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(Circulation. 1995;91:91-95.)
© 1995 American Heart Association, Inc.

A Prospective Randomized Comparison in Humans of Biphasic Waveform 60-µF and 120-µF Capacitance Pulses Using A Unipolar Defibrillation System

Gust H. Bardy, MD; Jeanne E. Poole, MD; Peter J. Kudenchuk, MD; G. Lee Dolack, MD; Rahul Mehra, PhD; Paul DeGroot, MS; Merritt H. Raitt, MD; Gregory K. Jones, MD; George Johnson, BSEE

From the Department of Medicine, University of Washington (Seattle).

Correspondence to Gust H. Bardy, MD, Mail Stop RG-22, University Hospital, Seattle, WA 98195.

	Abstract

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Background Improving unipolar implantable cardioverter-defibrillator (ICD) effectiveness has favorable implications for ICD safety, efficacy, and size. Advances in defibrillation efficacy would accelerate ICD ease of use by decreasing device size and by minimizing morbidity and mortality related to an improved defibrillation safety margin. The specific purpose of the present study was to determine whether unipolar defibrillation efficacy could be improved further in humans by lowering biphasic waveform capacitance.

Methods and Results We prospectively and randomly compared the defibrillation efficacy of a 60-µF and a 120-µF capacitance asymmetrical 65% tilt biphasic waveform using a unipolar defibrillation system in 38 consecutive cardiac arrest survivors before implantation of a presently available standard transvenous defibrillation system. The right ventricular defibrillation electrode had a 5-cm coil located on a 10.5F lead and was used as the anode. The system cathode was the electrically active 108-cm² surface area shell (or "can") of a prototype titanium alloy pulse generator placed in a left infraclavicular pocket. The defibrillation pulse was derived from either a 60-µF or a 120-µF capacitance and was delivered from RV CAN. Defibrillation threshold (DFT) stored energy, delivered energy, leading-edge voltage and current, pulse resistance, and pulse width were measured for both capacitances examined. The 60-µF capacitance biphasic pulse resulted in a stored-energy DFT of 8.5±4.1 J and a delivered-energy DFT of 8.4±4.0 J. In 34 of 38 patients (89%), the stored-energy DFT was <15 J. Leading-edge voltage at the DFT was 517±128 V. Mean pulse impedance for the 60-µF waveform was 60.6±7.1 . The 120-µF capacitance biphasic pulse resulted in a stored-energy DFT of 10.1±7.4 J and a delivered-energy DFT of 10.0±7.2 J (P=.13 and .13, respectively). In 28 of 38 patients (74%), the stored-energy DFT was <15 J (P=.052). Leading-edge voltage at the DFT with the 120-µF capacitance pulse was 386±142 (P<.00001). Mean pulse impedance for the 120-µF waveform was 60.7±7.0 (P=.80).

Conclusions The results of the present study suggest that a relatively small capacitance, 60 µF, can be used for unipolar defibrillation systems without compromising defibrillation energy requirements compared with more typical ICD capacitance values, but this will require a higher circuit voltage. The use of lower capacitance also provides a modest increase in the percent of patients who have very low energy defibrillation requirements, an important issue should maximum ICD energy be decreased from the present level of 34 J. Such a move to smaller output devices could allow significant decreases in device size, a necessary feature of making cardioverter-defibrillator implantation comparable to that of standard pacemaker surgery.

Key Words: death, sudden • fibrillation • tachycardia • defibrillation

	Introduction

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Unipolar transvenous defibrillation systems have been demonstrated to decrease defibrillation energy requirements into the range of epicardial systems and be nearly as easy to insert as a pacemaker.¹ This type of defibrillation system is potentially important in decreasing the mortality, morbidity, and costs of defibrillator use and may open the doors to prophylactic utilization of such devices for the prevention of sudden cardiac death. Despite an attractive size for an implantable cardioverter-defibrillator (ICD), the present unipolar system remains relatively large for pectoral implantation having a volume of 80 mL, similar to early pacemaker models, and consequently reduces the ease and safety of use. When ICD size approaches that of today's pacemaker, procedure morbidity and costs will decrease.

Improving defibrillation efficacy is one way to favorably influence defibrillator size. If a defibrillation pulse and lead system were capable of terminating ventricular fibrillation in 90% of individuals using 15 J, maximum ICD shock strength could decrease to 25 J from its present standard of 34 J and still provide an adequate safety margin in most patients. An ICD with a maximum output of 25 J would reduce overall ICD size by using a smaller capacitor and possibly a smaller battery.

