Comparison of Clinical Benefits and Outcome in Patients with Programmable and Nonprogrammable Implantable Cardioverter Defibrillators

Technological advances in implantable cardioverter defibrillators (ICDs) have provided a variety of programmable parameters and antitachycardia therapies whose utility and impact on clinical outcome is presently unknown. ICDs have capabilities for cardioversion defibrillation alone (first generation ICDs), or in conjunction with demand ventricular pacing (second generation ICDs), or with demand pacing and antitachycardia pacing (third generation ICDs). We examined the pattern of antitachycardia therapy use and long-term survival in 110 patients with sustained ventricular tachycardia (VT) or ventricular fibrillation (VF). Group I included 62 patients with nonprogrammable first generation ICDs that delivered committed shock therapy after ventricular tachyarrhythmia detection based on electrogram rate and/or morphology was satisfied. Group II included 48 patients with multiprogrammable ICDs (including second and third generation ICDs) that had programmable tachyarrhythmia detection based on rate and tachycardia confirmation prior to delivery of electrical treatment with either programmable shocks and/or, as in the third generation ICDs, antitachycardia pacing. Incidence and patterns of antitachycardia therapy use and long-term survival were compared in the two groups. The incidence of appropriate shocks in patients who completed 1 year of follow-up was significantly greater in group I (30 of 43 patients = 70% vs 11 of 26 patients = 42%; P less than 0.05). In the total follow-up period, a significantly larger proportion of group I patients as compared to group II patients used the shock therapies (46 of 62 patients = 74% vs 25 of 48 patients = 52%; P less than 0.01), with the majority doing so within the first year of implantation (96% and 92%, respectively). Although the frequency of antitachycardia therapy activation was similar, the number of shocks delivered per patient was lower in group II, particularly in the initial 3 months of follow-up (P = 0.06). No clinical variable aided in identifying users from nonusers of antitachycardia therapy. Arrhythmic mortality was virtually eliminated in both groups. Two-year actuarial cardiac survival in the two groups was similar (group I = 78% vs group II = 84%; P greater than 0.2). Survival from cardiac mortality in users and nonusers of antitachycardia therapies was also similar in both groups (P greater than 0.2) and in the total patient group (P greater than 0.2). We conclude that programmable ICDs continue to confer advantages in prevention of sudden death that were observed with nonprogrammable ICDs and can be expected to improve patient tolerance and physician acceptance of device therapy for VT/VF.     

Conversion Rates of Induced Versus Spontaneous Ventricular Tachycardia by a Third Generation Cardioverter Defibrillator

Seventy consecutive patients received the first VENTAK PRx pulse generators (Cardiac Pacemakers, Inc.) implanted in the United States. This multiprogrammable device has therapeutic options that include: (1) antitachycardia pacing; (2) low energy cardioversion; (3) defihrillation shocks; and (4) bradycardia pacing. There were 60 males and 10 females with a mean age of 65.3 ± 9.4 years. The anatomical diagnoses were coronary artery disease in 55 patients, cardiomyopathy in 7 patients, congenital heart disease in 2 patients, and miscellaneous disease in the remaining 6 patients. Thirty-six percent had a history of sudden cardiac death and 90% had documented monomorphic ventricular tachycardia. The mean ejection fraction was 32.7%± 12.2%. Thirty-three (49.3%) had an ejection fraction ≤ 30%. Electrophysiological testing was done preimplant, predischarge, and 1 to 2 months postimplant to define a specific electrical therapy and evaluate the efficacy of the device. Two hundred ninety-three of 367 (80%) episodes of induced ventricular tachycardia were successfully terminated by the VENTAK PRx programmed for antitachycardia pacing. There were 1,794 total therapy episodes for spontaneous ventricular tachycardia; 91% (1,641 episodes) were terminated by antitachycardia pacing and 153 episodes were converted by shocks during a minimal 6-month follow-up per patient. We conclude that documentation of a successful antitachycardia pacing modality in the electrophysiology laboratory predicts conversion of spontaneous episodes of ventricular tachycardia. Furthermore, antitachycardia pacing by the VENTAK PRx can terminate the majority of episodes of ventricular tachycardia.     

