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Notes: Atrial Fibrillation After Ventricular Defibrillation. Introduction: The induction of atrial fibrillation (AF) following implantable defibrillator therapy of ventricular fibrillation carries multiple risks. The frequency of shock-induced AF may be more problematic in patients with transvenous defibrillators because current is often delivered through atrial tissue. Thus, the purpose of this study was to determine the incidence of AF following transvenous ventricular defibrillation. Methods and Results: Atrial electrograms were recorded before and after energy delivery in patients undergoing intraoperative testing of transvenous defibrillation lead systems. A total of 114 tracings were examined from 21 patients following ventricular defibrillation. Transvenous deflbrillation shock strength ranged between 200–800 volts (2–40 joules). Bipolar atrial electrograms were obtained from atrial electrodes with 1-cm interelectrode spacing located on one of the defibrillation catheters. The timing of the ventricular defibrillation shock was expressed as a percentage of the preceding sinus PP interval. Three of the 114 transvenous shocks (2.6%) generated AF. Each episode of AF occurred in a different patient. The shocks responsible for AF occurred at 21%, 43%, and 84% of the preceding sinus PP interval. No relation was found between AF induction and the timing of pulse delivery, pulse strength, or pulse number. Conclusion: We conclude that transvenous ventricular defibrillation infrequently causes AF and that timing shock delivery to the atrial cycle is likely to be of marginal or no benefit in the prevention of shock-induced AF. (J Cardiovasc Electrophysiol, Vol. 3, pp. 411–417, October 1992)

Notes: Tranvenous Defibrillators Without EP Testing. Introduction: Baseline electrophysiologic study (EPS) is routinely performed in patients resuscitated from ventricular fibrillation (VF) to risk stratify and select patients for chronic antiarrhythmic drug therapy. The role of EP testing prior to insertion of a multiprogrammable implantable cardioverter defibrillator (ICD), however, is unclear. Methods and Results: This study was a retrospective review of outcome in 66 survivors of an initial episode of out-of-hospital VF not associated with a Q wave myocardial infarction or reversible causes, treated with transvenous ICDs as first-line therapy. Patients were excluded from the study if they had a previous history of monomorphic ventricular tachycardia (VT), a clinical history suggestive of supraventricular tachycardia, or had undergone preoperative EP testing. Fifty-two of the patients (79%) were male with an average age of 58 ± 11 years. Coronary artery disease was present in 43 patients (66%), cardiomyopathy in 15 patients (23%), and valvular heart disease in 1 patient (1.5%). Seven patients (11%) had no detectable structural heart disease. The mean left ventricular ejection fraction was 0.40 ± 0.16. With an average follow-up of 25 ± 12 months, survival free of death from any cause was 100%. Twenty-three patients (35%) experienced 48 episodes of recurrent rapid VT or VF (average cycle length: 236 ± 47 msec) treated by their device. The mean time to first therapy was 223 ± 200 days. Only one of these patients also received antitachycardia pacing for two episodes of VT. One patient (1. 5%) temporarily received amiodarone after removal of an infected device that was subsequently replaced. No other patient received antiarrhythmic drug therapy. Conclusion: After a cardiac arrest due to primary VF, select patients treated with multiprogrammable ICDs can be managed successfully without baseline EPS or antiarrhythmic drug therapy.

Notes: Effect of Defibrillation on Pacing Thresholds. Introduction: Significant increases in ventricular pacing threshold have been observed following monophasic waveform ventricular defibrillation shocks. High-output pacing is recommended to ensure consistent capture, particularly in pacemaker-dependent patients who are likely to be defibrillated. Whether biphasic waveform defibrillation compounds this problem is not known. The purpose of this prospective study was to examine serial changes in ventricular pacing thresholds following single, multiple, low- and high-energy biphasic defibrillation sbocks from an implanted defibrillator. Methods and Results: Bipolar pacing thresholds before and after defibrillation, and the adequacy of pacing capture at three times preshock threshold in the immediate aftermath of ventricular defibrillation, were prospectively evaluated in 67 consecutively tested recipients of a biphasic implanted cardioverter defibrillator. Overall, serial pacing thresholds following successful defibrillation were completely unchanged after 141 of 177 (80%) ventricular fibrillation inductions. In no case did the threshold pulse width increment 〉 0.06 msec from its baseline value after shock, nor did pacing at a pulse width of three times preshock threshold from dedicated bipolar pacing electrodes fail to result in successful ventricular capture. Changes in threshold were not related to when measured from the time of shock, defibrillation energy, number of shocks, electrode system, chronicity of leads, shock orientation, or to clinical factors. Conclusions: No clinically important changes in pacing threshold were observed after biphasic waveform defibrillation. Bradycardia pacing at conventional pacemaker outputs of three times baseline pulse width threshold from bipolar electrodes dedicated exclusively to pacing or sensing (but not defibrillation) consistently allowed for an adequate safety margin following defibrillation.

