Electrophysiological and anatomic differences between canine hearts with inducible ventricular tachycardia and fibrillation associated with chronic myocardial infarction

Statins and the reduction of sudden cardiac death: antiarrhythmic or anti-ischemic effect?

Sudden cardiac death is an important cause of cardiovascular mortality with the majority of cases occurring in low-risk groups. HMG-CoA reductase inhibitors (statins) have recently been shown to reduce the incidence of ventricular tachycardia (VT)/fibrillation (VF) and sudden cardiac death, and this has been attributed to their pleiotropic effects. However, it is unclear whether this occurs through an 'indirect' anti-ischemic or 'direct' antiarrhythmic effect. We systematically reviewed articles published on MEDLINE between January 1996 and December 2009 focusing on the reduction of VT/VF and sudden cardiac death by statins and the potential mechanisms. Studies reporting sudden cardiac death or VT/VF outcomes with statin use (n = 23) or the pathophysiology of sudden cardiac death reduction by statins (n = 19) were included. We found that statins have been shown to reduce VT/VF and sudden cardiac death only in subjects with underlying coronary artery disease or ischemic cardiomyopathy. No definite benefits were seen with statins in sudden cardiac death and VT/VF in patients with non-ischemic cardiomyopathy. There is insufficient evidence to point toward a benefit in populations at low risk for VT/VF. In conclusion, an anti-ischemic rather than a primary antiarrhythmic effect emerges as the likely mechanism of sudden cardiac death reduction with statins.

Cardiac expression of skeletal muscle sodium channels increases longitudinal conduction velocity in the canine 1-week myocardial infarction

BACKGROUND

Skeletal muscle sodium channel (Nav1.4) expression in border zone myocardium increases action potential upstroke velocity in depolarized isolated tissue. Because resting membrane potential in the 1-week canine infarct is reduced, we hypothesized that conduction velocity (CV) is greater in Nav1.4 dogs compared with in control dogs.

OBJECTIVE

The purpose of this study was to measure CV in the infarct border zone border in dogs with and without Nav1.4 expression.

METHODS

Adenovirus was injected in the infarct border zone in 34 dogs. The adenovirus incorporated the Nav1.4- and a green fluorescent protein (GFP) gene (Nav1.4 group, n = 16) or only GFP (n = 18). After 1 week, upstroke velocity and CV were measured by sequential microelectrode recordings at 4 and 7 mM [K(+)] in superfused epicardial slabs. High-density in vivo epicardial activation mapping was performed in a subgroup (8 Nav1.4, 6 GFP) at three to four locations in the border zone. Microscopy and antibody staining confirmed GFP or Nav1.4 expression.

RESULTS

Infarct sizes were similar between groups (30.6% +/- 3% of left ventricle mass, mean +/- standard error of the mean). Longitudinal CV was greater in Nav1.4 than in GFP sites (58.5 +/- 1.8 vs. 53.3 +/- 1.2 cm/s, 20 and 15 sites, respectively; P <.05). Transverse CV was not different between the groups. In tissue slabs, dV/dt(max) was higher and CV was greater in Nav1.4 than in control at 7 mM [K(+)] (P <.05). Immunohistochemical Nav1.4 staining was seen at the longitudinal ends of the myocytes.

CONCLUSION

Nav1.4 channels in myocardium surviving 1 week infarction increases longitudinal but not transverse CV, consistent with the increased dV/dt(max) and with the cellular localization of Nav1.4.

Mechanisms, prediction and treatment of ventricular tachyarrhythmias occurring late after myocardial infarction

Studies from the 1980s, refined in the intervening years, have examined the milieu for ventricular tachycardia (VT) and ventricular fibrillation (VF) occurring late after acute myocardial infarction (AMI). The arrhythmogenic substrate appears to be patchy areas of fibrous tissue interdigitating with viable bundles of myocardium which have distorted orientation and tortuous interconnections. These promote conduction delay in sinus rhythm. Factors found to promote induction of VT rather than VF are longer conduction delay in sinus rhythm, larger infarct size, a more ragged infarct edge and longer ventricular extrastimulus coupling intervals. Predictors of spontaneous VT and VF late after AMI include inducible VT at electrophysiological studies (EPS), delayed conduction in sinus rhythm detected as late potentials on signal-averaged surface electrocardiogram (ECG), and low left ventricular ejection fraction (LVEF). Treatments of propensity for VT or VF after AMI include insertion of a defibrillator (ICD), which has the best track record, antiarrhythmic medication (less reliable), and ablation or excision of arrhythmogenic substrate (for refractory VT and VF).