Ventricular Pacing Induced Ventricular Tachycardia in Patients with Implantable Cardioverter Defibrillators

Appropriately timed noncompetitive ventricular pacing potentially may initiate ventricular tachycardia in patients prone to these arrhythmias. The combination of bradycardia pacing and stored electrograms in a currently available cardioverter defibrillator provides an opportunity to evaluate the occurrence of such pacing induced ventricular tachycardia. During a surveillance period of 18.7 ± 11.4 months, stored electrograms documented 302 episodes of ventricular tachycardia in 77 patients. Five patients (6.5%) demonstrated 25 episodes (1–16 per patient) of ventricular tachycardia that were immediately preceded by an appropriately paced ventricular beat (8.3% of all episodes of ventricular tachycardia). All five patients had prior myocardial infarctions and a history of monomorphic ventricular tachycardia occurring both spontaneously and in response to programmed electrical stimulation. Antitachycardia pacing terminated pacing induced ventricular tachycardia in 22 episodes; in one episode antitachycardia pacing accelerated ventricular tachycardia. In two cases shock therapy was aborted for nonsustained ventricular tachycardia. We conclude that, in selected postinfarction patients with recurrent sustained monomorphic ventricular tachycardia treated with implantable cardioverter defibrillators, appropriately timed ventricular pacing may induce ventricular tachycardia.     

Catheter Ablation versus Standard Conventional Treatment in Patients with Left Ventricular Dysfunction and Atrial Fibrillation (CASTLE-AF) - Study Design

Background: Electrical isolation of the pulmonary veins by catheter ablation is an emerging treatment modality for the treatment of atrial fibrillation (AF) and is increasingly used in patients with heart failure.
Methods: The catheter ablation versus standard conventional treatment in patients with left ventricular dysfunction and atrial fibrillation trial (CASTLE-AF) is a randomized evaluation of ablative treatment of atrial fibrillation in patients with left ventricular dysfunction. The primary endpoint is the composite of all-cause mortality or worsening of heart failure requiring unplanned hospitalization using a time to first event analysis. Secondary endpoints are all-cause mortality, cardiovascular mortality, cerebrovascular accidents, worsening of heart failure requiring unplanned hospitalization, unplanned hospitalization due to cardiovascular reason, all-cause hospitalization, quality of life, number of therapies (shock and antitachycardia pacing) delivered by the implantable cardioverter-defibrillator (ICD), time to first ICD therapy, number of device-detected ventricular tachycardia and ventricular fibrillation episodes, AF burden, AF free interval, left ventricular function, exercise tolerance, and percentage of right ventricular pacing. CASTLE-AF will randomize 420 patients for a minimum of 3 years at 48 sites in the United States, Europe, Australia, and South America.     

''Long-Term Results of Antitachycardia Pacing in Patients with Supraventricular Tachycardia

Between 1979 and 1984 the Cybertach-60, (Intermedics, Inc. Model 262-01), a programmable, automatic antitachycardia pacemaker was implanted in 11 patients who had drug-refractory supraventricular tachycardia (SVT). The patients have been followed for a total of 64-108 (mean 84 months). All patients were symptomatic and had failed two or more drugs and six patients had required prior DC cardioversion. The mechanism of supraventricular tachycardia was atrioventricular (AV) nodal reentry in six patients, AV reentry in four patients, and atrial tachycardia in one patient. Preoperatively all patients had reliable termination of the tachycardia without induction of atrial fibrillation by pacing methods available to Cybertach-60. Postimplant, Cybertach-60 reliably terminated all episodes of tachycardia without ancillary drug therapy. Nevertheless, at long-term follow-up antitachycardia pacing was effective and safe in the minority (36%), with only four patients out of eleven still using a pacemaker for supraventricular tachycardia. One of these four patients required additional drug therapy. In one of the patients, the Cybertach-60 was replaced after 78 months by a more advanced device, (Intertach, Intermedics, Inc.) because of a depleted Cybertach-60 battery. In seven patients who no longer use antitachycardia pacing for termination of tachycardia, one patient developed atrial fibrillation during tachycardia termination (at 58 months postimplant). Three patients experienced induction of tachycardia or atrial fibrillation by the pacemaker due to undersensing of sinus P waves (at 36, 48, and 51 months).(ABSTRACT TRUNCATED AT 250 WORDS)     

Significance of Supraventricular Tachyarrhythmias in Patients with Implanted Pacing Cardioverter Defibrillators

