Clinical Predictors of Transvenous Biphasic Defibrillation Thresholds

Clinical Predictors of Transvenous Biphasic Defibrillation Thresholds Michael R. Gold, MD, PhD, Koroush Khalighi, MD, Neal G. Kavesh, MD, Barry Daly, Robert W. Peters, MD, and Stephen R. Shorofsky, MD, PhD

MD,

Transvenous lead systems have become routine for defibrillator placement. However, previous studies of clinical predictors of an adequate nonthoracotomy defibrillation threshold (DFT) evaluated monophasic waveforms or more complex lead systems, including subcutaneous patches. Accordingly, this study is a prospective evaluation of the predictors of an adequate biphasic DFT in 114 consecutive patients undergoing cardioverter-defibrillator implantation with a single transvenous lead. For each subject, 38 parameters were assessed, including standard demographic, electrocardiographic, echocardiographic, and radiographic measurements. An adequate DFT (°20 J) was achieved in 92% of patients. Multivariable analysis revealed 2 independent factors predictive of a high threshold: echocardiographic mea-

surements of left ventricular dilation (odds ratio Å 0.16, 95% confidence interval 0.05 to 0.53, p Å 0.003) and body size (odds ratio Å 0.36, 95% confidence interval 0.17 to 0.73; p Å 0.005). No patient with a normal left ventricular end-diastolic dimension had a high DFT, whereas 14% (9 of 66) of those with left ventricular dilation had elevated thresholds. When the DFT cutoff was lowered to 15 J, as is necessary with some downsized pulse generators, an adequate threshold was observed in 84% of patients and the same 2 independent predictors of high thresholds were found. These results indicate that an adequate transvenous DFT can be predicted from simple clinical parameters. Q 1997 by Excerpta Medica, Inc. (Am J Cardiol 1997;79:1623–1627)

onthoracotomy lead systems were developed to simplify implantation of cardioverter defibrilN lators. This surgical approach has been shown to

March 1996 at the University of Maryland. As part of the routine evaluation of these patients, an electrocardiogram and echocardiogram was obtained preoperatively and a chest x-ray was obtained postoperatively. All studies were evaluated without knowledge of the results of defibrillation testing. Informed consent was obtained from each patient for device implantation. Lead implantation and defibrillation testing: The transvenous defibrillation lead (Endotak Models 0062, 00064, 0074, or 0115) was placed under fluoroscopic guidance with the tip of the lead positioned at the right ventricular apex. The 60 and 70 series leads differ in the spacing between the distal electrode and the distal coil. There were no differences of defibrillation efficacy observed for the 2 series of leads, so the results were pooled for analyses. Defibrillation testing was performed either under general anesthesia (nitrous oxide, isoflurane, and vecuronium) or conscious sedation (fentanyl and midazolam). Ventricular fibrillation was induced with high-output ramp pacing through the defibrillation lead. After 10 seconds of fibrillation, defibrillation was attempted with an external defibrillator (CPI Model 2815, St. Paul, Minnesota). Shocks were delivered between the 2 coils of the transvenous lead. The biphasic shock waveform had a 125-mF capacitance with 60% first phase and 50% second phase tilts. The initial delivered shock energy was 15 J and the distal coil was the cathode for the first phase of the biphasic shock (i.e., normal polarity).14 If successful, the energy was decreased by a standard stepdown protocol.14 If the initial 15-J shock failed, then energy was increased to 20 J on the next trial. If this shock also failed, then shock polarity was reversed and testing at 20 J attempted on the subsequent trials.

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reduce perioperative morbidity, mortality, duration of hospitalization, and costs compared with the placement of epicardial patches.7 – 10 Initially, complex nonthoracotomy lead systems were used that included subcutaneous patches or arrays, but with optimization of shock waveform and polarity simple transvenous implantation is now the standard, and the requirement for thoracotomy has been virtually eliminated.5,6,11 – 16 Previous studies of clinical predictors of nonthoracotomy defibrillation implantation included multiple complex lead systems and monophasic waveforms.2 – 5 The only large series assessing a biphasic defibrillation threshold (DFT) evaluated a unipolar lead system, which included a pectoral pulse generator shell.17 However, the clinical predictors of transvenous DFTs have not been assessed previously. Accordingly, the present study is a prospective evaluation of DFT using a uniform testing protocol and biphasic shocks in 114 consecutive patients undergoing initial cardioverter-defibrillator placement with a single transvenous lead.