Surgery and ablative therapy for ventricular tachycardia

In the absence of acute ischaemia, ventricular tachycardia (VT) is the most common arrhythmia leading to cardiac arrest and death. This paper will describe the history of research into VT and the therapies that evolved. The contributions of John Uther and other members of the Department of Cardiology at Westmead Hospital will be outlined and placed into perspective.

Comparison of radiofrequency ablation in normal versus scarred myocardium

INTRODUCTION

Reentrant circuits causing ventricular tachycardia are closely associated with previously scarred myocardium. The presence of scar has been blamed for the poor success rate of radiofrequency ablation (RFA) in that context. This article investigates the in vivo effects of radiofrequency ablation in myocardium scarred from acute myocardial infarction.

METHODS AND RESULTS

Anterior myocardial infarction was induced in five dogs by ligating the left anterior descending artery. The mean left ventricular ejection fraction after infarction was 38%. At a mean of 15 weeks following myocardial infarction, 50 RFA lesions were created in random order, 25 in scarred and 25 in normal myocardium using a needle electrode (21 gauge, 5 mm in length) introduced from the epicardium of the left ventricle at thoracotomy. During unipolar temperature-controlled RFA (90 degrees C for 60 seconds), intramural temperatures were measured by thermistors at distances of 1, 2, 3, 4, and 5 mm from the ablating electrode. The margins of the lesions were clearly discernible in scar at histological examination in 64% of ablations where the scarring was patchy. There were no significant differences between lesion sizes, intramural temperatures at different distances, total energy required for ablation, or mean impedance during ablation of normal versus scarred myocardium.

CONCLUSIONS

Scar does not affect lesion size or intramural temperature profile during RFA if electrode size, tissue contact, and tip temperature are controlled. More radiofrequency energy is not required to maintain tip temperature at 90 degrees C in scar compared to normal myocardium.

ICD Therapy: What Have We Learned From the Clinical Trials?

Development of implantable cardioverter defibrillators (ICD) has been a dramatic advancement in the management of patients with life-threatening ventricular arrhythmias. We hereby reviewed the landmark clinical trials on ICD with special emphasis on late-breaking clinical trials and assessed their impact on every-day decision making and patient selection for ICD implantation.

Clinical predictors and prognostic significance of electrical storm in patients with implantable cardioverter defibrillators

AIMS

Insufficient data exists regarding predictors of electrical storms (ES) and clinical outcome in patients treated with an implantable cardioverter defibrillator (ICD). The purpose of this study was to delineate a subgroup of patients likely to experience ES and to determine the impact of ES on mortality in ICD recipients.

METHODS AND RESULTS

Baseline characteristics of 307 ICD-treated patients were retrospectively analysed. ES was defined as two or more ventricular tachyarrhythmias within 24 h leading to an immediate electrical therapy (antitachycardia pacing and/or shock), separated by a period of sinus rhythm. Clinical characteristics and survival of 123 patients experiencing a total of 294 episodes of ES (median 2 ES/patient, range 1-9), were compared with those of 184 ES-free patients during a median follow-up of 826 days (inter-quartile 1141 days). Median actuarial duration for the first ES occurrence after ICD implant was 1417 days [95% confidence interval (CI) 1061-2363] with a median follow-up of 816 days (7-4642 days) in ES-free patients. Univariate analysis identified older age, depressed left ventricular ejection fraction (LVEF), ventricular tachycardia (VT) as index arrhythmia, chronic renal failure and absence of lipid-lowering drugs as variables significantly associated with an increased risk of ES. Multivariable Cox analysis confirmed an independent predictive value for chronic renal failure [hazard ratio (HR) 1.54, 95% CI 0.95-2.51, P=0.052], VT (HR 2.20, 95% CI 1.44-3.37, P=0.0003), and LVEF (HR 0.98, 95% CI 0.97-0.99, P=0.027). In contrast, diabetics (HR 0.49, 95% CI 0.27-0.90, P=0.022) were less affected by ES. There was no difference in survival between both groups.

