Biphasic transthoracic defibrillation causes fewer ECG ST-segment changes after shock.

STUDY OBJECTIVE: Electrocardiographic abnormalities are common after transthoracic defibrillation. ECG ST-segment changes are especially problematic after defibrillation and may indicate ischemic or shock-induced cardiac dysfunction after resuscitation. Biphasic defibrillation waveforms, compared with monophasic waveforms, diminish shock-induced cardiac dysfunction in laboratory preparations. This effect has not been validated in human subjects. We therefore evaluated in a prospective, blinded fashion the effect of biphasic and monophasic transthoracic defibrillation on the ECG ST segment in 30 consecutive patients during surgery for the implantation of a cardioverter-defibrillator. METHODS: In each patient two low-energy truncated biphasic transthoracic defibrillation shocks (115 and 130 J) were compared with a standard clinical 200 J monophasic damped-sine wave shock. The biphasic shocks and the damped-sine wave shock have been demonstrated to have equal defibrillation efficacy of 97%. Fifteen-second ECG signals recorded across transthoracic defibrillation electrodes were digitized before ventricular fibrillation induction and immediately after each defibrillation attempt. The ST segments 80 msec after the J point were analyzed in a blinded fashion by two reviewers. The ST-segment deflection, QRS-interval duration, QT interval, and heart rate after each therapy were compared with baseline values. RESULTS: ECG ST-segment elevation was significantly greater with the 200-J damped-sine waveform than with either biphasic waveform. The ECG ST-segment levels were -.55 +/- .36 at baseline, -.76 +/- .36 mm after internal shock, -.02-.36 mm after 115-J biphasic shock, .21 +/- .38 mm after 130-J biphasic shock, and 2.09 +/- .37 mm after 200-J damped-sine wave shock (P<.0001). QRS-interval duration, QT interval, and heart rate did not change significantly with any waveform. CONCLUSION: Transthoracic defibrillation with biphasic waveforms results in less postshock ECG evidence of myocardial dysfunction (injury or ischemia) than standard monophasic damped sine waveforms without compromise of defibrillation efficacy.

Prospective evaluation of the effect of biphasic waveform defibrillation on ventricular 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 shocks 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.

Malignant sustained ventricular tachyarrhythmias in women: characteristics and outcome of treatment with an implantable cardioverter defibrillator.

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, nonarrhythmia 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.

Management of ventricular fibrillation with transvenous defibrillators without baseline electrophysiologic testing or antiarrhythmic drugs.

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.

A prospective randomized comparison in humans of 90-mu F and 120-mu F biphasic pulse defibrillation using a unipolar defibrillation system.

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-mu F capacitor compared to a 65% tilt biphasic pulse from a 120-mu F 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-mu F capacitance were: leading edge voltage 383 +/- 132 V; stored energy 7.4 +/- 5.0 J; and resistance 57 +/- 10 omega. The results for the 120-mu F 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 omega (P = 0.87). CONCLUSIONS: We conclude that 90-mu F, 65% tilt biphasic pulses used with unipolar pectoral defibrillation systems have equivalent stored energy defibrillation efficacy compared to 120-mu F, 65% tilt pulses. Use of lower capacitance is possible in present implantable defibrillators without compromising defibrillation.

A prospective randomized evaluation of implantable cardioverter-defibrillator size on unipolar defibrillation system efficacy.