METHODS Patient population: This prospective study included all patients undergoing defibrillator implantation with an Endotak C lead (Cardiac Pacemakers Inc., St. Paul, Minnesota) between February 1994 and From the Department of Medicine, Division of Cardiology, University of Maryland Medical System, Baltimore, Maryland. Manuscript received December 16, 1996; revised manuscript received and accepted February 27, 1997. Address for reprints: Michael R. Gold, MD, PhD, Division of Cardiology, N3W77 University of Maryland Hospital, 22 S. Greene Street, Baltimore, Maryland 21201. Q1997 by Excerpta Medica, Inc.

0002-9149/97/$17.00 PII S0002-9149(97)00210-5

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The DFT was defined as the lowest initial shock energy with either polarity that successfully terminated ventricular fibrillation. For thresholds of 15 or 20 J, a repeat trial at the DFT was given to confirm an adequate safety margin for implantation (DFT / method).18 Thus, an adequate implantation DFT was achieved if ¢2 successive first shocks were successful in either polarity with delivered energies of °20 J. Variables assessed: Thirty-eight parameters were analyzed for each patient. These included standard clinical features and electrocardiographic measurements. Coronary artery disease was considered present if there was a stenosis ú70% in ¢1 major epicardial coronary artery or if there was a history of myocardial infarction. Left ventricular ejection fraction was measured by radionuclide ventriculography or echocardiography. A patient was considered to be receiving amiodarone if the drug was taken long term or if a loading dose was administered within 1 week before implantation. Radiographic measurements were made from posteroanterior and lateral chest x-rays. The cardiothoracic ratio, thoracic diameter, and cardiac silhouette diameter were calculated by standard techniques.19 Cardiomegaly was defined as a cardiothoracic ratio of ú0.5. The anterior and lateral coil angles were the measured angles between the 2 coils of the Endotak lead in the posteroanterior and lateral views, respectively. The coil to apex and coil to sternum distances were measured from the distal coil tip to the cardiac apex and sternum, respectively. Echocardiographic dimensions were calculated as described previously.20,21 A dilated left ventricle was defined as an end-diastolic dimension of ú5.8 cm. Left ventricular mass was calculated by standardized measurements.21 Left atrial enlargement was defined as a parasternal width of ú4.5 cm. Intervals were measured from a 12-lead electrocardiogram. An intraventricular conduction delay was defined as a QRS duration of ú120 ms. Statistical analysis: For analysis, the patients were grouped according to whether there was an adequate (°20 J) or high (ú20 J) DFT. To assess the clinical predictors, the 38 parameters were analyzed. Univariate analysis was performed with t tests for continuous variables and Fisher’s exact test for categorical variables. All variables with p õ0.2 by univariate analysis were entered into a logistic regression method with principal components factor analysis.22 Factor analysis was employed because, as expected with the large number of parameters measured, some were highly correlated; this can confound the interpretation of results. With factor analysis, highly correlated parameters are grouped together as a factor. These factors are then used in the logistic regression method to identify independent predictors of a high DFT. Multivariable analysis was also performed with patients classified by a threshold of °15 J. This analysis was performed retrospectively because of the development of lower output pulse generators. A p value õ0.05 was con1624

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TABLE I Clinical Predictors of Defibrillation Threshold Parameter Number Gender (% men) Age (yrs) Coronary artery disease Coronary artery bypass surgery Ejection fraction Preoperative amiodarone Congestive heart failure History of cardiac arrest Weight (kg) Height (cm) Body surface area (m2)

High DFT (ú20 J)

Adequate DFT (°20 J)

p Value

9 89% 60 { 11 44% 22%

105 79% 63 { 13 64% 27%

0.68 0.55 0.29 0.99

0.25 { 0.11 44% 56% 22% 96.1 { 34.7 183 { 7 2.16 { 0.36

0.33 { 0.15 19% 38% 27% 81.2 { 15.4 174 { 10 1.96 { 0.21

0.13 0.09 0.31 0.47 0.02 0.01 0.01

sidered statistically significant. All results are expressed as mean { SD.