CONCLUSION

ES is frequent but does not increase mortality in ICD's recipients. Patients with severe systolic dysfunction, chronic renal failure and VT as initial arrhythmia are likely to experience ES. Diabetics are less affected by ES.

Mechanisms of enhanced shock-induced arrhythmogenesis in the rabbit heart with healed myocardial infarction

Shock-induced vulnerability and defibrillation have been mostly studied in structurally normal hearts. However, defibrillation therapy is normally applied to patients with diseased hearts, frequently those with prior myocardial infarction (MI). Shock-induced vulnerability and defibrillation have not been well studied under this condition. We sought to examine the mechanisms of shock-induced arrhythmogenesis and arrhythmia maintenance in a rabbit model of healed MI (4 wk or more postinfarction). Ligation of the lateral division or posterolateral division of the left coronary artery at a level of 40-70% from the apex was performed 53 +/- 21 days before acute experiments. Shock-induced vulnerability was assessed in infarcted (n = 8) and structurally normal (n = 8) hearts by delivering internal monophasic shocks at different shock strengths and delivery phases. Electrical activities from the anterior epicardium during shock application and during shock-induced arrhythmias were optically recorded and quantitatively analyzed. Ligation resulted in a transmural left ventricular free wall infarction mainly located at the apical region with a consistent endocardial border zone (BZ) as confirmed by histological studies. There were significant increases in the incidence, severity, and duration of shock-induced arrhythmias in the infarcted hearts versus controls due to 1) postshock break-excitation wavefronts that frequently originated near the infarction BZ and 2) the existence of an infarction BZ that created an anatomic reentry pathway and facilitated arrhythmia maintenance. In conclusion, the infarction BZ contributes to both increased shock-induced arrhythmogenesis and arrhythmia maintenance in the rabbit model of healed MI.

Cross-sectional infarct edge jaggedness does not influence ventricular electrical stability in a rabbit model of late myocardial infarct healing

Previous studies have suggested that the jaggedness of the healed or healing infarct edge influences cardiac electrical stability. However, these findings have been based on histological observations rather than quantitative measurements. The aim of this study was to assess infarct jaggedness by calculating its fractal dimension and to examine how this influences cardiac electrical stability during late infarct healing in the rabbit. Using programmed electrical stimulation, it was found that the fractal dimension did not differ significantly in 19 rabbits that had inducible ventricular tachycardia and 16 that did not. We conclude from these studies in the mature rabbit that infarct edge jaggedness does not influence the ease with which ventricular tachycardia is induced during late myocardial infarct healing.

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.

The signal-averaged electrocardiogram: update on clinical applications

The signal-averaged electrocardiogram (SAECG) facilitates noninvasive recording of low-amplitude cardiac signals such as ventricular late potentials. The SAECG has been used to accurately predict life-threatening ventricular tachyarrhythmias in patients after acute myocardial infarction and with nonischemic dilated cardiomyopathy, and to screen for inducible ventricular tachycardia in patients with unexplained syncope and with nonsustained ventricular tachycardia. This review focuses on currently accepted methodology and clinical and research applications of the SAECG.

Does the induction of ventricular flutter or fibrillation at electrophysiologic testing after myocardial infarction have any prognostic significance?