Previous efforts to improve defibrillation efficacy of unipolar defibrillation systems have not proved helpful and have included the addition of a superior vena caval or a coronary sinus electrode to the unipolar system as well as a change in the waveform tilt.^{2 3 4} Capacitance has previously been explored systematically as a defibrillation variable as early as 1970⁵ ; however, the effect of capacitance with the unipolar ICD system remains unexplored. Thus, the specific purpose of this study was to test in a prospective randomized fashion in humans whether a 60-µF capacitance biphasic waveform defibrillates better than the more standard 120-µF capacitance biphasic waveform when used with a unipolar defibrillation system. If so, it might be possible to further decrease defibrillator size.

	Methods

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Patient Population
After we obtained informed verbal and written consent from all participants, a comparison of 65% tilt 60-µF and 120-µF capacitance biphasic pulses (Fig 1) was undertaken in a randomized fashion in 38 consecutive patients with ventricular fibrillation (VF) and/or syncopal ventricular tachycardia (VT) before implantation of a standard transvenous defibrillator.^{6 7}

View larger version (13K): [in this window] [in a new window]   Figure 1. Oscilloscopic recordings of the 60-µF and 120-µF capacitance biphasic waveforms under study.

Unipolar Defibrillation Lead System
The unipolar transvenous defibrillation system has been fully described previously.¹ Briefly, it consists of a single 10.5F anodal 5-cm-long endocardial right ventricular (RV) defibrillation electrode (Medtronic model 6966) and a 108-cm²-surface-area cathodal pulse generator titanium shell electrode (Medtronic model 7219C) that was positioned in a left infraclavicular pocket. The RV endocardial electrode was inserted into the left cephalic vein when possible; otherwise, this lead was inserted into the left subclavian vein. The endocardial RV lead also had standard bipolar pace/sense electrodes at the tip. Ultimately, the RV lead was used as part of the permanently implanted standard transvenous defibrillation system.^{6 7}

Defibrillation Threshold Testing
Defibrillation threshold testing was performed in a randomized fashion for both waveforms under study. The first transvenous defibrillation test began with a 400-V leading-edge voltage pulse delivered 10 seconds after VF onset, including the time period during which alternating current was applied. If the transvenous pulse was unsuccessful, a 100- to 200-J transthoracic rescue pulse was delivered immediately via a precharged external defibrillator (Physio-Control LifePak 6s) and anteroposterior cutaneous pads (Darox Corporation).

After a minimum rest period of 3 minutes between VF inductions, pulse output was increased or decreased, depending on transvenous shock failure or success. Pulse voltages were changed in 100-V steps between voltages of 400 to 900 V and in 50-V steps below 400 V. Between each induction and termination of VF, care was taken to ensure that ECG ST-T segments, QRS duration, and arterial pressure had returned to baseline values before VF was reinitiated.

The defibrillation threshold was defined as the lowest pulse amplitude that could successfully terminate VF 10 seconds after its initiation. After the defibrillation threshold was determined for the first capacitance tested, the defibrillation protocol was repeated for the other capacitance. All defibrillation pulse characteristics were measured from oscilloscopic recordings of voltage and current waveforms as previously described.⁸

Statistical Analysis
The size of the study population was determined for a power of 80% likelihood to observe a 25% difference at a P<.05 level. The 25% difference, or 2.3 J, was deemed a clinically significant improvement from the present average defibrillation threshold value (9.3±6.0 J) for 120 µF 65% tilt unipolar biphasic RV CAN pulses.¹ The estimated standard deviation of the differences between the defibrillation threshold for the two capacitances in this study was 5.0 J. The minimum population sample size needed with these assumptions was 38 patients.

A paired two-tailed t test was used to compare defibrillation thresholds for the two biphasic waveform capacitances under study. A ² comparison for the paired data was done for percent defibrillation efficacy at 15 J for each capacitance. A linear regression analysis was conducted to determine whether a relation existed between pulse resistance and defibrillation efficacy.

