[Regular tachycardia with broad QRS complex: differential diagnosis on 12-lead ECG]. / Regelmässige Tachykardien mit breitem Kammerkomplex: Differenzialdiagnose mittels 12-Kanal-EKG.

Differential diagnosis of regular tachycardia with broad QRS complex can be challenging in daily practice. There are four different arrhythmias that have to be taken into account when being confronted with a broad QRS complex tachycardia: (1) ventricular tachycardia (VT); (2) supraventricular tachycardia (SVT) with bundle branch block (BBB); (3) SVT with AV conduction over an accessory AV pathway; (4) paced ventricular rhythm. Due to potentially fatal consequences, the correct diagnosis is important in view of both the acute treatment and the long-term therapy. Since SVT with accessory conduction is rare and a paced ventricular rhythm can be identified easily by stimulation artifacts, in most cases, a VT has to be differentiated from an SVT with BBB. Several ECG criteria can be helpful: (1) QRS complex duration > 140 ms in right BBB tachycardia or > 160 ms in left BBB tachycardia; (2) ventricular fusion beats; (3)"Northwest" QRS axis; (4) ventriculoatrial dissociation; (5) absence of an RS complex or RS interval > 100 ms in leads V(1)-V(6); (6) a positive or negative concordant R wave progression pattern in leads V(1)-V(6); (7) absence of an initial R wave or an S wave in lead V(1) in right BBB tachycardia; (8) absence of an R wave or an R/S ratio < 1 in lead V(6) in right BBB tachycardia; (9) absence or delay of the initial negative forces in lead V(1) in left BBB pattern (R wave duration > 30 ms in V(1); interval between onset of R wave and Nadir of S wave > 60 ms in V(1)); (10) presence of Q wave. Any of these variables favor VT. However, none of the criteria has both a sufficient sensitivity and specificity when utilized on its own. Therefore, various diagnostic algorithms have been proposed using a number of the above criteria consecutively. By doing so, the specificity and sensitivity of correctly identifying a VT or an SVT with BBB can be raised to > 95%.

Incidence and electrophysiological characteristics of spontaneous ventricular tachyarrhythmias in high risk coronary patients and prophylactic implantation of a defibrillator.

OBJECTIVES: To assess the incidence and electrophysiological characteristics of spontaneous ventricular tachyarrhythmias after implantable cardioverter-defibrillator (ICD) implantation for primary prevention. DESIGN: Prospective observational study. PATIENTS: 41 consecutive patients, who fulfilled MADIT (multicenter automatic defibrillator implantation trial) I criteria, except for suppressibility by procainamide, and who received a prophylactic ICD. INTERVENTIONS: Subpectoral implantation of an ICD. MAIN OUTCOME MEASURES: Incidence of ventricular tachyarrhythmias and their electrophysiological characteristics with respect to timing of the arrhythmia, tachyarrhythmia cycle length, mode of termination, and clinical relevance. RESULTS: During a mean (SD) follow up of 30 (21) months 18 of 41 (43.9%) patients experienced 142 appropriate ICD treatments. The mean (SD) time to first event was 9.6 (15.1) months. One patient had ventricular fibrillation (VF), 12 patients ventricular tachycardia (VT), and five both VT and VF. The mean (SD) cycle length of monomorphic VT was 306 (42) ms. Of 142 episodes, 117 (82.3%) were terminated by antitachycardia pacing and another 25 (17.6%) by ICD discharges. Cumulative survival of hypothetical death, defined as treated VT with a cycle length < 260 ms or VF, was 83.2% after one year and 78.4% after two years. CONCLUSIONS: Patients with a left ventricular ejection fraction < 35%, a history of myocardial infarction, non-sustained VT, and inducible VT/VF are at high risk of VT/VF early after implantation. Therefore, implantation of a tiered treatment defibrillator seems to be justified.

[Implantable cardioverter/defibrillator: long-term stability of the defibrillation threshold with a unipolar electrode configuration (active-can")]. / Implantierbarer Kardioverter-/Defibrillator: Langzeitstabilität der Defibrillationsschwelle mit einer unipolaren Elektrodenkonfiguration ("Active-Can").