This study examines the significance of inducing sustained ventricular fibrillation (VF) or ventricular flutter by programmed stimulation after infarction. Programmed ventricular stimulation was performed for prognostic reasons from the right ventricular apex at twice diastolic threshold using a protocol containing 4 extrastimuli. Of 502 patients tested 11 +/- 4 days after acute infarction, VF was induced in 164 (33%), ventricular flutter in 134 (27%), ventricular tachycardia (VT) in 44 (9%), and no arrhythmia in 160 (32%). All groups were similar in age, sex distribution, and sites of index infarction. Those with inducible VT had a higher incidence of multiple infarctions and a lower mean left ventricular ejection fraction at the time of testing. Without antiarrhythmic drug therapy, 8 patients (18%) with inducible VT experienced spontaneous VT or died instantaneously during the first year of follow-up. By contrast, only 1 (0.6%) patient with inducible VF, 1 (0.7%) with ventricular flutter, and 1 (0.6%) without any inducible arrhythmias experienced similar events in the same period (p < 0.001). By relating the cycle length of the induced monomorphic arrhythmia to later spontaneous electrical events, induced arrhythmias with cycle length as low as 230 ms still identified patients at high risk for spontaneous arrhythmias. Only the induction of sustained monomorphic VT with a cycle length > 230 ms indicates patients with ventricular electrical instability after infarction. The induction of VF or ventricular flutter is a negative test result with no adverse long-term prognostic significance.

A closed-chest pig model of sustained ventricular tachycardia

The goal of this study was to develop and explore a closed-chest animal model of sustained VT. Seven of 11 domestic pigs had successful induction of myocardial infarction by injection of agarose gel microbeads into the left anterior descending coronary artery through an inflated balloon angioplasty catheter. Four of the first five pigs died and seem to represent a "learning experience." During a 3- to 50-day follow-up period, each pig underwent 1-3 electrophysiological studies. Sustained, monomorphic VT was induced 1-4 times in 5 of the 7 pigs (a total of 19 episodes), was reproducible during the same study in all pigs, and could be repetitively induced during successive studies in some. Ventricular fibrillation was induced less frequently (nine episodes) and was successfully terminated by DC shock in eight episodes. We conclude that a closed-chest pig model of VT is feasible and is associated with a relatively high induction rate of sustained, monomorphic, and reproducible VT and a relatively low mortality rate.

Electrophysiologic characteristics at initiation of ventricular tachycardia and ventricular fibrillation in a canine infarct model

Local ventricular activation time and the conduction time during sinus rhythm at the induction of ventricular tachycardia (VT) and ventricular fibrillation (VF) were investigated using a canine model of chronic myocardial infarction. Of 26 dogs studied, 15 had inducible VT, 10 had inducible VF, and 1 had no inducible arrhythmias. Bipolar local ventricular electrograms were recorded during sinus rhythm from 136 sites in 10 dogs with VT and 164 sites in 11 dogs with VF. Mean activation time in dogs with inducible VT was significantly longer than in dogs with inducible VF. Furthermore, simultaneous local ventricular electrograms were recorded during the induction of VT (74 episodes) or VF (38 episodes) from the infarct border zone at the endocardium (B-EN), the epicardium (B-EP), and normal sites (N-EN, N-EP). During VT induction, the activation time at N-EN and N-EP was significantly longer than during VF induction (N-EN: 94 +/- 21, 70 +/- 19 ms; N-EP: 83 +/- 21, 64 +/- 10 ms; p < 0.05). Conduction time was measured at the initiation of VT or VF induced by orthodromic or antidromic pacing. The conduction times of the last paced beat between N-EN and B-EP (35 +/- 11, 62 +/- 24 ms), N-EN and N-EP (35 +/- 12, 14 +/- 13 ms), B-EN and B-EP (16 +/- 10, 38 +/- 25 ms), and B-EP and N-EP (77 +/- 27, 44 +/- 12 ms) were significantly different in dogs with inducible VT (p < 0.05), but not in dogs with VF.(ABSTRACT TRUNCATED AT 250 WORDS)

Ventricular defibrillation in canines with chronic infarction, and effects of lidocaine and procainamide

