreased current requirements suggests that biphasic shocks as compared with monophasic shocks are advantageous for transthoracic ventricular defibrillation.
Multiple studies have shown that for endocardial defibrillation, biphasic waveforms have a greater efficacy than monophasic waveforms (1–6). More recently, a 130-J trun-cated exponential biphasic waveform was shown to be as effective as a 200-J damped sine wave monophasic waveform for transthoracic ventricular defibrillation (7,8). In addition to increased efficacy, biphasic waveforms, which require less voltage and current for ventricular defibrillation than damped sine wave monophasic waveforms, may result in less post-shock myocardial dysfunction (8–12).
Despite the overall advantages of biphasic waveforms, the optimal type of biphasic waveform is unknown. Variables that may affect the efficacy of the biphasic waveform include phase duration (13,14), first-phase tilt (15)and second-phase peak voltage (16,17). A potential limitation of currently available truncated exponential biphasic waveforms is the large drop in current during energy delivery, which limits the average effective current delivered (18). As a result, to maintain an effective average current, a high initial peak current must be delivered, which may be detrimental for myocardial function.
In this study, we evaluated a novel rectilinear biphasic waveform that had a constant-current first phase. The potential advantage of a constant-current first phase is that it maintains an effective average current over the entire initial phase of energy delivery without exposing the myocardium to a high initial peak current. We compared the efficacy of this waveform with that of a damped sine wave monophasic waveform for transthoracic ventricular defibrillation.
This study was a prospective, randomized, multicenter trial comparing the efficacy of a 120-J rectilinear biphasic and a 200-J damped sine wave monophasic waveform during transthoracic ventricular defibrillation. During the study period between November 1997 and August 1998, 330 patients were enrolled from nine centers. Patients were eligible for study if they were undergoing an electrophysiologic study, transvenous implantable cardioverter-defibrillator (ICD) placement, ICD generator replacement or ICD testing. Patients were ineligible if they were <18 years of age, pregnant or currently enrolled in a research study for a medication or device. The Institutional Review Board at each participating institution approved the investigational protocol. Informed written consent was obtained from all patients.
The primary hypothesis of the study was that a 120-J biphasic shock would have the same first-shock defibrillation efficacy as a 200-J monophasic shock. Assuming a first-shock success rate of ≥85% with the 200-J damped sine wave monophasic waveform and ≥70% with the 120-J rectilinear biphasic waveform, a study sample of 154 patients was required to detect a >15% difference in efficacy between the monophasic and biphasic waveforms with a power of 80% and a significance level of 0.05.
Figure 1depicts a representative 200-J damped sine wave monophasic waveform and a 120-J rectilinear (constant-current first phase) biphasic waveform delivered across a 50-Ω load. The monophasic waveforms were generated from a ZOLL PD-2000 defibrillator (Burlington, Massachusetts) by delivering the stored charge on a 45-μF capacitor through a 20-mH inductor and an internal resistance of 14 Ω. Biphasic waveforms were generated from a 100-μF capacitor using the ZOLL PD-2100 defibrillator. The biphasic waveform consisted of a constant-current 6-ms first-phase pulse followed by a truncated exponential 4-ms second-phase pulse. The time between the trailing edge of the first phase and the leading edge of the second phase was 100 μs. For biphasic shocks, a constant-current first phase was maintained by automatically adjusting the variable internal impedance based on the patient’s transthoracic impedance, which was determined at the onset of shock delivery. Switching resistors 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. The effect of transthoracic impedance on delivered current for the monophasic and biphasic waveforms is outlined in Table 1.
An initial 120-J biphasic shock was chosen for several reasons. First, for the waveforms used in this study, the mean ratio of biphasic threshold current to peak damped sine wave monophasic current was ∼0.37 ± 0.04 (unpublished data). Because the mean current required for defibrillation with a monophasic damped sine wave is 25 to 30 A (19,20), we surmised that the biphasic waveform should deliver ∼9 to 11 A. The 120-J rectilinear biphasic waveform delivers between 12 and 16 A for transthoracic impedances between 50 and 125 Ω (Table 1). To confirm our assumptions, we conducted a pilot human study involving 20 patients (15 men, age 67 ± 11 years) during 22 episodes of an induced ventricular tachyarrhythmia requiring defibrillation. The first 10 patients (10 total episodes) were treated with a 150-J biphasic shock, which was effective in all of them. The next 10 patients (12 total episodes) were treated with a 120-J biphasic shock. The first-shock efficacy was 92% (11 of 12 episodes). We therefore elected to use a 120-J biphasic shock in the present study.