The majority of cardioverter/defibrillator (ICD) implantations are currently performed with a non-thoracotomy approach. From November 1993 to January 1995, 46 patients underwent implantation of a PCD 7219C with an "active-can" lead configuration at our institution. While the chronic stability of the defibrillation threshold (DFT) for an epicardial lead system is well established, the results are still inconsistent for non-thoracotomy lead systems. Accordingly, the aim of the present study was to compare the acute and chronic defibrillation thresholds of the ICDs implanted with an "active-can" lead system in order to assess the chronic stability of these systems. The defibrillation energy requirements were measured at implant, prior to hospital discharge, three, six and twelve months after implantation of the defibrillator. The patient group consisted of 8 females and 38 males with a mean age of 57.2 years. The mean left ventricular ejection fraction was 43.8%. The most frequent underlying heart disease was coronary artery disease in 31 of 46 patients. Eight patients had idiopathic dilated cardiomyopathy. In 39 of 46 patients, the defibrillation threshold could be successfully determined at all 4 time points after implantation. The mean defibrillation energy requirement at the time of implantation was 9.2 +/- 5.9 Joules (J). The subsequent mean energy requirements were 7.6 +/- 4.8 J at pre-hospital discharge, 8.6 +/- 5.7 J at the 3 month, 8.1 +/- 6.0 J at the 6 month and 8.6 +/- 5.8 J at the 12 month follow-up visits. The mean defibrillation threshold was lowest at the time of prehospital discharge, significantly lower than at the time of initial implantation (p = 0.021). However, at all later time points up to one year, there was no significant difference in the DFT as compared with the time of initial implantation. Comparing the DFT at the time of implantation and the DFT at all other time points, there were no significant differences (9.23 vs. 8.56 J, p = 0.291). Although there was an initial decrease in the DFT at seven to ten days, the long-term stability of the DFT up to one year remained stable in the devices with the "active-can" lead system.

Hospital readmission after transvenous cardioverter/defibrillator implantation; a single centre study.

AIMS: Hospital readmission after implantation of cardioverter/defibrillators has a major impact on quality of life and cost-effectiveness in defibrillator patients. Rehospitalization has not been studied in large patient populations with modern transvenous defibrillation systems. METHODS AND RESULTS: We report on incidence, reasons, time in follow-up, duration and predictors of hospital readmission in 180 patients after transvenous implantation of a cardioverter/defibrillator during a follow-up period of 25+/-18 months. There were 156 readmissions in 79 patients with a 0.87 readmission rate per patient during the time followed, a 0.46 readmission rate per patient-year of follow-up and a 0.38 readmission rate per patient-year of follow-up for cardiac reasons. The majority of readmissions was caused by multiple appropriate shock interventions (26%), battery depletion (19%) and lead- and device-related complications (14%). The time to first hospital readmission was 12+/-9 months for arrhythmia-related and 20+/-16 months for other cardiac-related reasons (P<0.05), and could not be predicted by clinical variables, respectively. The duration of rehospitalization was 14+/-15 days for cardiac-related reasons and 12+/-17 days for arrhythmia-related reasons. Age >60 years was an independent predictor of rehospitalization time per patient-year of follow-up for both cardiac-related (P<0.005) and arrhythmia-related reasons (P<0.05). CONCLUSION: The rate of hospital readmission per patient-year of follow-up is as high as 0.46 after implantation of a modern cardioverter/defibrillator. Rehospitalization time in such patients is significantly longer in the patient cohort >60 years. The majority of readmissions is caused by multiple appropriate shock treatments. Further studies are needed to systematically investigate strategies for the prevention of rehospitalization in modern ICD therapy.

Clinical experience with implantable atrial and combined atrioventricular defibrillators.