Eighty-six patients were treated with an implantable cardioverter defibrillator (ICD) because of sustained ventricalar tachycardia (VT) or ventricular fibrillation (VF). In 27 patients an epicardial system was used, in 59 patients a transvenous system with a subcutaneous patch electrode was implanted. During a mean follow-up time of 17 ± 9 months, inappropriate activations of the ICD due to supraventricular tachycardia were documented by Holter monitoring in 14 patients (16%). In 8 patients paroxysmal atrial fibrillation (AF), in 2 patients chronic AF, in 1 patient atrial flutter, and in 3 patients sinus tachycardia triggered antitachycardia pacing functions (12 patients) or internal defibrillation (2 patients). In 3 patients (5%) VT was induced by inappropriate antitachycardia pacing. In an additional 18 patients (21%) inappropriate activation of antitachycardia functions due to atrial tachyarrhythmias were suspected based on telemetry readouts or the patient's history. Inappropriate activation of ICD therapy triggered by intermittent supraventricular tachyarrhythmias is common. Further improvements of detection algorithms for supraventricular tachycardia are required in future device generations.     

Optimization of Pacing Algorithms to Prevent and Treat Supraventricular Tachyarrhythmias

Preventive atrial pacing and antitachycardia pacing have been proposed for the treatment of atrial fibrillation and associated arrhythmias in patients with indications for device implantation. Preventive algorithms provide overdrive atrial pacing, reduction of atrial premature beats, and prevent short-long atrial cycles with good patient tolerance. However, clinical trials testing preventive algorithms have shown contradictory results, possibly because of different trial designs, end points and patient populations. Factors probably responsible for neutral results include an already high atrial pacing percentage with the conventional DDDR mode, suboptimal atrial pacing site, and the deleterious effects of high percentages of right ventricular apical pacing. Atrial antitachycardia pacing therapies are effective in treating organized atrial tachyarrhythmias (that precede atrial fibrillation), mainly when delivered early after the onset particularly if the tachycardia is relatively slow. Antitachycardia pacing therapies might influence atrial fibrillation burden, but clinical studies have shown conflicting results about this issue. Consistent monitoring of atrial and ventricular rhythm including progression to persistent forms of atrial arrhythmias, variability of atrial arrhythmia recurrence patterns and onset mechanisms as well as antitachycardia pacing efficacy should be recorded in the stored device memory and used for optimal individual programming of these new functions.     

Clinical Experience with the Intertach 262-12 Pulse Generator in Patients with Recurrent Supraventricular and Ventricular Tachycardia

FROMER, M., ET AL.: Clinical Experience with the Intertach 262-12 Pulse Generator in Patients with Recurrent Supraventricular and Ventricular Tachycardia. An antitachycardia pulse generator, the Intermedics Intertach 262-12 was implanted in 16 patients (14 patients with supraventricular tachycardia of various origins and two patients with recurrent ventricular tachycardia), who were not responsive to various antiarrhythmic drug regimens. The follow-up was from 6–49 months (mean 30.9 ± 13.8). Five patients had a follow-up of over 3 years. The device was used in all patients. One patient with ventricular tachycardia died from a nonarrhythmic cause. Loss of responsiveness to burst pacing was observed in 1/14 patients with supraventricular tachycardia and nontolerance of antitachycardia pacing in one patient. Overall clinical success of pacing was observed in 13/16 patients = 81%. The pacemaker proved to be a versatile system with reliable tachycardia detection and termination functions.     

Paced Beats Following Single Nonsensed Complexes in a "Codependent" Cardioverter Defibrillator and Bradycardia Pacing System: Potential for Ventricular Tachycardia Induction

Patients with implantable defibrillators often require bradycardia pacemakers. Adverse interactions between separate defibrillator and bradycardia pacing units have occurred, including failure to detect ventricular fibrillation due to persistent bradycardia pacing during the arrhythmia. A device with combined bradycardia pacing and antitachycardia therapy capability may obviate adverse device interactions. We describe a previously unrecognized phenomenon that may occur in a combined device when the algorithms for sensing bradycardia and tachycardia are "codependent"; that is, the circuitry for brady- and tachyarrhythmia detection relies on the same automatic gain sense amplifier. Three of 37 patients in whom the device was implanted had ventricular tachycardia initiated when bradycardia pacing stimuli were delivered by the device after probable nonsensed sinus beats. In each case, nonsensed beats appeared to have a markedly diminished amplitude, occurred after ventricular premature depolarizations that produced large amplitude electrograms, and had an electrogram morphology that matched that of sinus rhythm. In each case, the bradycardia pacing interval was at least 1,200 msec (range 1,200 to 1,714 msec). In two of the three patients, large amplitude ventricular premature depolarizations or nonsustained ventricular tachycardia caused an adjustment of the gain control that potentiated the failure to sense the subsequent lower amplitude signal. In all three patients, the induced arrhythmia was rapidly terminated by pacing or cardioversion. Decreasing the bradycardia pacing interval by 110-514 msec has prevented recurrence during short-term follow-up. Our findings suggest that codependent bradycardia and antitachycardia devices may have their own unique potential difficulties in adapting to rapid changes in rate and signal amplitude.     