RESULTS Patient population: Of the 114 patients studied, 91 were men (80%). The mean age of this cohort was 62 { 13 years and the mean left ventricular ejection fraction was 0.32 { 0.14. As expected in patients undergoing defibrillator implantation, there were high incidences of coronary artery disease (63%) and congestive heart failure (39%). Cardioverter-defibrillator implantation: All patients underwent successful nonthoracotomy defibrillator implantation. There was 1 perioperative death (0.8%) from complications of chronic hemodialysis 1 week after defibrillator implantation. An adequate transvenous DFT (°20 J) was achieved in 105 patients (92%) at the initial implant procedure. In 7 patients, a subcutaneous patch (n Å 4) or array (n Å 3) was required to achieve an adequate DFT. In the remaining 2 patients a high threshold was observed initially, but they underwent lead alone implantation because the DFT was lower at a second assessment after washout of amiodarone and further treatment of congestive heart failure. However, only the results from the initial testing are included in subsequent analyses. Predictors of adequate defibrillation threshold: Of the 11 clinical variables evaluated, only body size predicted a high DFT (Table I). All 3 measures of body size were correlated with a high DFT. Although patients receiving preoperative amiodarone had higher thresholds than those not receiving this drug (16.0 { 12.1 vs 11.5 { 6.5 J, p Å 0.017), the proportions of patients with an adequate DFT did not differ significantly. The electrocardiographic measurements are presented in Table II. The QRS and corrected QT intervals as well as the presence of an intraventricular conduction delay were associated with a high DFT. A comparison of radiographic parameters in the 2 groups is shown in Table III. The cardiac silhouette diameter, cardiothoracic ratio, and coil to apex distance were all increased in patients with a high DFT. The echocardiographic measurements are shown in JUNE 15, 1997

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related parameters, as independent predictors of a high threshold. These factors were body size (odds ratio Å 0.36, 95% confidence interval 0.17 to 0.73, High DFT Adequate DFT p p Å 0.005), which included weight, height, and body Parameter (ú20 J) (°20 J) Value surface area as parameters, and echocardiographic Heart rate (beats/min) 78 { 11 75 { 14 0.66 measures of left ventricular dilation (odds ratio Å PR interval (ms) 205 { 40 186 { 42 0.26 0.16, 95% confidence interval 0.05 to 0.53, p Å QRS duration (ms) 144 { 46 117 { 27 0.008 0.003), which included end-diastolic and systolic diQT interval (ms) 419 { 80 394 { 50 0.17 mensions and end-diastolic dilation (ú5.8 cm). As QTc interval (ms) 471 { 67 435 { 34 0.008 an example of this relation, the DFT was signifiIVCD (%) 78% 41% 0.04 Left bundle branch block 22% 12% 0.34 cantly higher in patients with a dilated left ventricle Right bundle branch block 11% 10% 0.99 than in those with a normal diastolic dimension (14.4 { 9.4 vs 9.8 { 4.4 J, p Å 0.024). Moreover, no IVCD Å intraventricular conduction delay. patient with a normal left ventricular diastolic size had a high threshold (0 of 48), whereas 14% (9 of 66) TABLE III Radiographic Predictors of Defibrillation Threshold of those with left ventricular dilaHigh DFT Adequate DFT p tion had elevated thresholds. Parameter (ú20 J) (°20 J) Value Pulse generator size has been Cardiac silhouette diameter (cm) 20.1 { 2.2 16.7 { 2.2 0.001 downsized sufficiently to allow for Cardiothoracic ratio 0.63 { 0.05 0.53 { 0.07 0.001 routine pectoral implantation. With Cardiomegaly 100% 67% 0.20 further downsizing, the maximal Thoracic diameter (cm) 31.9 { 3.6 31.8 { 2.8 0.91 output of some implantable defibrilCoil to apex (cm) 8.8 { 2.6 5.8 { 1.2 0.001 Coil to sternum (cm) 3.5 { 2.0 2.9 { 1.0 0.15 lator systems has been reduced, thus Anterior coil angle (deg) 97 { 11 100 { 19 0.67 requiring a lower implant DFT. To Lateral coil angle (deg) 120 { 39 118 { 30 0.84 assess if the use of lower output devices would affect the predictors of an adequate threshold, the data were TABLE IV Echocardiographic Predictors of Defibrillation Threshold reanalyzed with a cutoff of 15 rather than 20 J. A DFT °15 J was High DFT Adequate DFT Parameter (ú20 J) (°20 J) p Value achieved in 84% of patients. The univariate analyses were similar to Left ventricular end-diastolic 7.8 { 1.5 5.9 { 0.9 0.0001 those noted above, except that dimension (cm) Left ventricular end-systolic 6.2 { 1.5 4.7 { 1.1 0.0003 amiodarone use and left ventricular dimension (cm) ejection fraction were now signifi100% 57% 0.01 Dilated left ventricle (endcant predictors of an adequate diastolic dimension ú5.8 cm) threshold. However, multivariate Left ventricular hypertrophy 22% 27% 0.99 Intraventricular septum (cm) 1.1 { 0.4 1.2 { 0.8 0.72 analysis again demonstrated that Left ventricular posterior wall (cm) 1.1 { 0.3 1.1 { 0.2 0.99 only body size (odds ratio 0.41, p Å Left atrial size (cm) 4.8 { 0.7 4.2 { 0.8 0.03 0.02) and left ventricular dilation Left atrial dilation 67% 30% 0.06 (odds ratio 0.48, p Å 0.01) were inLeft ventricular mass (g) 546 { 306 359 { 358 0.13 dependent predictors of an adequate Left ventricular mass index 245 { 117 185 { 191 0.35 DFT. TABLE II Electrocardiographic Predictors of Defibrillation Threshold