Prior studies in dogs with normal hearts have demonstrated that lidocaine increases but procainamide does not change the energy required for successful defibrillation. Because many postinfarct patients receiving implantable cardioverter defibrillator devices require adjunctive antiarrhythmic therapy, we have studied the effects of lidocaine and procainamide on the relationship between delivered voltage and defibrillation success in mongrel dogs 21 +/- 3 days following ligation of the left anterior descending and first diagonal coronary arteries. Internal defibrillation testing using a patch-patch electrode configuration was performed before and during the administration of saline controls (n = 10), lidocaine (n = 10) and procainamide (n = 10). The mean infarct size as determined by staining with tetrazolium was 13.4% +/- 8.3% of right and left ventricles, and did not differ significantly between groups. The 50% effective defibrillation (ED50) voltage increased with infusions of saline (16% +/- 15%), lidocaine (40% +/- 22%), and procainamide (13% +/- 15%) and the ED50 energy increased 41% +/- 44%, 104% +/- 62%, and 35% +/- 36%, respectively. However, the increase in ED50 voltages and energies were significantly greater in animals receiving lidocaine compared to those receiving either saline control or procainamide (P < 0.01). There were trends toward change of hemodynamic parameters in all animals following baseline defibrillation testing; stroke volume declined 21% +/- 16%; and mean pulmonary artery and aortic pressure increased by 22% +/- 25% and 11% +/- 15%, respectively. In conclusion, unlike our previous studies in dogs with normal hearts, in this model hemodynamic deterioration occurred with repeated fibrillation and defibrillation, and defibrillation voltage requirements increased in the control series. Taking into consideration the increase in defibrillation voltage requirements over the duration of the experiments, lidocaine increases and procainamide does not change ED50; thus, their effects are similar in normal and infarcted canine hearts.

The automatic implantable defibrillator is the most realistic and cost-effective way of preventing sudden cardiac death

Judgement about cost-effectiveness of medical treatments depends upon the criteria used to define effectiveness. The most rational single criterion is the Quality Adjusted Life Year gained (QALY). Comparisons of cost-effectiveness of treatments may be made by comparing the dollar costs per QALY. Both the dollar and the QALY comparisons are subject to all errors of biological variable measurement and bias introduced by experimental design seen in other biological experiments. The proper methodology for comparison is the prospective randomised controlled clinical trial. Such trials using the automatic implantable defibrillator (AICD) as prophylaxis against sudden cardiac death (SCD) have not yet been performed. The major underlying cause of SCD is coronary artery disease with previous myocardial infarction (MI) and provocative tests for ventricular arrhythmias are the most powerful predictors of SCD in infarction survivors. AICD implantation carriers a mortality of about 2%, and survival after successful implantation is about 89% at one, and 84% at two years. By comparison, infarction survivors with left ventricular ejection fraction < 0.4 and inducible slow monomorphic ventricular tachycardia have a survival rate of 70% at one, and 54% at two years. Antiarrhythmic drugs have not proven effective. There is an urgent need to confirm the advantage of the AICD by proper randomised controlled trial.

Differences between predictive characteristics of signal-averaged electrocardiographic variables for postinfarction sudden death and ventricular tachycardia

Several studies indicate that the electrophysiologic substrate for sustained ventricular tachycardia differs from that of ventricular fibrillation. This prospective study examined whether there were clinically relevant differences between the predictive values of the standard time-domain signal-averaged (SA) electrocardiographic (ECG) variables for ventricular tachycardia and sudden death after myocardial infarction. Predischarge SA electrocardiograms were recorded in 332 patients after infarction. During a follow-up period of greater than or equal to 6 months, there were 12 sudden deaths (3.6%), 14 patients (4.2%) developed spontaneous sustained ventricular tachycardia and 20 patients (6%) died of circulatory failure. The sensitivity, specificity and positive predictive accuracy of the numerical values of the time-domain SA electrocardiographic variables for predicting sudden death and ventricular tachycardia were compared. The optimal criteria for predicting ventricular tachycardia required the positivity of greater than or equal to 2 of the standard time-domain SA variables, whereas the optimal criteria for predicting sudden death required the positivity of all 3 variables. A high specificity was sustained over a wider range of sensitivity for sudden death than it was for ventricular tachycardia and the values of the variables which provided the same sensitivity for sudden death and ventricular tachycardia were different. For a sensitivity of 70%, the positive predictive accuracy was 31% for predicting sudden death and 13% for predicting ventricular tachycardia. The study concludes that differences in the predictive characteristics of variables for ventricular tachycardia and sudden death may be used to refine postinfarction risk stratification.