BACKGROUND: The active can unipolar implantable cardioverter-defibrillator (ICD) has been shown to defibrillate efficiently, but its current 80-cc size limits use in the pectoral position in many patients. Decreasing can size will facilitate pectoral insertion and will soon be feasible as an inevitable consequence of technological advancements. However, decreasing the can size has the potential to compromise unipolar defibrillation efficacy. It is the purpose of this study, therefore, to prospectively and randomly compare unipolar defibrillation efficacy with 80-cc, 60-cc, and 40-cc can sizes in patients immediately before ICD surgery in anticipation of advances in technology that will make smaller ICDs possible. METHODS AND RESULTS: Twenty-four consecutive patients underwent prospective, randomized evaluation of the effect of ICD can size on defibrillation efficacy during standard ICD surgery. Each patient had the unipolar defibrillation threshold (DFT) measured with 80-cc, 60-cc, or 40-cc active can placed in the left subcutaneous infraclavicular region. The system included a 10.5F tripolar right ventricular electrode that served as the shock anode. The shock waveform used in each instance was a single capacitor biphasic 65% pulse delivered from a 120-microF capacitor. Stored energy at the DFT for the 80-cc, 60-cc, and 40-cc cans were 8.1 +/- 4.7 J, 8.7 +/- 5.8 J, and 9.5 +/- 4.8 J, respectively. There was no statistical significant difference between the DFTs for the three unipolar can electrodes (P = 39). Leading edge voltage also did not differ significantly among the three unipolar cans (356 +/- 92 V, 365 +/- 110 V, and 387 +/- 94 V, respectively, P = .29). There was, however, a slight progressive increase in resistance with decreasing can size (57 +/- 7 omega, 60 +/- 9 omega, and 65 +/- 9 omega, respectively, P < .001). CONCLUSIONS: Decreasing can volume from 80 cc to 60 cc to 40 cc does not compromise unipolar defibrillation efficacy despite a slight rise in shock resistance. These findings indicate that technological advances that allow for smaller-volume ICDs will not compromise defibrillation efficacy for unipolar systems.

Anatomical findings in patients having had a chronically indwelling coronary sinus defibrillation lead.

The purpose of this report is to review the gross and histological cardiac anatomical findings in patients with chronically indwelling coronary sinus leads at the time of autopsy or cardiac transplantation. Transvenous cardioverter defibrillators offer effective protection against sudden death. The use of a coronary sinus electrode has been shown in some patients to decrease the defibrillation threshold. The anatomical consequences of chronically indwelling coronary sinus cardioversion/defibrillation electrodes in patients having transvenous implantable cardioverter defibrillators is unknown. The hearts of seven patients with chronically indwelling coronary sinus electrodes were evaluated following autopsy (n = 2) or cardiac transplantation (n = 5). The coronary sinus electrode in each case was a 6.5 French silicone lead with a 5-cm long defibrillation coil (Medtronic CS lead model 6933) that was positioned as distally as possible within the coronary sinus at the time of implantable cardioverter defibrillator surgery. The seven hearts examined were derived from patients whose age ranged between 49 and 69 (mean 56 +/- 7 years). Six had coronary artery disease and one had idiopathic dilated cardiomyopathy. The time from implant to death or cardiac transplantation was 8 +/- 6 months, range 1-18 months. In all seven patients, there was no evidence of any significant damage from the presence of the coronary sinus lead. The only finding in each case was the scattered presence of a thin white fibrous sheath over the lead that intermittently adhered to the coronary sinus endothelium and, in the two patients transplanted 1-3 months after implantable cardioverter defibrillator insertion, a mild inflammation reaction adjacent to the leads in the coronary sinus endothelium. There was no evidence of coronary sinus occlusion, adjacent coronary artery injury, coronary sinus perforation, coronary sinus burn, or myocardial injury adjacent to the lead. Cause of death was due to end-stage congestive heart failure and thrombotic stroke, respectively, in the two patients examined at autopsy. Coronary sinus defibrillation leads can be used safely without harmful anatomical effect.

Mechanical complications after implantation of multiple-lead nonthoracotomy defibrillator systems: implications for management and future system design.

Nonthoracotomy lead system (NTL) implantable cardioverter defibrillators (ICDs) provide excellent protection against sudden death from ventricular tachyarrhythmias. However, these devices have unique mechanical complications and management issues. We reviewed the major complications occurring in 159 patients who underwent attempted implantation of a multilead NTL system. Successful implantation was obtained in 98% of patients. Two-year, all-cause actuarial survival on an intention-to-treat basis was 94%. Major complications occurred in 28 (17.6%) patients over a follow-up period of 21 +/- 10 months. Complications included 11 (6.9%) lead dislodgements, 10 (5.7%) lead fractures in 9 patients, 2 (1.3%) pocket infections, 1 frozen shoulder, 1 right ventricular perforation, 1 pneumothorax, 1 bleed requiring transfusion, 1 thromboembolism, and 1 "twiddle"-induced torsion of leads. Most of the lead dislodgements and fractures were identified by routine x-ray surveillance. Single-lead systems may significantly reduce complication rates in the future and maintain excellent survival rates.