DISCUSSION The major finding of this study Factor Parameters p Value Odds Ratio Wald was that an adequate biphasic transLeft ventricular Left ventricular end-diastolic, left 0.002 0.16 9.3 venous DFT for implantation (°20 dilation ventricular end-systolic, dilated J) can be attained in a vast majority left ventricle (92%) of patients, and that routinely Body size Weight, height, body surface area 0.007 0.29 7.3 obtained clinical factors predict a high threshold. Specifically, in our Table IV. Again, measures of an enlarged heart in- series of 114 consecutive patients, increasing body cluding left ventricular end diastolic or systolic di- size and echocardiographic measurements of left mension, or a dilated left atrium or ventricle were ventricular dilation were independently associated associated with a high threshold. Despite signs of with a high DFT. These results can help risk stratify cardiomegaly in those patients with a high DFT, the patients for transvenous implantation since no pashock impedance did not differ between groups (41 tient without left ventricular diastolic dilation had an elevated threshold. { 7 vs 45 { 7, p Å 0.13). The only previous detailed study of the clinical Multivariable analysis was performed to identify independent predictors of a high DFT. The algorithm predictors of DFTs with biphasic waveforms emidentified 2 factors, each comprised of 3 highly cor- ployed a unipolar lead system consisting of a right TABLE V Multivariate Factor Analysis

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ventricular coil and a left pectoral shell.17 Body size, QRS duration, heart size, left ventricular mass, gender, QT interval, and heart rate correlated with thresholds. Heart rate and ventricular mass were the only independent predictors, but all correlations were relatively weak. Raitt et al17 analyzed these data as a continuous function of DFT. However, a DFT defined by a single successful shock at the threshold energy can be a very inaccurate measure of defibrillation energy requirements.18 Accordingly, our results were analyzed based on clinical practice (i.e., that an adequate DFT is °20 J by the DFT / method) rather than as a function of the measured threshold. While this may affect the correlations observed, the results can be used more effectively to predict patients at increased risk for complicated implants. Nevertheless, despite these differences in lead systems and analytic techniques, overall the results of these 2 studies were similar. This suggests that the factors affecting transvenous thresholds are similar to those affecting thresholds with an active pectoral pulse generator, despite important differences in current vectors. Several previous studies have evaluated the predictors of successful nonthoracotomy cardioverterdefibrillator implantation with monophasic shocks. Brooks and colleagues2 evaluated 101 patients and 2 lead systems, and they observed that the independent predictors of a high DFT were increased radiographic cardiac size and male gender. However, antiarrhythmic drug use, left ventricular end-diastolic dimension, and QRS duration were all univariate correlates of an adequate threshold. In another study of 100 patients with the same 2 lead systems,4 only preoperative amiodarone therapy predicted a high DFT. Schwartzman and colleagues5 analyzed 145 patients undergoing nonthoracotomy implantation using 3 different lead systems. Increased body surface area and congestive heart failure were associated with the need for thoracotomy. Finally, we recently reported that amiodarone therapy, body size, and left ventricular dilation were the predictors of an adequate monophasic DFT, using the same transvenous lead system as in the present study.23 Our results demonstrate that, in general, the predictors of an adequate biphasic transvenous DFT are similar to those for nonthoracotomy defibrillation with monophasic waveforms.2 – 5,23 Left ventricular size and amiodarone use, but not ejection fraction or underlying heart disease tend to predict high thresholds. However, biphasic shocks may be particularly beneficial in patients receiving amiodarone as this was no longer a predictor of high thresholds as observed previously in studies of monophasic shock waveforms.4,23 Our results must be interpreted in the light of certain methodologic limitations. First, despite evaluating a large cohort of consecutive patients, only a relatively small number (n Å 9) had a high biphasic DFT. This could potentially affect the multivariable analysis. However, the same independent factors were noted when 18 patients with a more stringent DFT criteria (°15 J) were analyzed. Only a single 1626