UNLABELLED: The high prevalence of atrial fibrillation (AF) and its clinical complications, the poor efficacy of medical therapy for preventing recurrences, and dissatisfaction with alternative modes of therapy stimulated interest in implantable atrial and combined atrioventricular defibrillators. In a multicenter study, the safety and efficacy of a stand alone implantable atrial defibrillator, the Metrix system, were evaluated. The device was implanted in 51 patients with highly symptomatic episodes of AF refractory to pharmacological treatment. During a follow-up of 9 months, 96% of 227 spontaneous AF episodes were successfully converted to sinus rhythm in 41 patients. In 62 episodes (27%), several shocks and/or additional drug treatment were required to maintain stable sinus rhythm because of early recurrences of AF. A total of 3719 shocks were delivered and no induction of ventricular proarrhythmia or inaccurately synchronized shocks occurred. The AF detection algorithm exhibited a 100% specificity for the recognition of sinus rhythm and a 92.3% sensitivity for the detection of AF. The combined atrioventricular defibrillator, Jewel AF 7250, was evaluated in a multicenter, randomized, cross-over trial. The primary study objectives included: overall safety as determined by complications-free survival at 6 months, efficacy of tiered atrial pacing and defibrillation therapies for termination of spontaneous atrial tachycardias (AT) and AF, and relative sensitivity of a new dual-chamber detection algorithm. The device was implanted in 211 patients with either a history of ventricular tachyarrhythmias (VT/VF) alone or with a history of both AT/AF and VT/VF. During a mean follow-up of 4.5 months, it has been shown that the Jewel AF is safe and effective in treating atrial and ventricular tachyarrhythmias. Pace termination of 85% of AT episodes were achieved with painless delivery of antitachycardia pacing; additional 35% of AT episodes were terminated by high frequency burst pacing. CONCLUSIONS: The stand alone implantable atrial defibrillator may be safe and clinically useful in selected patients for the treatment of highly symptomatic, drug resistant recurrences of AF. The combined atrioventricular defibrillator may be particularly indicated in patients presenting with both a history of atrial and ventricular tachyarrhythmias.

[Clinical experience with the implantable atrial defibrillator (atrioverter) in patients with atrial fibrillation. Metrix Investigators]. / Klinische Erfahrungen mit dem implantierbaren atrialen Defibrillator (Atrioverter) bei Patienten mit Vorhofflimmern. Metrix Investigators.

Due to the limited efficacy of drug therapy in atrial fibrillation and the high rate of recurrence, strong efforts were made to find non-pharmacological strategies. For three years now, the implantable atrial defibrillator Metrix from InControl has been available as an alternative therapy. From October 1995 to the present the atrial defibrillator was implanted in 179 patients worldwide. The sensitivity of the system and its algorithms to detect atrial fibrillation is 90%; the specificity to detect sinus rhythm is 100%. In 121 of 179 patients, 748 episodes of spontaneous atrial fibrillation were treated with 2.4 shocks per episode. No proarrhythmic event or stroke was seen. A cardioversion to sinus rhythm could be achieved in 95% of patients; the overall clinical success rate was 88%. In 7% of all patients, early recurrence of atrial fibrillation (ERAF) occurred that could not be converted into stable sinus rhythm after further cardioversions and antiarrhythmic therapy. In 4.1% there were lead-related complications, in 4 patients the device had to be explanted because of ineffective therapy, and in 3 patients the device had to be changed because of loss of telemetry or early depletion of battery. In 8 patients, postoperative complications were seen (infections, pneumothorax and thrombosis of the subclavian vein). Overall, the implantable atrial defibrillator Metrix is an effective and safe alternative in treating atrial fibrillation.

Inappropriate implantable defibrillator discharge caused by a retained pacemaker lead fragment.

INTRODUCTION: Inappropriate discharge is still a major issue of implantable cardioverter defibrillator therapy. The diagnostic options of modern devices facilitate classification of the underlying abnormality. METHODS AND RESULTS: A 65-year-old woman with depressed left ventricular performance received spurious shocks from an ICD, implanted for ventricular tachycardia. A lead fragment of an explanted VVI pacemaker system could be identified as cause of erroneous ventricular fibrillation detection by the ICD. The electrical noise caused by interaction between the lead remnant and the ICD lead was detectable even on the surface ECG. CONCLUSION: Based on our findings, removal of fragmented lead material should be considered prior to ICD implantation, to avoid potentially adverse and harmful interactions with ICD systems.

Case report: inappropriate discharge of an implantable cardioverter-defibrillator caused by the combined count criterion.

We describe a rare case of an inappropriate defibrillator discharge during a sinus tachycardia with single fast sensed beats fulfilling the ventricular fibrillation rate criterion, activating the "combined count criterion," although the prevalent sinus tachycardia was inside a ventricular tachycardia rate zone, programmed to monitor only mode. The "combined count criterion" is a safety feature that ensures defibrillation therapy in case of a tachyarrhythmia inside the ventricular tachycardia detection zone, that is temporarily crossing the ventricular fibrillation detection rate. This case reports shows that there is a low but unpredictable risk of device therapy even in a zone programmed to monitor only mode. This phenomenon should be considered when a monitor only zone is installed for revelation of slower tachyarrhythmias.

Electrical proarrhythmia: induction of inappropriate atrial therapies due to far-field R wave oversensing in a new dual chamber defibrillator.