Prospective randomized comparison of biphasic waveform tilt using a unipolar defibrillation system.

BACKGROUND: A unipolar defibrillation system using a single right ventricular (RV) electrode and the active shell or container of an implantable cardioverter defibrillator situated in a left infraclavicular pocket has been shown to be as efficient in defibrillation as an epicardial lead system. Additional improvements in this system would have favorable practice implications and could derive from alterations in pulse waveform shape. The specific purpose of this study is to determine whether defibrillation efficacy can be improved further in humans by lowering biphasic waveform tilt. METHODS: We prospectively and randomly compared the defibrillation efficacy of a 50% and a 65% tilt asymmetric biphasic waveform using the unipolar defibrillation system in 15 consecutive cardiac arrest survivors prior to implantation of a presently available standard transvenous defibrillation system. The RV defibrillation electrode has a 5-cm coil located on a 10.5 French lead and was used as the anode. The system cathode was the active 108 cm2 surface area shell (or "CAN") of a prototype titanium alloy pulse generator placed in the left infraclavicular pocket. The defibrillation pulse derived from a 120-microF capacitor and was delivered from RV-->CAN, with RV positive with respect to the CAN during the initial portion of the cycle. Defibrillation threshold (DFT) stored energy, delivered energy, leading edge voltage and current, pulse resistance, and pulse width were measured for both tilts examined. RESULTS: The unipolar single lead system, RV-->CAN, using a 65% tilt biphasic pulse resulted in a stored energy DFT of 8.7 +/- 5.7 J and a delivered energy DFT of 7.6 +/- 5.0 J. In all 15 patients, stored and delivered energy DFTs were < 20 J. The 50% tilt biphasic pulse resulted in a stored energy DFT of 8.2 +/- 5.4 J and a delivered energy DFT of 6.1 +/- 4.0 J; P = 0.69 and 0.17, respectively. As with the 65% tilt pulse, all 15 patients had stored and delivered energy DFTs < 20 J. CONCLUSION: The unipolar single lead transvenous defibrillation system provides defibrillation at energy levels comparable to that reported with epicardial lead systems. This system is not improved by use of a 50% tilt biphasic waveform instead of a standard 65% tilt biphasic pulse.

Clinical predictors of the defibrillation threshold with the unipolar implantable defibrillation system.

OBJECTIVES: The purpose of this study was to determine the relation between clinical variables and the defibrillation threshold by using a standardized testing protocol and a uniform implantable defibrillator system. BACKGROUND: Past studied have not revealed useful correlations between clinical variables and the energy required to terminate ventricular fibrillation. Most of these studies did not use a uniform implantable defibrillator system or a standardized protocol to measure the defibrillation threshold and, thus, did not control for the influence of these technical influences. We postulated that defibrillator and defibrillation threshold measurement-based sources of variability overshadowed important clinical predictors. METHODS: The defibrillation threshold was measured by using a standardized protocol in 101 consecutive patients. We used a transvenous unipolar pectoral defibrillation system that employed a single endocardial right ventricular defibrillation coil as the anode and the shell of an 80-cm3 pulse generator as the cathode to deliver a 65% tilt biphasic pulse. RESULTS: Several clinical variables were found to be significantly associated with the defibrillation threshold: patient gender, height, weight, body surface area, heart rate at rest, QRS and corrected QT (QTc) intervals, left ventricular mass and several measures of heart and chest size by chest roentgenogram. None of these variables had a correlation coefficient > 0.45 with the defibrillation threshold. On multivariate analysis, left ventricular mass and heart rate at rest were the only independent predictors of the defibrillation threshold and explained only 25% of the observed variability. CONCLUSIONS: Despite the use of a uniform transvenous defibrillation system and a standardized protocol to measure the defibrillation threshold, no clinically relevant correlation was found between clinical variables and the defibrillation threshold. The defibrillation threshold is probably a function of a complex interaction of anatomic, physiologic and cellular variables that are not adequately represented by easily obtainable clinical information. It is probably not possible to predict defibrillation outcome from standard clinical variables.