Notes: Implantable Defibrillators in Women. Clinical rhythm, heart disease, ejection fraction, defibrillation threshold, recurrent arrhythmias, and mortality were compared in 268 consecutive recipients (213 men and 55 women) of their first implantable cardioverter defibrillator for life-threatening ventricular tachycardia or fibrillation. Women were younger than men, less likely to have structural heart disease, and more likely to have clinical ventricular fibrillation, a higher ejection fraction, and a lower defibrillation threshold. Complications of defibrillator placement were similar in both sexes. Unadjusted survival tended to be higher in women than in men (97% vs 90%, respectively, at 2 years, P = 0.08), largely due to fewer deaths from noncardiac causes or cardiac causes other than arrhythmia (P = 0.04). Women also tended to be at lower, albeit still substantial, risk for recurrent arrhythmias during follow-up (37% vs 52% in men at 2 years, P = 0.11). After adjustment for baseline differences, overall survival, arrhythmia death-free survival, nouarrhythmia death-free survival, and frequency of recurrent arrhythmias were not found to be gender related. Despite their apparent “lower risk” status on initial presentation, women remained at substantial risk for recurrent arrhythmias. This underscores the need to avoid being unduly biased by the “appearance” of health in managing women with malignant arrhythmias. That survival and other clinical endpoints were all ultimately independent of gender emphasizes the importance of other clinical variables in assessing risk from ventricular tachyarrhythmias.

Notes: Damped Sine Wave Defibrillation Pulses. Introduction: Damped sine wave pulses have been used for nearly 50 years in transthoracic defibrillation systems. The purpose of this study was to determine whether damped sine wave pulses have a role in implantable defibrillators. Methods and Results: In 21 survivors of cardiac arrest, we prospectively compared defibrillation efficacy of a standard truncated capacitor (RC) monophasic pulse with a damped sine wave inductor-capacitor (LRC) pulse using a right ventricular-left ventricular epicardial patch-patch electrode system. The RC pulse was a standard 65% tilt monophasic waveform generated from a 120μF capacitor. The LRC pulse was designed to simulate the waveform currently used in transthoracic defibrillators and was generated by passing the charge stored on a 40μF capacitor through a 37-mH inductor. Capacitor voltage, peak delivered voltage, peak delivered current, discharge pathway resistance, delivered energy, and stored energy were compared for the two waveforms at the defibrillation threshold. There was no difference in defibrillation efficacy for the two waveforms. Peak delivered voltage was similar at the defibrillation threshold: 313 ± 101 V for the RC pulse and 342 ± 119 V for the LRC pulse (P = 0.16). Similarly, no differences were found in defibrillation threshold peak delivered current: 8.6 ± 2.5 (RC) versus 9.3 ± 2.7 (LRC) amperes (A) (P = 0.20); discharge pathway resistance: 37 ± 11 (RC) versus 38 ± 13 (LRC) O (P = 0.71); delivered energy: 7.0 ± 4.5 (RC) versus 7.0 ± 4.0 (LRC) joules (J) (P = 0.88); and stored energy: 8.7 ± 5.7 (RC) versus 9.8 ± 5.4 (LRC) J (P = 0.35). Although both waveforms performed the same, it was necessary to use substantially higher stored voltages with the damped sine wave delivery system than with the truncated waveform delivery system: 356 ± 110 V for the RC pulse and 675 ± 192 V for the LRC pulse (P 〈 0.0001). Conclusion: This study demonstrates that RC monophasic pulses provide equally effective epicardial defibrillation as LRC pulses with respect to delivered voltage and current and stored and delivered energy. However, in order for LRC pulses to provide comparable delivered voltage, current, and energy to that of RC pulses, nearly twice the voltage must be stored on the capacitor to accomplish the same task. These findings suggest that despite the nearly 50-year experience with damped sine wave pulses with transthoracic defibrilliitors, there is no need to begin using damped sine wave pulses for implantabte defibrillators. Moreover, these data raise a question regarding the need for inductors in transthoracic defibrillators.