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lead system was evaluated, so it is unknown whether these findings can be generalized to other lead systems, including those using active pectoral pulse generators.24,25 In addition, only 1 biphasic waveform was evaluated. Although somewhat controversial, shock tilt26,27 and capacitance28 – 30 may affect defibrillation efficacy, although the impact of these parameters on clinical predictors of the DFT is unknown. The results of our study have important clinical implications. These results demonstrate that transvenous defibrillation is possible in ú90% of patients. Left ventricular dilation and large body habitus identifies a high-risk cohort for an elevated DFT that likely requires prolonged implantation testing.

1. Block M, Hammel D, Isbruch F, Borggrefe M, Wietholt D, Hachenberg T, Scheld HH, Breithardt. Results and realistic expectations with transvenous lead systems. PACE 1992;15:665–670. 2. Brooks R, Garan H, Torchiana D, Vlahakes GJ, Jackson G, Newell J, McGovern BA, Ruskin JN. Determinants of successful nonthoracotomy cardioverter-defibrillator implantation: experience in 101 patients using two different lead systems. J Am Coll Cardiol 1993;22:1833–1842. 3. Sra JS, Natale A, Axtell K, Maglio C, Jazayeri M, Deshpande S, Dhala A, Blanck Z, Akhtar M. Experience with two different nonthoracotomy systems for implantable defibrillator in 170 patients. PACE 1994;17:1741–1750. 4. Kopp DE, Blakeman BP, Kall JG, Olshansky B, Kinder CA, Wilber DJ. Predictors of defibrillation energy requirements with nonepicardial lead systems. PACE 1995;18:253–260. 5. Schwartzman D, Concato J, Ren J-F, Callans DJ, Gottlieb CD, Preminger MW, Marchlinski FE. Factors associated with successful implantation of nonthoracotomy defibrillation lead systems. Am Heart J 1996;131:1127–1136. 6. Gold MR, Shorofsky SR. Transvenous defibrillation lead systems. J Cardiovasc Electrophysiol 1996;7:570–580. 7. Saksena S, The PCD Investigators. Defibrillation thresholds and perioperative mortality associated with endocardial and epicardial defibrillation lead systems. PACE 1993;16:202–207. 8. Kleman JM, Castle LW, Kidwell GA, Maloney JD, Morant VA, Trohman RG, Wilkoff BL, McCarthy PM, Pinski SL. Nonthoracotomy-versus thoracotomy-implantable defibrillators. Intention-to-treat comparison of clinical outcomes. Circulation 1994;90:2833–2842. 9. Zipes DP, Roberts D. Results of the international study of the implantable pacemaker cardioverter-defibrillator. A comparison of epicardial and endocardial lead systems. Circulation 1995;92:59–65. 10. Venditti FJ, O’Connell M, Martin DT. Transvenous cardioverter defibrillators: cost implications of a less invasive approach. PACE 1995;18:711–715. 11. Block M, Hammel D, Bocker D, Borggrefe M, Budde T, Isbruch F, Wietholt D, Scheld HH, Breithardt G. A prospective randomized cross-over comparison of mono- and biphasic defibrillation using nonthoracotomy lead configurations in humans. J Cardiovasc Electrophysiol 1994;5:581–590. 12. Natale A, Sra J, Axtell K, Maglio C, Dhala A, Deshpande S, Jazayeri M, Wase A, Akhtar M. Preliminary experience with a hybrid nonthoracotomy defibrillating system that includes a biphasic device: comparison with standard monophasic device using the same lead system. J Am Coll Cardiol 1994;24:406–412. 13. Strickberger SA, Hummel JD, Horwood LE, Jentzer J, Daoud E, Niebauer M, Bakr O, Man KC, Williamson BD, Kou W, Morady F. Effect of shock polarity on ventricular defibrillation threshold using a transvenous lead system. J Am Coll Cardiol 1994;24:1069–1072. 14. Shorofsky SR, Gold MR. Effects of waveform and polarity on defibrillation thresholds in humans using a transvenous lead system. Am J Cardiol 1996;78:313–316. 15. Saksena S, An H, Mehra R, DeGroot P, Krol RB, Burkhardt E, Mehta D, John T. Prospective comparison of biphasic and monophasic shocks for implantable cardioverter-defibrillators using endocardial leads. Am J Cardiol 1992;70:304–310. 16. Wyse DG, Kavanagh KM, Gillis AM, Mitchell LB, Duff HJ, Sheldon RS, Kieser TM, Maitland A, Flanagan P, Rothschild J, Mehra R. Comparison of biphasic and monophasic shocks for defibrillation using a nonthoracotomy system. Am J Cardiol 1993;71:197–202. 17. Raitt MH, Johnson G, Dolack GL, Poole JE, Kudenchuk PJ, Bardy GH. Clinical predictors of the defibrillation threshold with the unipolar implantable defibrillation system. J Am Coll Cardiol 1995;25:1576–1583. 18. Singer I, Lang D. Defibrillation threshold: clinical utility and therapeutic implications. PACE 1992;15:932–949. 19. Squire LF, Novelline RA. The Heart in Fundamentals of Radiology. Cambridge: Harvard University Press, 1988:128–129.

