Repetitive Ventricular Response Research Articles

Effect of anisotropy on ventricular vulnerability to unidirectional block and reentry by single premature stimulation during normal sinus rhythm in rat heart.

Single high-intensity premature stimuli when applied to the ventricles during ventricular drive of an ectopic site, as in Winfree's "pinwheel experiment," usually induce reentry arrhythmias in the normal heart, while single low-intensity stimuli barely do. Yet ventricular arrhythmia vulnerability during normal sinus rhythm remains largely unexplored. With a view to define the role of anisotropy on ventricular vulnerability to unidirectional conduction block and reentry, we revisited the pinwheel experiment with reduced constraints in the in situ rat heart. New features included single premature stimulation during normal sinus rhythm, stimulation and unipolar potential mapping from the same high-resolution epicardial electrode array, and progressive increase in stimulation strength and prematurity from diastolic threshold until arrhythmia induction. Measurements were performed with 1-ms cathodal stimuli at multiple test sites (n = 26) in seven rats. Stimulus-induced virtual electrode polarization during sinus beat recovery phase influenced premature ventricular responses. Specifically, gradual increase in stimulus strength and prematurity progressively induced make, break, and graded-response stimulation mechanisms. Hence unidirectional conduction block occurred as follows: 1) along fiber direction, on right and left ventricular free walls (n = 23), initiating figure-eight reentry (n = 17) and tachycardia (n = 12), and 2) across fiber direction, on lower interventricular septum (n = 3), initiating spiral wave reentry (n = 2) and tachycardia (n = 1). Critical time window (55.1 ± 4.7 ms, 68.2 ± 6.0 ms) and stimulus strength lower limit (4.9 ± 0.6 mA) defined vulnerability to reentry. A novel finding of this study was that ventricular tachycardia evolves and is maintained by episodes of scroll-like wave and focal activation couplets. We also found that single low-intensity premature stimuli can induce repetitive ventricular response (n = 13) characterized by focal activations.NEW & NOTEWORTHY We performed ventricular cathodal point stimulation during sinus rhythm by progressively increasing stimulus strength and prematurity. Virtual electrode polarization and recovery gradient progressively induced make, break, and graded-response stimulation mechanisms. Unidirectional conduction block occurred along or across fiber direction, initiating figure-eight or spiral wave reentry, respectively, and tachycardia sustained by scroll wave and focal activations.

Catheter ablation in an 80-year-old male with late-onset Wolff-Parkinson-White syndrome

Wolff-Parkinson-White syndrome is a congenital condition involving abnormal conductive cardiac tissue between atria and ventricles, that provides a pathway for a re-entrant tachycardia circuit or rapid repetitive ventricular response during atrial fibrillation. Clinical onset is usually from childhood to middle age, and symptoms vary in severity from palpitations (with or without syncope) to cardiac arrest. Presentation varies depending on the patient’s age and presence of comorbidities, as arrhythmias are less tolerated in case of an associated heart disease. In individuals who do not experience symptoms, the condition may go unnoticed for years. We describe the case of an 80-year-old Caucasian male with late-onset of Wolff-Parkinson-White syndrome. He presented with a regular, hemodynamically unstable wide-complex tachycardia with left bundle branch morphology, successfully treated with medical therapy. After recurrence of tachycardia, he underwent an electrophysiological study that showed an orthodromic atrioventricular re-entrant tachycardia using a posterolateral accessory pathway and with functional left bundle branch block. Transseptal radiofrequency catheter ablation of the accessory pathway was successfully performed. After 15 months of follow-up, there was no tachycardia recurrence or findings of pre-excitation on the electrocardiogram. The treatment of both supraventricular and ventricular arrhythmias may be challenging especially in elderly patients. Catheter ablation of left accessory pathways with transseptal approach is feasible and effective in old patients with late-onset Wolff-Parkinson-White syndrome.

Transthoracic cardiac stimulation thresholds for short pulses.

The most common cause of death due to electric shock is ventricular fibrillation (VF). This work reviews applicable results from the literature and provides an estimation model for the risk of VF with short-duration pulses. For 1 ms pulses, the predicted current and charge thresholds required for successful transthoracic cardiac stimulation were 1.12 A and 1.12 mC, respectively. For pulses of 0.1 ms durations, the transthoracic current and charge thresholds predicted by the model are 10.9 A and 1.09 mC, respectively. In humans, the charge required for single-response cardiac capture using transthoracic electrodes and 0.1 ms pulses is at least 0.5 mC. The transthoracic charge required to trigger repetitive ventricular responses in humans is at least several times higher than that for single responses. Hence, in adult humans, the transthoracic charge threshold required to induce repetitive ventricular responses, tachycardia, or fibrillation, with 0.1 ms pulses is expected to be significantly greater than 1 mC.