	Results

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Patient Clinical Characteristics
Of the 38 patients studied, 28 (74%) were men. Mean age was 61.3±10.0 years with a range of 38 to 79 years. Coronary artery disease was the primary structural heart disease in 20, 6 had a dilated cardiomyopathy, 8 had both coronary disease and a dilated cardiomyopathy, 2 had primary electrical disease, 1 had hypertrophic cardiomyopathy, and 1 had long QT syndrome. Mean left ventricular ejection fraction was 0.37±0.17 (range, 0.11 to 0.75). The index arrhythmia leading to device implantation was VF in 17 patients, VT in 13 patients, and both VT and VF in 8 patients.

Defibrillation Thresholds
The defibrillation threshold data are given in detail in the Table. The defibrillation threshold stored-energy for the reference 120-µF capacitance biphasic 65% tilt unipolar defibrillation system was 10.1±7.4 J, with a range of 1.7 to 38.4 J (Fig 2). The defibrillation threshold delivered energy for the 120-µF capacitance biphasic pulse was 10.0±7.2 J, with a range of 1.7 to 37.8 J. The measured leading-edge voltage defibrillation threshold for the 120-µF capacitance biphasic unipolar defibrillation system was 386±142 V, with a range of 169 to 800 V (Fig 3). The measured leading-edge current defibrillation threshold for the 120-mF capacitance biphasic pulse was 6.5±2.6 A, with a range of 2.6 to 13.8 A. The measured-leading edge resistance at the defibrillation threshold was 60.7±7.0 , with a range of 38.8 to 73.1 . Total pulse width was 17.6±1.8 milliseconds.

View this table: [in this window] [in a new window]   Table 1. Pulse Measures at the Defibrillation Threshold

View larger version (19K): [in this window] [in a new window]   Figure 2. Paired defibrillation threshold stored-energy data for the two biphasic waveform capacitances studied.

View larger version (23K): [in this window] [in a new window]   Figure 3. Paired defibrillation threshold leading-edge voltage data for the two biphasic waveform capacitances studied.

In the case of the 60-µF capacitance 65% tilt biphasic waveform, the mean defibrillation threshold stored-energy was 8.5±4.1 J, with a range of 2.2 to 19.2 J (P=.13) (Fig 2). The defibrillation threshold delivered-energy for the 60-µF capacitance biphasic pulse was 8.4±4.0 J, with a range of 2.2 to 18.9 J. The measured leading-edge voltage at the defibrillation threshold for the 60-µF capacitance biphasic waveform was 517±128 V, with a range of 273 to 800 V (P<.00001) (Fig 3). The measured leading edge current defibrillation threshold for the 60-F capacitance biphasic pulse was 8.7±2.5 A, with a range of 4.3 to 14.0 A (P<.00001). Pulse resistance at the defibrillation threshold was 60.6±7.1 , with a range of 39.5 to 79.5 (P=.80). Total pulse width was 8.8±1.1 milliseconds (P<.00001).

Percent Defibrillation Efficacy
For the 120-µF capacitance biphasic pulse, 28 of 38 patients (74%) were defibrillated by <15 J stored energy. For the 60-µF capacitance biphasic pulse, 34 of 38 patients (89%) were defibrillated by <15 J stored energy (P=.052).

Effect of Resistance on Defibrillation Efficacy
Because resistance can alter the waveform decay constant, the relation of resistance to defibrillation efficacy was examined. There was no relative benefit of one capacitance over the other regarding stored-energy defibrillation threshold and pulse resistance. The ratio of stored-energy defibrillation thresholds for the two capacitance outputs failed to show a relation to pulse impedance (r=-.10, P=.56; Fig 4).

View larger version (10K): [in this window] [in a new window]   Figure 4. Plot of effect of pulsing resistance on the relative efficacy of 60-µF and 120-µF capacitance biphasic pulses as expressed in the ratio of stored-energy defibrillation threshold data.

	Discussion

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Role of Capacitance
The theoretical basis for the effect of capacitance on the defibrillation threshold for a truncated exponentially decaying waveform is not established. One might postulate, however, an improvement in defibrillation for lower capacitance pulses based on the average current hypothesis.⁹ This hypothesis, although conceptually useful in contemplating the effect of capacitance on monophasic shock efficacy, antedates the use of biphasic shocks and is probably inapplicable to biphasic waveforms, or any waveform with multidirectional current flow, thereby explaining why the empirical findings of this study failed to confirm this concept. It may also be the case that the impact of biphasic pulsing is strong enough to overwhelm any effect of capacitance on outcome and override the average current hypothesis.