Inappropriate Shock Despite Successful Termination of Supraventricular Tachycardia by Antitachycardia Pacing during Charging

We present a case of a 70‐year‐old man who received an inappropriate implantable cardioverter‐defibrillator shock for sinus tachycardia falling outside of the ventricular tachycardia zone. This occurred after termination of supraventricular tachycardia falling into the ventricular fibrillation zone by antitachycardia pacing. Particularities of the programming algorithms are reviewed. (PACE 2010; 33:e81–e83)     

Holter Documented Sudden Death in a Patient with an Implanted Defibrillator

A 68-year-old man with recurrent attacks of monomorphic ventricular tachycardia (VT) received a pacer cardioverter defibrillator featuring antitachycardia pacing and cardioversion/defibrillation. Over 300 episodes of VT were successfully terminated by antitachycardia pacing. During Holter monitoring the patient experienced supraventricular tachycardia with delivery of multiple antitachycardia pacing, cardioversion, and defibrillation therapies ending with the death of the patient. The following factors played a role in the unfortunate outcome of this patient: 1. triggering of VT therapy by an unexpected high sinus rate; 2. atrial fibrillation induced by cardioversion therapy; 3. a gradual and continuous increase in rate during atrial fibrillation possibly caused by repeated VT and ventricular fibrillation therapies and/or by a thrombus, found at autopsy, in a bypass graft; and 4. the limited ability of presently available defibrillators to distinguish between ventricular and supraventricular arrhythmias.     

Combination of Antitachycardia Pacemaker and Automatic Implantable Cardioverter/Defibrillator for Ventricular Tachycardia

Antitachycardia pacing for ventricular tachycardia (VT) is associated with the possibility of fibrillating the heart; on the other hand, the frequency of VT and patient discomfort can limit treatment with the automatic implantable cardioverter/defibrillator (AICD). To contribute to the further development of a universal pacemaker, we evaluated the combined use of the antitachycardia pacemaker ("tachylog") and the AICD in five patients with recurrent VT. In the automatic mode, the "tachylog" worked as a bipolar VVI pacemaker. For antitachycardia pacing, a burst of rapid ventricular pacing was delivered at about 80% of the cycle length. During a follow-up period of 5 +/- 2 months (range, 3 to 8) two to 291 successful interventions of antitachycardia pacing were counted from diagnostic data which had been collected by the pulse generator during the course of treatment. When the antitachycardia pacemaker failed to terminate VT, the AICD was activated. In the individual case, between 0 and 41 discharges of the AICD were delivered. The high pulse energy of the AICD did not damage the antitachycardia pacemaker; no interference of the two devices was observed. Future antitachycardia systems should be more flexible with regard to detection and termination modes, combining antitachycardia pacing with back-up defibrillation.     

Pacing for tachydysrhythmias

Advances in cardiac electrophysiology have clarified some of the mechanisms of tachydysrhythmias and have characterized tachycardias that are amenable to pace termination. Tachydysrhythmias most likely to be terminated by pacing techniques tend to be slow, re-entrant tachycardias. Although single extrastimulation can be effective, its yield is relatively low. Delivery of multiple extrastimuli or burst pacing are more effective than single extrastimulation at the expense of a higher risk of tachycardia acceleration or transformation of the rhythm to fibrillation. The use of pacing for the termination of tachydysrhythmias has primarily been limited to the acute care setting, although permanently implantable antitachycardia pacemakers are being used in increasing numbers. While these devices have been approved by the Food and Drug Administration (FDA) for the treatment of supraventricular tachydysrhythmias, their use to treat ventricular tachydysrhythmias has produced mixed results and remains investigational. In some cases of sustained ventricular tachycardia, automatic cardioverter/defibrillators have been implanted along with antitachycardia pacemakers to provide high-energy shock back-up in case of tachycardia acceleration by the pacemaker. Soon, devices will be available which will combine, in a single unit, antitachycardia pacing and high-energy cardioversion/defibrillation.     