Ventricular arrhythmias detected after transvenous defibrillator implantation in patients with a clinical history of only ventricular fibrillation. Implications for use of implantable defibrillator.

BACKGROUND: Patients with a history of ventricular fibrillation (VF) have been shown to have a clinical profile, response to electrophysiological testing (EPS), and response to antiarrhythmic therapy that distinguishes them from patients with a history of sustained monomorphic ventricular tachycardia (MVT). Despite these differences, it is not clear whether VF in these patients is triggered by MVT or occurs de novo. The incidence of MVT and VF in such patients after their index VF event has important implications for therapeutic decisions regarding implantable defibrillator selection and programming. METHODS AND RESULTS: The records of 111 consecutive patients who had undergone transvenous cardioverter/defibrillator (ICD) implantation for malignant ventricular arrhythmias were reviewed retrospectively. For each patient, all device tachyarrhythmia detections were examined and classified as VF, MVT, rapid polymorphic VT, or other. The number of events, time to first arrhythmia detection, and cycle length of MVTs were recorded. There were 55 patients with a history of only VF and 56 with a history that included an episode of MVT. Over 14 months of follow-up, with all patients initially off of antiarrhythmic medications, MVT was detected by only 18% of patients with a history of only VF compared with 54% of those with a history that included MVT (P = .002). Among patients who did detect MVT, those with a history of only VF had fewer episodes (7 +/- 7 versus 20 +/- 31, P = .001) and a shorter mean MVT cycle length (279 versus 314 ms, P = .03) than those with a clinical history of MVT. Abrupt onset of VF not preceded by MVT was detected in 11% of patients with VF only. In addition to a history of MVT, male sex, age < 60 years, and MVT inducible on EPS were all significantly associated with an increased likelihood of MVT detection. On multivariate analysis, the inducibility of MVT was the primary independent predictor of MVT detection but was of minimal incremental predictive value in the subgroup of patients with a history of only VF. When EPS results were not considered, arrhythmia history was the primary independent predictor of MVT detection. CONCLUSIONS: Patients with a history of only VF infrequently have MVT detected by their defibrillators. When these patients do detect MVT, it is faster than that detected in patients with a clinical history of MVT before ICD surgery. A significant percentage of VF survivors detected the abrupt onset of VF not preceded by MVT, suggesting that the deterioration of rapid MVT to VF is not the only clinically important mechanism of VF induction. These findings may have important implications for the understanding of the mechanism of VF induction and for use of an implantable defibrillator.

Truncated biphasic pulses for transthoracic defibrillation.

BACKGROUND: Early defibrillation is the single most important factor for improving out-of-hospital ventricular fibrillation resuscitation rates. To achieve the earlier response times required for survival, typically < 6 minutes from time of collapse, it will be necessary to equip a far wider network of first responders (firefighters, police, and other individuals with responsibility for public safety) with small, lightweight, and inexpensive automatic external defibrillators (AEDs). An important step in reducing the size and cost of AEDs will be to improve defibrillation efficacy. Because biphasic waveform defibrillation has had a favorable impact on implantable cardioverter-defibrillators (ICDs), there are reasons to believe that biphasic waveforms would also improve transthoracic defibrillators. Our purpose, therefore, was to examine the efficacy of two different low-energy biphasic truncated waveforms referenced to a standard damped sine waveform for transthoracic defibrillation in humans. METHODS AND RESULTS: We prospectively and randomly compared the transthoracic defibrillation efficacy of two different truncated biphasic waveforms, 115 J (70 microF) and 130 J (105 microF), with that of a standard 200-J (36-microF, 28-mH) damped sine wave pulse using right anterior and left lateral thoracic pads (R2 Medical Systems) in 30 cardiac arrest survivors during transvenous ICD surgery. The right anterior patch electrode was used as the cathode and the left lateral thoracic pad as the anode. Transthoracic ventricular defibrillation rescue shocks were tested after a failed transvenous defibrillation shock delivered in the course of ICD testing. Each of the three different rescue shocks was tested in random order in each patient. All shocks were delivered at end expiration. The investigators responsible for determining transthoracic shock efficacy were blinded throughout the study to the transthoracic rescue waveform used. A total of 33 patients were considered for study, but three patients failed to satisfy all entry criteria or did not have a sufficient number of ventricular fibrillation inductions to allow for testing of all three waveforms. Percent efficacy for the three waveforms was then compared in the 30 patients who satisfied entry criteria and completed the protocol. The study population had a mean age of 61 +/- 11 years, with 22 (73%) being men. The mean left ventricular ejection fraction was 0.39 +/- 0.14. Coronary artery disease was present in 22 (73%). The 115-J (70-microF) biphasic pulse, the 130-J (105-microF) biphasic pulse, and the 200-J (36-microF, 28-mH) damped sine wave pulse were equally effective, resulting in a 97% first-shock ventricular defibrillation efficacy rate. Each waveform failed to defibrillate once, with each waveform failing in a different patient. CONCLUSIONS: The results of this study suggest that biphasic truncated transthoracic shocks of low energy (115 and 130 J) are as effective as 200-J damped sine wave shocks used in standard transthoracic defibrillators. This finding may contribute significantly to the miniaturization and cost reduction of transthoracic defibrillators, which could enable the development of a new generation of AEDs appropriate for an expanded group of out-of-hospital first responders and, eventually, layperson use.