This case report describes delivery of atrial therapies during a sinus tachycardia in a new dual chamber implantable cardioverter defibrillator inappropriately caused by far-field oversensing of ventricular beats in the atrial channel. Upon classification of the PR interval pattern, the rate criterion for an atrial tachycardia was fulfilled, and the device initiated high-frequency burst pacing as the first stage of programmed tiered atrial therapies. Atrial fibrillation subsequently was induced by high-frequency burst pacing, and eventually a programmed 10-J shock was delivered for successful termination of atrial fibrillation. The phenomenon of far-field oversensing of ventricular beats could be repeatedly observed during exercise testing and abolished by decreasing the atrial sensitivity.

Specific considerations with the automatic implantable atrial defibrillator.

INTRODUCTION: Internal atrial defibrillation has been evaluated as an alternative approach to the external technique for more than two decades. Previous studies in animals and humans have shown that internal atrial defibrillation is feasible with relatively low energies. The promising results achieved with internal atrial defibrillation have facilitated the development of an implantable atrial defibrillator (IAD). METHODS AND RESULTS: For any new therapy, it is imperative to demonstrate safety, efficacy, tolerability with improvement in quality of life, and cost-effectiveness compared with therapeutic options already available. Maintenance of sinus rhythm or prolonged duration in arrhythmia-free intervals should be demonstrated clearly with an IAD. Initial clinical experience with the Metrix system indicates stable atrial defibrillation thresholds, appropriate R wave synchronization markers, no shock-induced ventricular proarrhythmia, and excellent detection of atrial fibrillation (AF) with a specificity of 100%. Ventricular proarrhythmia has not been reported for correctly R wave synchronized low-energy shocks when closely coupled to RR intervals, and long-short cycles are avoided. CONCLUSION: Preliminary experience with the Metrix system suggests that the IAD may offer a therapeutic alternative for a subgroup of patients with drug-refractory, symptomatic, long-lasting, and infrequent episodes of AF. Further efforts must be undertaken to reduce the patient discomfort associated with internal atrial defibrillation in an attempt to make this new therapy acceptable to a larger patient population with AF.

[Circadian and weekly distribution of malignant ventricular arrhythmias in patients with coronary heart disease or dilatative cardiomyopathy who have an implanted cardioverter-defibrillator]. / Zirkadiane und wöchentliche Verteilung maligner ventrikulärer Tachyarrhythmien bei Patienten mit koronarer Herzkrankheit oder dilatativer Kardiomyopathie mit implantiertem Kardioverter-Defibrillator.

BACKGROUND AND OBJECTIVE: Epidemiological studies have demonstrated a circadian distribution of sudden cardiac death (SD) and acute myocardial infarction (AMI), with a maximum frequency of events during the morning hours. Recently an analysis of computer recordings of implanted cardioverter/defibrillators has confirmed these findings with respect to SD. The majority of these studies concerned patients with coronary heart disease. In a prospective study we evaluated the circadian and weekly distribution of malignant ventricular tachyarrhythmias (VTA) in patients with a nonischaemic cardiac disease, namely dilated cardiomyopathy (CMP), and those with coronary heart disease (CHD). PATIENTS AND METHODS: Over a mean period of 25 +/- 9 months computer-stored data and cardiac electrograms recorded from the implanted cardioverter/defibrillators (ICD) were analysed in 28 patients with CHD and 11 with DCMP. The circadian and weekly distribution of VTA was obtained, using customary arrhythmia classification and the stored and timed events. RESULTS: Patients with CHD had a maximal frequency of VTA or ventricular fibrillation (VF) during the morning, with a peak between 9 and 10 o/c, while those with DCMP were at a higher risk of VTA or VF in the later afternoon and early evening. Regarding the frequency of events during the week, a significantly higher incidence of VTA was recorded on Saturdays in those with CHD, but on Mondays and Wednesdays in those with DCMP. INTERPRETATION: The circadian and weekly distribution of VTA differs significantly between patients with CHD and those with DCMP. A possible reason for this difference may be different trigger mechanisms in the two types of cardiac disease, since transient ischaemia is unlikely to be the cause in patients with nonischaemic DCMP.

A new classification algorithm for discrimination of ventricular from supraventricular tachycardia in a dual chamber implantable cardioverter defibrillator.