On a practical level, one of the goals of using a smaller capacitance in the present study was to allow for a reduction in ICD size. However, altering capacitance will not decrease ICD size unless the energy required for storage on the capacitor can be decreased because capacitor size is proportional to the energy stored on it. Therefore, reducing the capacitance to 60 µF is not meaningful if it does not reduce the stored-energy needed for defibrillation.

Although the outcome of the study with respect to defibrillation thresholds (DFTs) does not strongly argue for the use of lower capacitance, it is interesting to note that a greater percentage of patients could be defibrillated with 60-µF than with 120-µF output capacitance at a 15-J output (89% versus 74%, P=.052). A comparison of capacitance effect based on a 15-J cutoff is important when considering ICD size. For example, the 60-µF output capacitance would have allowed device implantation in a greater percentage of patients than a 120-µF capacitance device if one were to use an ICD with a maximum output that had been reduced to 25 J. For patients with a DFT 15 J, a 25-J device would be clinically acceptable as it would provide a defibrillation safety margin within the context of current practice. A lower maximum ICD energy would result in a smaller capacitor volume and therefore a smaller ICD.

Corresponding Effect of Tilt and Pulse Width
In any study of capacitance on defibrillation efficacy, one must consider the independent defibrillation variables of tilt and pulse width as each affects the amount of charge and energy delivered to the tissues. Consequently, the optimal tilt and pulse width for a 60-µF capacitance pulse may be different than that for a 120-µF capacitance output. Use of a different tilt than the 65% used in the present study, or use of a fixed pulse width, may have changed our study outcome. The selection of a 65% tilt for this study was based on an earlier investigation examining the effect of tilt on RV CAN defibrillation: it showed no significant difference between 65% and 50% tilt at 120-µF capacitance.⁴ The pertinent question for the present study is whether the defibrillation thresholds with 60-µF capacitance and 120-µF capacitance would be different if measurements would have been made at their respective optimal tilts or pulse widths. The optimal tilt or pulse width at each of these capacitances for unipolar defibrillation systems is not known and could differ. That said, it is important to recognize that most of the clinical studies indicate that there is no significant difference in defibrillation thresholds with 120-µF capacitance when 50% and 65% tilts are compared using several different lead systems.^{4 10 11} Another recent clinical study differs from these reports, however, arguing that 42% tilt biphasic pulses reduce the defibrillation threshold compared with 65% tilt biphasic pulses when a 120-µF capacitance was used.¹² This finding may have been a consequence of the significantly lower average resistance seen in this study (28±6 , J.F. Swartz, personal communication, unpublished data), which may have resulted from the simultaneous, dual-pathway coronary sinus defibrillation that was used. The average impedance in the other studies ranged between 54 and 65 .^{4 10 11} These findings suggest that tilt may need to be customized to lead system impedance whenever capacitance is altered.¹³

There have been few clinical studies controlling for the effect of pulse width on defibrillation, a waveform characteristic that can be pertinent to the present study if one considers the potential for high capacitance pulses to prolong the pulse duration. In one earlier clinical study, we examined the effect on defibrillation of varying pulse width for sequential 60-µF monophasic pulses when using an epicardial three-patch system.¹⁴ We were able to demonstrate a progressive deterioration in defibrillation efficacy as a function of delivered-energy as pulse width increased from 2.5 to 5.0 to 7.5 milliseconds. Although the lead system and waveform were considerably different than those used in the present study, this study does supply evidence to suggest that a pulse that is too long is detrimental for defibrillation. These findings are consistent with studies in dogs indicating that excessively long pulses may be less effective.^{5 15} In this light, one must consider the possibility that high capacitance shocks can prolong pulse width for any particular tilt and may prove to be a disadvantageous waveform design despite yielding higher shock energies.

Can Size and Defibrillation Efficacy
Any effort to diminish ICD size for an active can system, regardless of means, has the potential to affect defibrillation adversely by diminishing the electrically active can surface area. If one were to use a smaller capacitance and ICD size were consequently reduced, the resulting reduction in ICD surface area could be countered to some degree by changing ICD shape, for example, by flattening the ICD more than diminishing its frontal area or by increasing the surface complexity, ie, with fractal folds. Although this problem will ultimately require attention, it is unlikely to be clinically evident for several years. We have looked at defibrillation efficacy for 40-mL and 60-mL cans of similar size, shape, and surface material to the 80-mL can used in the present study and have not found a decrease in defibrillation efficacy.¹⁶ We suspect the knee of the defibrillation efficacy curve, as referenced to ICD volume, will eventually be encountered, although, as can volumes get smaller than 40 mL.