A prospective randomized comparison in humans of biphasic waveform 60-microF and 120-microF capacitance pulses using a unipolar defibrillation system.

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-microF and a 120-microF 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-microF or a 120-microF 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-microF 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-microF waveform was 60.6 +/- 7.1 omega. The 120-microF 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-microF capacitance pulse was 386 +/- 142 (P < .00001). Mean pulse impedance for the 120-microF waveform was 60.7 +/- 7.0 omega (P = .80). CONCLUSIONS: The results of the present study suggest that a relatively small capacitance, 60 microF, 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.

Serial defibrillation threshold measures in man: a prospective controlled study.

UNLABELLED: 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 nonthoracotomy/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.(ABSTRACT TRUNCATED AT 250 WORDS)

A prospective randomized comparison of defibrillation efficacy of truncated pulses and damped sine wave pulses in humans.

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 120 mu F 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 40 mu F 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) omega (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 defibrillators, there is no need to begin using damped sine wave pulses for implantable defibrillators. Moreover, these data raise a question regarding the need for inductors in transthoracic defibrillators.

Runaway pacemaker during high-energy neutron radiation therapy.

Runaway pacemaker occurred in a patient undergoing high-energy neutron radiation therapy despite adherence to published safety guidelines. The very low estimated dose of 0.9 Gy received by the pacemaker demonstrates the extreme sensitivity of integrated circuits to this new modality of radiation therapy.

Efficacy of a single-lead unipolar transvenous defibrillator compared with a system employing an additional coronary sinus electrode. A prospective, randomized study.

BACKGROUND: Recent development of a prototype single-lead unipolar transvenous defibrillator offers the possibility of device implantation with the ease of a permanent pacemaker. Lowering defibrillation energy requirements would allow for a further reduction in defibrillator generator size and enhance the feasibility of pacemaker-like placement. However, if achieving a lower defibrillation energy requires placing additional intracardiac leads, the potential advantage of a smaller generator may be offset by the disadvantages of a more complex lead system. The purpose of this study was to compare defibrillation energy requirements of a single-lead unipolar defibrillator with a three-electrode system employing an additional lead in the coronary sinus. METHODS AND RESULTS: Testing of a single-lead unipolar biphasic pulse defibrillation system versus a three-electrode system with an additional coronary sinus lead was performed in prospective, randomized fashion in 15 patients with a history of ventricular tachycardia or fibrillation. Ventricular fibrillation was induced with alternating current, and defibrillation threshold was measured by a pulse given 10 seconds after arrhythmia induction. The mean defibrillation threshold stored energy and mean leading edge voltage did not significantly differ between the two systems (11.3 +/- 5.9 J versus 9.9 +/- 5.2 J and 418 +/- 118 V versus 390 +/- 112 V, respectively; P > .4). Using either defibrillation system, all patients were successfully defibrillated by < 24 J and over half of patients by < 10 J. CONCLUSIONS: A unipolar transvenous biphasic defibrillation system is an effective means of treating ventricular fibrillation. The added complexity of additional leads is not offset by any significant improvement in defibrillation efficacy or energy requirements. Given the simplicity and effectiveness of a single-lead system coupled with a small generator, placement of defibrillation systems may now approach the ease of pacemaker implantation.