INTRODUCTION: The high incidence of inappropriate therapies due to supraventricular tachycardia remains a major unsolved problem of implantable cardioverter defibrillators. We report a new detection formula for discrimination of ventricular tachycardia from supraventricular tachycardia in a patient with a dual chamber implantable cardioverter defibrillator and a new atrioventricular classification algorithm. METHODS AND RESULTS: The enhanced detection algorithm performs a stepwise arrhythmia analysis. The rhythm is first classified on the basis of cycle length. Each episode is then classified as supraventricular or ventricular on the basis of atrioventricular association, stability of circle length, and origin of acceleration. Sophisticated diagnostic information is provided by atrioventricular markers and electrogram recordings. Successful discrimination of two spontaneous episodes of ventricular tachycardia and supraventricular tachycardia is demonstrated. CONCLUSION: This new dual chamber detection algorithm may significantly improve the specificity of tachyarrhythmia detection without sacrificing sensitivity, thereby reducing the number of spurious shocks in patients with recurrent supraventricular tachycardias. Further studies are needed to assess the sensitivity and specificity of this detection algorithm.

Prospective evaluation of the quality and long-term stability of atrial signals in non-thoracotomy defibrillation electrodes: comparison of four different endocardial electrograms.

UNLABELLED: Documentation of atrial signals in stored endocardial electrograms of modern implantable cardioverter-defibrillators (ICD) is a useful tool to classify the underlying arrhythmia leading to device therapy. Newest generations of ICD provide near- and far-field electrograms derived between various endocardial electrodes. The aim of this prospective study was to assess the quality and long-term stability of atrial signals in different far-field configurations including the active can housing. METHODS AND RESULTS: A total of 300 real-time endocardial electrogram recordings in 60 consecutive patients with a modern ICD in subpectoral position were analysed at the time of implant, pre-hospital discharge, 1, 3 and 12 months follow-up. Four different configurations were evaluated: right ventricular coil to can housing, can housing to pace/sense ring, right ventricular coil to pace/sense tip, and pace/sense tip to pace/sense ring. The best visibility of p-waves at an ECG-resolution of 0.5 mV/mm was seen in the can to coil configuration (77% of the patients). In the can to pace/sense ring electrogram p-waves could be observed in 58% of the patients. No p-waves were visible to pace/sense tip to pace/sense ring. At a resolution of 1.0 mV/mm p-waves were only visible in 10% of all patients exclusively in the can housing to right ventricular coil configuration. The results were stable (100% of the patients) over a follow-up of one year. CONCLUSIONS: Endocardial far-field electrograms, derived from the can housing and the right ventricular coil provide a p-wave visibility in 77% of the patients and demonstrate a long-term stability over at least one year, provided that the ECG-resolution is set at 0.5 mV/mm. Since the electrogram resolution of stored electrograms depends on the EGM-range, and the ECG-resolution at an EGM-range of 15 mV would be 1 mV/mm, the EGM-range is recommended to be programmed to 7.5 mV to ensure an ECG-resolution of at least 0.5 mV/mm for stored electrograms.

[Circadian variation of ventricular tachyarrhythmias in patients with an implantable cardioverter-defibrillator]. / Zirkadiane Variation ventrikulärer Tachyarrhythmien bei Patienten mit implantierbarem Kardioverter-Defibrillator.

For the acute myocardial infarction and sudden cardiac death a circadian variation can be observed. There are several, mostly epidemiological studies that demonstrate this non-uniform distribution of events in different day-time periods. These studies are in the vast majority based on eye-witness reports, medical documents and retrospective evaluation of the timing of the event. Moreover, they represent only singular observations in a heterogeneous population. With the introduction of implantable cardioverter-defibrillators which provide extensive diagnostic features such as stored RR-intervals, endocardial electrograms of each episode and internal time storage, an exact analysis of the circadian variation of malignant ventricular tachyarrhythmias became feasible. Ventricular tachyarrhythmias follow a different circadian distribution with increased number of events at certain day-time periods. For patients with a coronary artery disease a significantly higher risk to experience a potentially fatal arrhythmia could be shown for the mid-morning hours and a secondary peak occurrence for the late afternoon and the early evening. Patients with idiopathic dilated cardiomyopathy tend to have a higher probability of events in the evening hours. With respect to beta-blockers and other antiarrhythmic drugs most of the studies failed to confirm a blunting influence on the degree of variability for fast arrhythmias. Others showed similar patterns for fast and slow tachycardias. Another clinical parameter, the degree of congestive heart failure was in some studies found to influence the pattern in the sense of causing a distinct difference in occurrence frequency. A higher vairation was determined for lower NYHA classes < III. Other studies were contrary to these findings. The activity level before the onset of arrhythmias has yet been only insufficiently analyzed regarding the day-time-variability. But first results make believe, that patients younger than 50 years and still involved in the working process seem to follow a more varying circadian distribution. More substudies will have to be performed to compare working and nonworking patients and patients with different structural heart diseases. Furthermore ther influence of antiarrhythmic agents on the day-time-variation of ventricular tachyarrhythmias should be assessed. Finally, there, should be systematic prospective studies that evaluate the influence of drug administration on triggers of ventricular tachyarrhythmias such as sympathoadrenergic activity, premature ventricular beats, reduction of heart rate variability and others with respect to their day-time-variability.