Ventricular Fibrillation Triggered during Radiofrequency Energy Delivery for Verapamil-Sensitive Idiopathic Left Ventricular Tachycardia

Verapamil-sensitive idiopathic left ventricular tachycardia (VT) can be suppressed by radiofrequency ablation (RF) on the Purkinje network. RF delivered on the Purkinje fibers triggers premature ventricular contractions or repetitive ventricular response and disappear after cessation of RF. We present a case with idiopathic left VT who represented ventricular fibrillation (VF) during RF on the Purkinje network. Case: A 15-year-old male admitted our hospital because of recurrent sustained VT with QRS morphology of right bundle branch block and left axis deviation. Intravenous verapamil terminated VT. However, electrophysiologic study could not induce VT even if isoproterenol infusion. We delivered RF energy at the distal left posterior fascicular region where Purkinje potentials were recorded during sinus rhythm with pace-mapping QRS morphology mimicking VT-QRS. Shortly after RF delivery, polymorphic VT induced and degenerated into VF. RF deliveries around the VF site induced similar responses. Then, we created RF lesions on the Purkinje fibers more proximal to the VF-induced sites. No VT was documented 3 years after ablation. Conclusion: Purkinje network appears to have potential role for VF induction. RF ablations at the proximal site of the critical Purkinje network can prevent VT/VF occurrence in such case.

The Significance of Repetitive Ventricular Responses Induced by Radiofrequency Energy Application for Idiopathic Left Ventricular Tachycardia

In radiofrequency (RF) ablation for idiopathic left ventricular tachycardia (ILVT), the termination of tachycardia during RF ablation is considered a hallmark of success. However, in cases of patients with difficulty of induction of ventricular tachycardia (VT), the evaluation of procedural success can be problematic. We have observed thermal responses reflected as ventricular rhythm change to RF energy delivered on sinus rhythm for ILVT. We therefore describe the significance of repetitive ventricular responses. The study subjects were 11 ILVT patients for whom RF energy was delivered during sinus rhythm because of difficulty in re-induction of tachycardia. During each energy delivery, we focused on the occurrence of repetitive ventricular responses especially exhibiting a similar morphology to clinical VT. The repetitive ventricular responses were noted in 10 of 11 patients. Two patients received a second procedure due to the recurrence of ILVT. The mean follow-up period was 36.2±12.8 months. The clinical course of the remaining patients was favorable and without recurrence of ILVT. Based on the favorable clinical outcomes, ablation-induced repetitive ventricular responses with similar QRS morphology to clinical ILVT are useful markers for selecting an ablation site and could be used as an additional mapping method, termed as "thermal mapping".

Clinical Significance of the Electrophysiologic Study (EPS)-Guided Therapy for the Secondary Prevention of Ventricular Tachycardia

Although electrophysiologic study (EPS) is one of the most reliable methods for selecting preventive therapy for patients with sustained ventricular tachycardia (VT), VT may recur during EPS-guided effective therapy; therefore, the importance of implantable cardioverter-defibrillator (ICD) has been emphasized. In this study, the prognoses of VT patients were evaluated to clarify the importance of EPS-guided therapy for the secondary prevention of VT. The study population consisted of 99 consecutive patients with a history of sustained VT, which was inducible in EPS. The VT induction protocol used 1-3 extrastimuli and rapid ventricular pacing at 2 right ventricular sites and included additional isoproterenol infusion. ICD implantation was applied to all patients with an episode of hemodynamically unstable VT, regardless of the result of preventive therapy. For preventive therapy, an antiarrhythmic drug and/or catheter ablation were selected, and they were defined as being effective in the EPS-guided therapy when the induction of VT was completely prevented. When no therapy was effective for prevention, an antiarrhythmic drug was prescribed under ICD implantation. During the follow-up period of 19+/-20 months, VT recurred in 17 of 32 patients (53%) in the ineffective group and in 10 of 67 patients (15%) in the effective group (p=0.0001). The therapies used in the effective group were class I antiarrhythmic drug in 9, class III in 15, and catheter ablation in 35 patients. Between the patients with and without VT recurrence, there were no significant differences in the left ventricular ejection fraction and the maximum number of repetitive ventricular responses that remained in VT induction in EPS. Although VT may recur in up to 15% of patients with EPS-guided effective therapy, the recurrence rate was significantly reduced in comparison to that in the ineffective group. EPS-guided therapy may be useful to reduce the clinical recurrence of VT, as well as the action of ICD.

Electrophysiologic parameters to predict clinical recurrence of ventricular tachycardia in patients under electrophysiologic study-guided effective pharmacological therapy.