Prospective, randomized comparison in humans of a unipolar defibrillation system with that using an additional superior vena cava electrode.

BACKGROUND: A unipolar defibrillation system using a single right ventricular (RV) electrode and the active shell or "CAN" of the implantable cardioverter-defibrillator itself situated in a left infraclavicular pocket has been shown to be as efficient in defibrillation as an epicardial lead system. The purpose of this study was to determine whether defibrillation efficacy can be improved further by adding a superior vena cava (SVC) electrode to this already efficient defibrillation system. METHODS AND RESULTS: We prospectively and randomly compared the defibrillation efficacy of a simplified unipolar defibrillation system, RV-->CAN, with that of one incorporating a high SVC electrode, RV-->SVC + CAN, in 15 consecutive cardiac arrest survivors undergoing implantation of a presently available transvenous defibrillation system. The RV defibrillation electrode was a 5-cm coil located on a 10.5F lead used as the anode in both lead configurations examined. The active CAN was a 108-cm2 surface area shell of a titanium alloy pulse generator used as the cathode in both configurations and placed in a left infraclavicular pocket. The SVC electrode was a 6F 5-cm-long coil and was used as an additional cathode positioned at the junction of the SVC and the left innominate vein. The defibrillation pulse used was a 65% tilt, asymmetric biphasic waveform delivered from a 120-microF capacitor. The defibrillation threshold (DFT) stored energy, leading edge voltage, current, and pulsing resistance were measured for both lead systems. The single-lead unipolar system, RV-->CAN, resulted in a stored energy DFT of 7.4 +/- 5.2 J, and the three-electrode dual pathway system, RV-->SVC + CAN, resulted in a DFT of 6.0 +/- 3.4 J (P = .20). There was no difference in defibrillation efficacy with the more complicated three-electrode system over the unipolar system despite a decrease in pulsing resistance to 48.6 +/- 3.5 omega compared with 61.2 +/- 5.9 omega for the unipolar system (P < .0001) and a slight rise in delivered current to 6.3 +/- 1.8 A compared with 5.5 +/- 2.0 A for the unipolar system (P = .062). CONCLUSIONS: The unipolar single-lead transvenous defibrillation system provides defibrillation at energy levels comparable to that reported with present epicardial lead systems. Coupling of this lead system to a third SVC electrode increases system complexity but offers little defibrillation advantage despite a large decrease in pulsing resistance and a modest increase in delivered current.

Clinical predictors of implantable cardioverter-defibrillator shocks (results of the CASCADE trial). Cardiac Arrest in Seattle, Conventional versus Amiodarone Drug Evaluation.

The Cardiac Arrest in Seattle, Conventional Versus Amiodarone Drug Evaluation (CASCADE) study evaluated antiarrhythmic drug therapy in high-risk survivors of out-of-hospital ventricular fibrillation. Antiarrhythmic drug therapy for 228 patients was randomized to amiodarone or conventional antiarrhythmic drugs. Additional therapy with an implantable cardioverter-defibrillator was provided to 105 of these patients. Clinical predictors of shocks were evaluated for the 88 patients with coronary artery disease (amiodarone 46, conventional 42), treated with an implantable cardioverter-defibrillator. Survival free of all shocks at 2 years was 77% for patients taking amiodarone and 42% for those receiving conventional therapy (p = 0.014). Two-year survival free of syncopal shocks was 98% for amiodarone-treated patients and 81% for those receiving conventional agents (p = 0.01). Multiple clinical factors were evaluated by Cox analysis for potential clinical predictors of shocks. The independent clinical predictors of shocks were low ejection fraction (p = 0.002), female gender (p = 0.007) and conventional antiarrhythmic drug therapy (p = 0.015). The only independent predictor of a shock associated with syncope was conventional antiarrhythmic drug therapy (p = 0.035). Patients treated with amiodarone receive fewer shocks than patients treated with conventional drug therapy.