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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: 90-μF and 120-μF Biphasic Pulse Transvenous Defibrillation. Introduction: Capacitance is known to influence defibrillation. Optimal biphasic waveform capacitance for transvenous unipolar defibrillation systems in man is currently being defined. In an effort to improve defibrillation efficacy, we examined the relative defibrillation efficacy of a 65% tilt biphasic pulse from a 90-μF capacitor compared to a 65% tilt biphasic pulse from a 120-μF capacitor in a prospective, randomized fashion in 16 consecutive cardiac arrest survivors undergoing defibrillator surgery. Methods and Results: The transvenous unipolar pectoral defibrillation system uses a single endocardial RV anodal defibrillation coil and the shell of an 80-cc volume (88 cm2 surface area) pulse generator (Medtronic Model 7219C PCD “active CAN”) as the cathode for the first phase of the biphasic shock: RV+→ CAN−. Defibrillation thresholds for each capacitance were determined prospectively in a randomized fashion. The defibrillation threshold results for the 90-μF capacitance were: leading edge voltage 383 ± 132 V; stored energy 7.4 ± 5.0 J; and resistance 57 ± 10 ω. The results for the 120-μF capacitance were: leading edge voltage 315 ± 93 V (P = 0.002); stored energy 6.5 ± 3.7 J (P = 0.21); and resistance 57.0 ± 11 ω (P = 0.87). Conclusions: We conclude that 90-μF, 65% tilt biphasic pulses used with unipolar pectoral defibrillation systems have equivalent stored energy defibrillation efficacy compared to 120-μF, 65% tilt pulses. Use of lower capacitance is possible in present implantable defibrillators without compromising defibrillation.

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: Serial DFT Measures in Man. Introduction; The defibrillation threshold (DFT) may change throughout the first year following implantation of a cardioverter defibrillator, but it remains uncertain if changes are a consequence of changes in clinical condition or are related to fundamental alterations at the electrode-tissue interface. The purpose of this study was to evaluate the extent and time course of DFT changes over the first year following implantable cardioverter defibrillator (ICD) surgery when extraneous clinical and device variables potentially affecting the DFT were excluded. Methods and Results.: We prospectively enrolled 61 patients undergoing epicardial or non-thoracotomy/transvenous ICD therapy into a series of follow-up studies where the DFT was measured at implant and at 1, 6,12, and 52 weeks following implantation in a uniform manner. Stored energy DFT was measured and recorded for all patients. Patient exclusion criteria were: (1) inability to complete all five measures of the DFT; (2) institution of Class I or Class III antiarrhythmic drugs at any time during the study; (3) lead system changes (relocation or new leads) or programming changes in pulse width or current pathway; or (4) development of a significant change in their clinical status, such as decompensated congestive heart failure or acute ischemia. Only 20 of the 61 patients satisfied the criteria required to complete the study. Two of the excluded patients developed high DFTs, which required reprogramming of the current pathway. Eight patients had an epicardial lead system, and 12 had a nonthoracotomy lead system. The rise in DFT over the first 12 weeks was significant for the eight epicardial lead system patients (P = 0.05) and for the 12 nonthoracotomy lead system patients (P = 0.004). The peak rise in DFT occurred at 1 week for the patients with an epicardial lead system (3.4 ± 1.8 J to 7.9 ± 3.8 J) and at 12 weeks for the patients with a transvenous lead system (10.3 ± 5.3 J to 16.1 ± 7.4 J). Conclusions: This study confirms a transient significant rise in the DFT in the first 12 weeks following ICD surgery that partially returns to the implant value over the remainder of the year. Because specific clinical and technical variables were excluded from this study, the observations made in this patient population suggest that the rise in DFT may be a consequence of changes at the electrode-tissue interface.