Effectiveness of Noninvasive Programmed Stimulation for Initiating Ventricular Tachyarrhythmias in Patients with Third-Generation Implantable Cardioverter Definrillators

Previous generations of implantable cardioverter defibrillators (ICDs) required invasive electrophysiological testing to assess defibrillator function. Newer third-generation ICDs include the capability for performing noninvasive programmed stimulation (NIPS) and may reduce the need for invasive studies to assess tachycardia recognition and antitachycardia therapy algorithms. The effectiveness of ICD-based NIPS for the induction of ventricular arrhythmias has not, however, been formally assessed. Third-generation ICDs were implanted in 79 patients, who underwent a total of 166 postoperative defibrillator tests. NIPS with rapid ventricular pacing was performed in all patients in an attempt to induce ventricular fibrillation. In patients with prior sustained uniform ventricular tachycardia, programmed stimulation with up to three extrastimuli was performed in order to attempt to initiate the clinical ventricular tachcardia. Ventricular fibrillation was induced with NIPS in 146 of 166 studies (88%). Ventricular tachycardia was initiated with NIPS in 104 of 123 studies (85%). The type of defibrillator and the use of endocardial or epicardial rate sensing/ pacing leads did not influence the efficacy of NIPS. NIPS with third-generation ICDs is generally effective at inducing ventricular fibrillation and clinically relevant ventricular tachycardias, and reduces the need to perform invasive electrophysiological testing following device implantation. In a minority of patients temporary transvenous pacing catheters must still be used to facilitate arrhythmia induction.     

Short-and Long-Term Performance of a Tripolar Down-Sized Single Lead for Implantable Cardioverter Defibrillator Treatment: A Randomized Prospective European Multicenter Study

A new, thinner (10 Fr) and more flexible, single-pass transvenous endocardial ICD lead, Endotak DSP, was compared with a conventional lead, Endotak C, as a control in a prospective randomized multicenter study in combination with a nonactive can ICD. A total of 123 patients were enrolled, 55 of whom received a down-sized DSP lead. Lead-alone configuration was successfully implanted in 95% of the DSP patients vs 88% in the control group. The mean defibrillation threshold (DFT) was determined by means of a step-down protocol, and was identical in the two groups, 10.5 ± 4.8 J in the DSP group versus 10.5 ± 4.8 J in the control group. At implantation, the DSP mean pacing threshold was lower, 0.51 ± 0.18 V versus 0.62 ± 0.35 V (p < 0.05) in the control group, and the mean pacing impedance higher, 594 ± 110 Ω vs 523 ± 135 Ω (p < 0.05). During the follow-up period, the statistically significant difference in thresholds disappeared, while the difference in impedance remained. Tachyarrhythmia treatment by shock or antitachycardia pacing (ATP) was delivered in 53% and 41%, respectively, of the patients with a 100% success rate. In the DSP group, all 28 episodes of polymorphic ventricular tachycardia or ventricular fibrillation were converted by the first shock as compared to 57 of 69 episodes (83%) in the control group (p < 0.05). Monomorphic ventricular tachycardias were terminated by ATP alone in 96% versus 94%. Lead related problems were minor and observed in 5% and 7%, respectively. In summary, both leads were safe and efficacious in the detection and treatment of ventricular tachyarrhythmias. There were no differences between the DSP and control groups regarding short- or long-term lead related complications.     

A Single Atrial Extrastimulus Can Distinguish Sinus Tachycardia from 1:1 Paroxysmal Tachycardia

We have developed a tachycardia detection scheme for use in an antitachycardia pacemaker in which the use of a properly timed atrial extrastimulus provides a means of discriminating sinus tachycardia from pace-terminable 1:1 tachycardias. An atrial extrastimulus is delivered in late diastole (80 ms premature), and the ventricular response is monitored. In sinus tachycardia, the ventricular response is expected to appear early as well, but in pace-terminable tachycardias, such as AV reentrant and ventricular with VA conduction, the ventricular rhythm will be unperturbed. Testing of the algorithm was performed in 34 patients. In 29 patients, atrial extrastimuli were delivered during sinus tachycardia, and in 22 patients during various types of 1:1 paroxysmal tachycardia. In one patient the procedure was completely automated, i.e., delivery of the atrial extrastimuli and diagnosis were microcomputer controlled. In 28/29 cases, the delivery of an atrial extrastimulus 80 to 120 ms early during sinus tachycardia elicited a ventricular response at least 28 ms early. In 22/22 patients with 1:1 paroxysmal tachycardia, atrial extrastimuli 80 to 120 ms early failed to produce a significant change in ventricular cycle length. This technique appears to be promising for prevention of inadvertent pacing of sinus tachycardia in an antitachycardia pacemaker.     