Adhesive pads (ZOLL Radiolucent Stat-Padz) for transthoracic defibrillation were applied to the cardiac apex, to the left fifth intercostal space in the anterior axillary line and to the left scapula posteriorly. The apical electrode had an active surface area of 78 cm2and the posterior electrode had an active surface area of 113 cm2. The defibrillation pads were connected to a multiple-defibrillation interface unit, which in turn was connected to the monophasic and biphasic defibrillators. The interface unit measured the voltage and current delivered to the patient as well as the patient’s impedance. This information was transmitted to a laptop personal computer that was also connected to the interface unit.
During electrophysiologic study or ICD testing, ventricular fibrillation, ventricular flutter or polymorphic or rapid monomorphic (cycle length ≤300 ms) ventricular tachycardia was induced with either alternating current, programmed stimulation, a T-wave shock or burst ventricular pacing.
Patients were randomized, using a simple block randomization scheme, to either the monophasic (Fig. 2)or biphasic waveform (Fig. 3)protocol. In brief, patients randomized to the monophasic protocol received sequential shocks of 200 J, 300 J and 360 J, if necessary. Patients randomized to the biphasic protocol received sequential shocks of 120 J, 150 J and 170 J, if necessary.
To obtain maximal experience with the biphasic waveform, all patients, regardless of randomization, were treated according to the biphasic waveform protocol during subsequent inductions of a ventricular tachyarrhythmia. No patient underwent defibrillation with a previously unsuccessful waveform. Successful defibrillation was defined as conversion of ventricular tachycardia or ventricular fibrillation to a supraventricular or paced rhythm associated with a measurable blood pressure and pulse that persisted for at least 30 s post shock. A minimum of 3 min was allowed between serial inductions of ventricular fibrillation.
All continuous variables are expressed as the mean value ± SD. Comparisons of dichotomous and continuous variables between the monophasic and biphasic waveform groups were calculated using the chi-square test and the Student ttest, respectively. The first-shock efficacy of the 120-J biphasic and 200-J monophasic shocks were compared using the Fisher exact test. For all comparisons, p ≤ 0.05 was considered to be statistically significant.
Of the 330 patients enrolled in the study, 194 required transthoracic defibrillation. The 136 patients who did not undergo defibrillation were excluded from further analysis. An additional 10 patients were excluded owing to cardioversion of monomorphic ventricular tachycardia with a cycle length >300 ms (n = 5) (21), insufficient data collection (n = 2), incorrect randomization (n = 2) and inadvertent duplicate enrollment (n = 1). The remaining 184 patients constituted the study group.
The patients’ mean age was 63 ± 14 years and the left ventricular (LV) ejection fraction was 30 ± 15%. Coronary artery disease was present in 69% of patients. Of the 184 patients who underwent defibrillation, 98 (53%) were randomized to the biphasic group and 86 (47%) to the monophasic group. The two groups were similar with respect to age, gender, weight, body surface area, LV ejection fraction, underlying cardiac disease, type of study performed, use of medications (including amiodarone) and type of induced ventricular arrhythmia (Table 2).
The 184 patients underwent defibrillation of 219 episodes of a ventricular tachyarrhythmia. For the first induction of a ventricular tachyarrhythmia, first-shock efficacy was observed in 97 (99%) of 98 patients receiving a 120-J biphasic shock. In contrast, first-shock efficacy was observed in only 80 (93%, p = 0.05) of 86 patients receiving a 200-J damped sine wave monophasic shock. In addition, successful defibrillation with biphasic shocks was achieved with 58% less voltage (915 ± 168 vs. 2166 ± 262 V, p < 0.0001) and 58% less current (14 ± 1 vs. 33 ± 7 A, p < 0.0001) than that delivered with monophasic shocks (Table 3).