Routine programmed electrical stimulation in survivors of acute myocardial infarction for prediction of spontaneous ventricular tachyarrhythmias during follow-up: results, optimal stimulation protocol and cost-effective screening

Of 3,286 consecutive patients treated for acute myocardial infarction, electrophysiologic testing was performed in 1,209 survivors (37%) free of significant complications at the time of hospital discharge to determine their risk of spontaneous ventricular tachyarrhythmias during follow-up. Sustained monomorphic ventricular tachycardia was inducible by programmed electrical stimulation in 75 (6.2%). Antiarrhythmic therapy was not routinely prescribed regardless of the test results. During the 1st year of follow-up, 14 infarct survivors (19%) with inducible ventricular tachycardia experienced spontaneous ventricular tachycardia or fibrillation in the absence of new ischemia compared with 34 (2.9%) of those without inducible ventricular tachycardia (p less than 0.0005). During the extended follow-up period (median 28 months) of those with inducible ventricular tachycardia, 19 (25%) had a spontaneous electrical event; 37% of these first events were fatal. These results suggest that the most cost-effective strategy for predicting arrhythmia will be obtained by restricting electrophysiologic testing to infarct survivors whose left ventricular ejection fraction is less than 40% and using a stimulation protocol containing four extrastimuli. Electrophysiologic testing is the single best predictor of spontaneous ventricular tachyarrhythmias during follow-up in infarct survivors. The majority (94%) with a negative test benefit from the more reliable reassurance that all is well, whereas the 25% risk of electrical events in those with inducible ventricular tachycardia justifies a prospective trial of effective prophylactic antiarrhythmic interventions.

Reduction in incidence of inducible ventricular tachycardia after myocardial infarction by treatment with streptokinase during infarct evolution

The aim of this study was to determine whether intravenous streptokinase administered with or without oral aspirin to patients with evolving myocardial infarction reduces the inducibility of ventricular tachycardia at electrophysiologic study and thus the risk of sudden death in infarct survivors. Of 159 patients randomized at Westmead Hospital to the multicenter Second International Study of Infarct Survival (ISIS-2) after streptokinase and aspirin in acute myocardial infarction, 87 underwent electrophysiologic testing 6 to 28 days after infarction to determine their risk of subsequent ventricular arrhythmias (streptokinase 20 patients; aspirin 25 patients; streptokinase and aspirin 21 patients; both placebos 21 patients). Patients who underwent electrophysiologic testing had similar clinical characteristics to those of patients who did not. The stimulation protocol comprised up to and including four extrastimuli applied to the right ventricular apex at twice diastolic threshold. An abnormal result was defined as ventricular tachycardia with a cycle length greater than or equal to 230 ms lasting greater than or equal to 10 s. Ventricular tachycardia was inducible at electrophysiologic study in 8 patients who received placebo streptokinase, but in no patient who received active streptokinase (8 of 46 versus 0 of 41; p = 0.005, Fischer's exact test). Ventricular tachycardia was inducible in 4 patients who received aspirin therapy and 4 who did not (4 of 41 versus 4 of 46; p = NS). During a mean follow-up period of 39 +/- 9 months, there were no spontaneous episodes of ventricular tachycardia, ventricular fibrillation or witnessed sudden death in the streptokinase-treated group compared with three such events in the placebo-treated group (p = 0.13). When compared with placebo therapy, intravenous streptokinase substantially reduced the incidence of inducible ventricular tachycardia in infarct survivors. No similar benefit was attributable to aspirin therapy.

Inducible monomorphic sustained ventricular tachycardia in the conscious pig

Sustained monomorphic ventricular tachycardia (VT) is of clinical importance but has not been readily modeled in conscious animals. Eleven pigs had myocardial infarction induced by pulling snares previously placed around the left anterior descending (LAD) coronary artery. Six days after occlusion, bipolar pacing catheters were inserted in the right ventricular apex for induction of VT. Testing was repeated in conscious pigs on 6 out of 8 to 19 days after infarction. Monomorphic VT was induced in each animal during each session, using three to four extrastimuli. VT was terminated by burst pacing in 74% of trials; average VT rate was 362 +/- 26 beats/min. VT was prevented in four of eight animals by procainamide and in five of eight animals by magnesium, but was not prevented by lidocaine or metoprolol. The model may be useful in the study of potentially malignant ventricular tachyarrhythmias, important prodromes for sudden death.