Incidence of ICD lead related complications during long-term follow-up: comparison of epicardial and endocardial electrode systems.

UNLABELLED: The aim of this study was to evaluate the long-term stability of epicardial and endocardial lead systems for third-generation cardioverter defibrillators (ICDs) and to assess the usefulness of diagnostic tools. One hundred forty patients with 61 epicardial (43.6%) and 79 nonthoracotomy systems (56.4%) were followed for 25 +/- 19 months. A total of 18 (12.9%) lead related complications were documented. Complications of epicardial systems were detected in 10 patients (16.4%) during a follow-up time of 36 +/- 8 months: crinkling of patch electrodes in 6 patients (9.8%), insulation breakage of sensing electrodes in 2 patients (3.3%), and adapter defect in 2 patients (3.3%). Eight of the patients (10.1%) with transvenous-subcutaneous systems had lead related complications during a 13 +/- 6 months follow-up: fracture of the subcutaneous patch lead in 2 patients (2.5%), dislodgement of the right ventricular lead in 2 patients (2.5%), dislodgement of the superior vena cava lead in 2 patients (2.5%), insulation breakage of sensing electrodes in 1 patient (1.3%), and connector defect in 1 patient (1.3%). There was no significant difference in the incidence of lead related complications between epicardial and endocardial systems (P > 0.05). Fractures, dislodgements, and crinklings were documented within the first 8 +/- 5 months by regular chest X ray. Defects of insulation, adapter, or connector were detected 22 +/- 10 months after implantation and were associated with delivery of multiple inappropriate ICD therapies. An operative lead revision was indicated for 4 epicardial (6.6%) and 6 endocardial (7.6%) lead systems. CONCLUSIONS: Endocardial lead systems offer a similar long-term stability as compared to epicardial lead systems. Chest X ray is the most useful tool to detect lead fracture, dislodgment, and patch crinkling. Marker recordings or real-time electrograms have not been helpful in this series to identify patients with suspected lead defects prior to the experience of inappropriate ICD discharges.

Clinical efficacy of shock waveforms and lead configurations for defibrillation.

A randomized, prospective comparison of the defibrillation efficacy of various shock waveforms and nonthoracotomy lead configurations was performed in five distinct patient groups undergoing implantation of a cardioverter defibrillator. In the first group using a bidirectional lead configuration, there was no significant difference in the mean defibrillation threshold (DFT) between simultaneous and sequential monophasic shocks (17.8 +/- 5.8 joules versus 17.3 +/- 2.7 joules). In the second group using a bidirectional lead configuration, the mean DFT was 21.9 +/- 7.3 joules with monophasic shocks and 14.9 +/- 5.0 joules with biphasic shocks (p < 0.001). In the third group using a unidirectional lead configuration, the mean DFT was significantly higher (p < 0.001) with monophasic shocks (22.1 +/- 4.2 joules) compared with biphasic shocks (15.0 +/- 5.4 joules). In the fourth group, an intraindividual comparison with monophasic shock waveforms showed no significant differences in DFT using either a bidirectional (21.3 +/- 5.8 joules) or a unidirectional (21.7 +/- 2.6 joules) lead configuration. In the fifth group, a simplified unipolar transvenous defibrillation lead system ("active can") demonstrated significant lower DFTs (9.7 +/- 3.8 joules) compared with a standardized unidirectional lead configuration (18.0 +/- 6.8 joules). It is concluded that: (1) there seems to be no significant difference in the DFT between simultaneous and sequential monophasic shocks; (2) biphasic waveforms require significantly less energy for defibrillation than their corresponding monophasic waveforms; and (3) the unipolar single-electrode defibrillation system is easy to implant and provides DFTs at energies comparable with epicardial lead systems.