Defibrillation for a second induction of a ventricular tachyarrhythmia was required in 29 patients and for a third induction in six patients. No patient was induced more than three times. Twenty-six of the 29 patients who underwent defibrillation for a second induction were treated with the 120-J biphasic waveform, which was effective in all 26 patients. Three of the 29 patients during a second induction were inadvertently defibrillated with a 200-J monophasic shock rather than a 120-J biphasic shock. The shock was effective in two of the three patients. However, because repeat monophasic shocks constituted a protocol violation, these three patients were excluded from further analysis. All six patients who underwent a third induction were treated with the 120-J biphasic waveform, which was effective in all six patients. Therefore, the first-shock success for all inductions treated with the biphasic waveform (129 [99%] of 130 episodes) was significantly greater than that treated with the monophasic waveform (80 [93%] of 86 episodes, p = 0.02).
The only patient who initially failed a 120-J biphasic shock was successfully defibrillated with a second 150-J biphasic shock. No patient in this study required a 170-J biphasic shock. Six patients failed an initial 200-J monophasic shock. Of these 6 patients, 3 were converted with a second 300-J monophasic shock, 1 required a 360-J monophasic shock (after an additional failure with 300 J) and 2 received a rescue ICD shock before delivery of a 300-J monophasic shock. There were no complications related to either monophasic or biphasic shocks.
The patients’ age, gender, weight, body surface area, LV ejection fraction and underlying cardiac disease did not affect defibrillation efficacy. However, defibrillation efficacy was significantly influenced by the patient’s impedance. In patients with a transthoracic impedance <70 Ω, the first-shock efficacy of the monophasic waveform (40 [95%] of 42 episodes) was comparable to that of the biphasic waveform (58 [100%] of 58 episodes, p = NS). However, in patients with a transthoracic impedance ≥70 Ω, the first-shock efficacy of the biphasic waveform was greater than that of the monophasic waveform (67 [99%] of 68 vs. 24 [86%] of 28 episodes, p = 0.02). A similar advantage for the biphasic waveform was observed in patients with an impedance ≥90 Ω (19 [100%] of 19 vs. 5 [63%] of 8 episodes for the monophasic waveform, p = 0.02).
The principal finding of this study is that transthoracic ventricular defibrillation with 120-J rectilinear biphasic shocks is more efficacious than that with 200-J damped sine wave monophasic shocks, particularly in patients with a high transthoracic impedance. In addition, successful defibrillation with biphasic shocks could be achieved with nearly 60% less current than that delivered with monophasic shocks.
Although most currently available defibrillators use an energy-calibrated method for defibrillation, transthoracic current appears to be a more precise electrical descriptor of defibrillation threshold (19,22). Defibrillation using an energy-calibrated method may be suboptimal because, for a given energy, current is dependent on transthoracic impedance (Table 1). Therefore, insufficient current may be delivered to patients with high impedance (decreased efficacy), and excessive current may be delivered to patients with low transthoracic impedance (increased toxicity). Manifestations of toxicity include myocardial dysfunction (23,24), reflected by ST segment changes as well as a decline in the cardiac index, and provocation of arrhythmias (10,25), including ventricular tachyarrhythmias and atrioventricular block. Efforts to circumvent this limitation have resulted in defibrillator designs that vary the waveform duration, automatically increase energy output in patients (26)with high impedance or deliver a fixed current dose independent of the patient’s impedance (18,26).
The results of this study also suggest that a rectilinear biphasic waveform offers an advantage with respect to patients with high impedance, because it is not affected by changes in the transthoracic impedance to the same degree as the damped sine wave monophasic waveform (Table 1). For example, the peak current delivered to a patient with 125-Ω impedance from a 200-J damped sine wave monophasic shock is only 56% of that delivered to patient with 50-Ω impedance. In contrast, the first-phase current delivered by a 120-J rectilinear biphasic shock to a patient with 125-Ω impedance is 75% of that delivered to a patient with 50-Ω impedance, thus reducing the adverse effect of an increased transthoracic impedance on delivered current (27). Consistent with this potential advantage was the greater first-shock success of the biphasic waveform in patients with ≥70 Ω impedance. It is important to note that on the basis of the spectrum of transthoracic impedance observed in this study, this advantage is applicable to ∼50% of patients undergoing defibrillation.