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20. Devereux R, Reichek N. Echocardiographic determination of left ventricular

mass in man. Circulation 1977;55:613–618. 21. Schiller NB, Shah PM, Crawford M, DeMaria A, Devereux R, Feigenbaum H, Gutfessell H, Reichek N, Sahn D, Schnittger I, Silverman NH, Tajik AJ. Recommendations for quantitation of the left ventricle by two-dimensional echocardiography. J Am Soc Echocardiogr 1989;2:358–367. 22. Statistical Package for the Social Sciences Incorporated (SPSS). Chicago, 1995. 23. Khalighi K, Daly B, Leino EV, Shorofsky SR, Kavesh NG, Peters RW, Gold MR. Clinical predictors of transvenous defibrillation energy requirements. Am J Cardiol. 1997;79:150–153. 24. Bardy GH, Johnson G, Poole JE, Dolack GL, Kudenchuk PJ, Kelso D, Mitchell R, Mehra R, Hofer B. A simplified, single-lead unipolar transvenous cardioversion-defibrillation system. Circulation 1993;88:543–547. 25. Gold MR, Foster AH, Shorofsky SR. Effects of an active pectoral-pulse generator shell on defibrillation efficacy with a transvenous lead system. Am J Cardiol 1996;78:540–543.

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for human nonthoracotomy defibrillation. Circulation 1993;88:2646–2654. 27. Poole JE, Bardy GH, Kudenchuk PJ, Dolack GL, Raitt MH, Mehra R,

Johnson G. Prospective randomized comparison of biphasic waveform tilt using a unipolar defibrillation system. PACE 1995;18:1369–1373. 28. Bardy GH, Poole JE, Kudenchuk PJ, Dolack GL, Mehra R, DeGroot P, Raitt MH, Jones GK, Johnson G. A prospective randomized comparison in humans of biphasic waveform 60-mF and 120-mF capacitance pulses using a unipolar defibrillation system. Circulation 1995;91:91–95. 29. Block M, Hammel D, Bocker D, Borggrefe M, Seifert T, Fastenrath C, Scheld HH, Breithardt G. Internal defibrillation with smaller capacitors: a prospective randomized cross-over comparison of defibrillation efficacy obtained with 90-mF and 125-mF capacitors in humans. J Cardiovasc Electrophysiol 1995;6:333–342. 30. Swerdlow CD, Kass RM, Davie S, Chen P-S, Hwang C. Short biphasic pulses from 90 microfarad capacitors lower defibrillation threshold. PACE 1996;19:1053–1060.

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