Permanent Antitachycardia Pacemaker Therapy for Ventricular Tachycardia

This article describes our experience with an antitachycardia pacemaker alone (N = 3) or in combination with an automatic implontoble cardioverter defibrillator (AICD, N = 8) in the treatment of ventricular tochycardia. EJeven patients (mean ejection fraction 31%, mean oge 67 years) received an antitachycardia pacemaker. Nine had their units programmed for automatic antitachycardia pacing, one unit was programmed to automatic antitachycardia pacing by magnet activation only, and one to tachycardia detection and bradycardia support. Of the nine patients with automatic antitachycardia pacing, seven received appropriate and successful pace termination of spontaneous ventricular tachycardia at up to 120 times per month. Eight of these nine have had AICD implantations as well. There were no operative complications. Over a mean (± SD) follow-up of 12.1 ± 9.3 months (range 3–29 months), there have been two deaths, both due to heart failure. There have been four AICD discharges in three patients. Two units discharged in a clinically appropriate setting. The other two units, both with rate cutoffs <200 beats/min, were inadvertently triggered by the antitachycardia pacemaker and/or the underlying rate. In addition to the careful selection of the defibrillator rate cutoff, adverse device-device interactions were avoided by careful intraoperative lead positioning, and the disabling of bradycardia pacing when not needed or contraindicated. Antitachycardia pacing, with the safety provided by the AICD, is an effective treatment for patients with medically refractory ventricular tachycardia.     

Clinical predictors and efficacy of antitachycardia pacing in patients with implantable cardioverter defibrillators: the importance of the patient's sex

The ICD has become accepted as primary therapy for malignant ventricular arrhythmias. The incorporation of antitachycardia pacing into ICDs has provided a better tolerated alternative to shocks but has the potential disadvantage of delaying definitive therapy. Accordingly, we sought to delineate the characteristics of patients likely to experience unsuccessful termination of pacing and to identify ineffective pacing strategies. Of 519 patients who received ICDs, 11 clinical and tachycardia characteristics in the 162 who received antitachycardia pacing therapy for sustained ventricular arrhythmias were evaluated. Tachycardia episodes were grouped according to outcome of pacing (successful, unsuccessful, acceleration). Of 1,946 episodes, 1,502 (77.2%) were successfully reverted with pacing, 322 (16.5%) were unsuccessful, and 121 (6.2%) were accelerated. Antitachycardia pacing was less successful in women, patients with a history of myocardial infarction, those with more severe left ventricular dysfunction, those who received antiarrhythmic drugs, and those programmed to ramp pacing. Tachycardia acceleration was inversely related to tachycardia cycle length and was more frequent in patients programmed to more aggressive ramp pacing protocols. Women had an almost threefold incidence of tachycardia acceleration compared with men (14% vs 5%, P < 0.001). Antitachycardia pacing is generally successful in terminating ventricular tachycardia and has a low incidence of tachycardia acceleration. Caution should be used with rapid tachycardias and aggressive ramp pacing protocols because of an increased risk of acceleration. Antitachycardia pacing appears less successful and has a higher incidence of complications in women.     

Inducibility of Sustained Ventricular Tachycardia in a Closed-Chest Ovine Model of Myocardial Infarction

The two goals of this study were (1) to develop a closed-chest animal model of monomorphic ventricular tachycardia; and (2) to investigate the effect of dual site pacing on inducibility of ventricular tachycardia. In the first part of the study, 10 of 14 sheep underwent successful induction of myocardial infarction by temporary balloon occlusion of the left anterior descending coronary artery. After a follow-up period of 21–43 days, sustained monomorphic ventricular tachycardia could be induced during programmed electrical stimulation using a "clinical" stimulation protocol in 8 of the 10 sheep. The number of ventricular tachycardia episodes per animal varied between 5 and 70. Ventricular fibrillation was never induced during programmed electrical stimulation. Ventricular tachycardia episodes lasted from 30 seconds up to 15 minutes and were terminated by antitachycardia pacing or DC cardioversion. In the second part of the study, the effect of dual site stimulation on ventricular tachycardia inducibility was investigated. High current stimuli from an area within the infarcted zone were given with the S1 programmed stimulation protocol. This dual site stimulation showed no effect on ventricular tachycardia induction during programmed electrical stimulation. This animal model shows a high induction rate of sustained monomorphic ventricular tachycardia in the chronic phase of myocardial infarction. The high incidence of ventricular tachycardia inducibility provides a reliable tool to study new techniques for the prevention of ventricular tachyarrhythmias.