Several studies of endocardial defibrillation as well as one recent study of transthoracic defibrillation have demonstrated lower defibrillation threshold voltage and current with biphasic waveforms as compared with monophasic waveforms (1–8). Regardless of the mechanism for this benefit, which may be related to recovery of sodium channel activity during the first phase of the biphasic shock (28), it is generally accepted that defibrillation with lower voltage and current is desirable owing to a decreased likelihood of post-shock myocardial dysfunction (8–12).
There is one other randomized study that compared the efficacy of monophasic versus biphasic waveforms during transthoracic ventricular defibrillation (8). This study compared truncated exponential biphasic waveforms (115 J and 130 J) generated with a 95-μF capacitor with damped sine wave monophasic waveforms (200 J and 360 J) generated with a 32-μF capacitor through a 50-mH inductor with 10-Ω internal resistance. The individual phase duration, total duration and tilt of the biphasic waveform varied according to the transthoracic impedance of the patient.
An equivalent first-shock efficacy was demonstrated for the 130-J biphasic (86% success) and the 200-J (86% success) waveforms. The greatest efficacy was demonstrated for the 360-J damped sine wave monophasic shock, which had a first-shock efficacy of 96%. This has raised concern that low energy biphasic shocks may be less efficacious than 360-J damped sine wave monophasic shocks (29). However, the first-shock efficacy for the 120-J biphasic waveform in our study was 99%, which approximates the efficacy of the 360-J damped sine wave monophasic shock in the previous study. It remains to be determined whether differences in efficacy between our study and the previous study are related to differences in the biphasic waveforms. A direct comparison between the truncated exponential and rectilinear biphasic waveforms was not currently possible because the former was not available for hospital-based defibrillation.
This study demonstrates that 120-J rectilinear biphasic shocks have a greater efficacy than 200-J damped sine wave monophasic shocks for transthoracic ventricular defibrillation, particularly in patients with a high transthoracic impedance. In addition, ventricular defibrillation was achieved with less delivered current. The combination of increased efficacy and decreased current requirements suggests that biphasic shocks as compared with monophasic shocks are advantageous for transthoracic ventricular defibrillation.
Investigators and participating institutions
Hugh Calkins, MD, and Rozann Hansford, RN (Johns Hopkins University Medical Center, Baltimore, Maryland); Bradley P. Knight, MD, Fred Morady, MD, and Karin M. Brinkman, MS, (University of Michigan Medical Center, Ann Arbor, Michigan); Bruce B. Lerman, MD, Kenneth M. Stein, MD, Steven M. Markowitz, MD, Suneet Mittal, MD, David J. Slotwiner, MD, Maliza Sarmiento, RN, MA, ANP, and Mary Wong, RN, MSN, ANP (New York Hospital–Cornell Medical Center, New York, New York); John M. Miller, MD, and Lisa Thome (Temple University Medical Center, Philadelphia, Pennsylvania); Edward V. Platia, MD, Jean Fenton, RN, MSN, and Dulce Manno, RN, MHSA (Washington Hospital Center, Washington, DC); David Schwartzman, MD, and Doris Cavlovich, RN, BSN (University of Pittsburgh Medical Center, Pittsburgh, Pennsylvania); Ruey J. Sung, MD, and Michelle Lamoureux, RN, MS (Stanford University Hospital, Palo Alto, California); Patrick J. Tchou, MD, and Donald R. Holmes, RN, MSN (Cleveland Clinic Foundation, Cleveland, Ohio); and J. Marcus Wharton, MD, and Catherine Grill, RN, BSN (Duke University Medical Center, Durham, North Carolina).
☆ This work was supported in part by grants from the National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland (RO1–HL56139), the Rosenfeld Foundation, the Michael Wolk Foundation, New York, New York, and the Zoll Medical Corporation, Burlington, Massachusetts.
- implantable cardioverter-defibrillator
- left ventricle or ventricular
- Received February 11, 1999.
- Revision received June 18, 1999.
- Accepted June 30, 1999.
- American College of Cardiology
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