Study Limitations
One limitation of the present study relates to the type of pulsing technique and lead system used to examine the relative efficacy of the two capacitances under study. It is conceivable that a different electrode configuration with different impedance characteristics or a more dramatic variation in waveform capacitance or fixation of pulse width rather than tilt would have resulted in a different conclusion to that found in the present study. This said, the system used is a pragmatic one and has the advantage of being in widespread clinical practice.

An additional limitation regards the statistical power of our results. Because of the high variability of the difference in defibrillation thresholds for 60 µF and 120 µF, the statistical power of the present study is such that the minimum detectable difference is 2.9 J, or 29% of the mean defibrillation threshold for 120 µF. This is higher than the stated goal of detecting a 2.3-J difference and is significantly greater than the 1.6-J mean difference actually observed in the study. In fact, using the results for power calculations rather than previously published data, as done at the outset of this study, the power of establishing the stated goal is 61%. To significantly establish a difference of <2.9 J, a greater number of patients would have been required than our power calculations initially indicated.

Conclusions
The results suggest that a relatively small capacitance can be used for unipolar defibrillation systems without compromising defibrillation energy requirements. It does not, however, significantly decrease DFT stored-energy requirements. The use of lower capacitance, on the other hand, is potentially important in that it appears to increase the percentage of patients with very low defibrillation requirements. This is important should maximum ICD energy capabilities be decreased from their present 34-J level to a 25-J level. Such a move to lower energy output ICDs would, in turn, decrease device size, an essential feature of making ICD surgery comparable to that of standard pacemaker surgery. The practical procedural advantages of a smaller ICD are obvious: local anesthesia, a subcutaneous pocket, outpatient insertion, decreased morbidity, and decreased costs. Smaller capacitance devices may modestly favor such a development.

	Acknowledgments

 
This study was supported in part by a National Institutes of Health grant (RO1-HL-48814-01); by the Tachycardia Research Foundation, Seattle, Wash; and by Medtronic Corporation. The authors thank Joan McDaniel for secretarial assistance.

Received May 27, 1994; accepted July 31, 1994.

	References

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2. Bardy GH, Dolack GL, Kudenchuk PJ, Poole JE, Mehra R, Johnson G. Prospective randomized comparison in man of a unipolar defibrillation system with that utilizing an additional superior vena cava electrode. Circulation. 1994;89:1090-1093. [Abstract/Free Full Text]

3. Kudenchuk PJ, Bardy GH, Dolack GL, Poole JE, Mehra R, Johnson G. Efficacy of a single lead unipolar transvenous defibrillator compared with a system employing an additional coronary sinus electrode: a prospective randomized study. Circulation. 1994;89:2641-2644. [Abstract/Free Full Text]

4. Bardy GH, Dolack GL, Poole JE, Kudenchuk PJ, Johnson G, Raitt MH, Mehra R, DeGroot P, Hofer BO. A prospective randomized comparison in humans of 50% vs 65% tilt biphasic pulse defibrillation using the unipolar pectoral transvenous defibrillation system. Circulation. 1993;88(suppl I):I-113. Abstract.

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13. Swerdlow CD, Kass RM, Hwang C, Chen PS. Effect of capacitor size and pathway resistance on defibrillation threshold in humans. PACE Pacing Clin Electrophysiol.. 1994;17:851. Abstract.

14. Bardy GH, Ivey TD, Stewart RB, Fishbein DP, Poole JE, Kudenchuk PJ, Adhar GC, Greene HL. Sequential pulse defibrillation thresholds in man: effect of pulse width variation. J Am Coll Cardiol. 1987;9:166A. Abstract.

15. Chapman PD, Wetherbee JN, Vetter JW, Troup P, Souza J. Strength-duration curves of fixed pulse width varable tile truncated exponential waveforms for nonthoracotomy internal defibrillation in dogs. PACE Pacing Clin Electrophysiol.. 1988;11:1045-1050. [Medline] [Order article via Infotrieve]

16. Jones GK, Poole JE, Kudenchuk PJ, Dolack GL, Johnson G, DeGroot P, Gleva M, Raitt M, Bardy GH. A prospective randomized evaluation of implantable cardioverter-defibrillator size in unipolar defibrillation system efficacy in man. Circulation 1994;90(suppl I):I-227. Abstract.

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