Although an electrophysiologic study (EPS) is the most reliable method for selecting the treatment for a patient with sustained ventricular tachycardia (VT), VT recurrence may occur even during EPS-guided effective therapy. Electrophysiologic parameters were compared between patients with and without arrhythmic events under EPS-guided effective therapy to identify the predictive parameters of VT recurrence during the clinical course. The study population consisted of 77 consecutive patients with sustained VT who were receiving long-term pharmacological therapy that was demonstrated to be effective by the EPS assessment. The VT induction protocol employed 1-3 extrastimuli and rapid ventricular pacing at 2 right ventricular sites and 1 left ventricular site, and isoproterenol was infused when VT was not induced. To determine the 'effective' antiarrhythmic drug, all sustained ventricular arrhythmias had to be prevented during the whole induction protocol, but repetitive ventricular responses (RVR) were allowed to remain for up to 5 beats when they were in the same QRS configurations as the clinical VT and up to 12 beats when they were in polymorphic QRS configurations. The effective refractory periods (ERPs) at the 3 ventricular pacing sites and their difference (i.e., ERP-dispersion) and the maximum number of RVR beats were evaluated in an EPS during the control state and at the time of drug assessment. In the comparison of patients with and without VT recurrence, there was no significant difference in clinical characteristics or ERPs, but the deltaERP-dispersion (i.e., the increase in ERP-dispersion caused by the antiarrhythmic drug) and the maximum number of RVRs were significantly smaller in the group of patients without VT recurrence (deltaERP-dis, -3+/-8 vs. 6+/-12, p = 0.0027; maxRVR, 3+/-3 vs. 5+/-4, p = 0.0160). The VT recurrence rate was significantly lower in the patients with deltaERP-dis < or =0 or maxRVR<6 in comparison with the others (p = 0.01 14 and p = 0.0360). Patients with VT recurrence showed greater deltaERP-disp and a longer duration of RVRs at the time of drug assessment in comparison with the patients without VT recurrence. The prognosis of patients under EPS-guided therapy may be improved by the use of stricter criteria for drug assessment in the EPS, although this may decrease the number of drug responders determined in the EPS.

Repetitive ventricular responses induced by radiofrequency ablation for idiopathic ventricular tachycardia originating from the outflow tract of the right ventricle.

In 23 consecutive patients, radiofrequency (RF) ablation was used as treatment for idiopathic ventricular tachycardia (VT) originating from the outflow tract of the right ventricle. In this study, we focused on the repetitive ventricular response (> 5 consecutive QRS beats during RF application). The incidence and clinical implications of the repetitive ventricular response were examined through the results of endocardial mapping and RF ablation. VT origin was mapped as the earliest activation site during VT, and it was determined within 0.5 x 0.5 cm (narrow site) in 13 patients and wider than 0.5 x 0.5 cm (wide origin) in the other 10 patients. The repetitive ventricular response was induced during application of RF current in 14 of 23 patients (61%), and it was more frequently observed in VT from a wide origin (100%) than in the VT from a narrow site (31%). The QRS morphology of the repetitive ventricular response was identical to that of clinical VT. As RF application was continued and/or repeated, the RR interval of the repetitive ventricular response was gradually prolonged, the number of consecutive QRS complexes was decreased, and clinical VT was finally eliminated. The overall success rate of RF ablation was 96% (22/23 patients), and no complications were observed. In conclusion, a repetitive ventricular response was frequently observed in idiopathic right VT. The changing pattern of repetitive ventricular response, slowly and/or disappearing was consistent with successful RF ablation.

Definition of Predicted Effective Antiarrhythmic Drug Therapy for Ventricular Tachyarrhythmias by the Electrophysiologic Study Approach: Randomized Comparison of Patient Response Criteria

Objectives. We sought to compare efficacies of therapy for ventricular tachyarrhythmias selected by programmed stimulation using two different patient response efficacy criteria: <5 versus <16 repetitive ventricular responses.Background. Therapy selection for ventricular tachyarrhythmias by programmed stimulation requires definition of a patient response that predicts long-term efficacy. Such definitions have not been previously compared prospectively.Methods. Patients with sustained ventricular tachyarrhythmias were randomized to therapy selection using either the <5 or <16 repetitive response criterion of predicted effective therapy. The primary end point was sudden death or recurrence of ventricular tachyarrhythmia requiring intervention.Results. Predicted effective drug therapy was found for 23 (34%) of 68 patients randomized to the <5 criterion and 29 (36%) of 81 patients randomized to the <16 criterion (p = NS). Definition of therapy required 3.0 ± 1.6 drug trials (mean ± SD) in patients randomized to the <5 criterion and 2.9 ± 1.8 trials in patients randomized to the <16 criterion (p = NS). Patients randomized to the <5 criterion had a lower 2-year probability of the primary end point (0.20 ± 0.05) than did patients randomized to the <16 criterion (0.33 ± 0.05, one-tailed p = 0.004). The advantage of the <5 criterion was also seen in subgroup analyses involving patients with and without an initial drug efficacy prediction.Conclusions. The programmed stimulation approach to the selection of antiarrhythmic therapy for ventricular tachyarrhythmias using a patient response criterion of <5 repetitive ventricular responses results in a lower probability of recurrence of ventricular tachyarrhythmia than does use of a <16 repetitive response criterion.

Induction of polymorphic ventricular tachycardia by programmed ventricular stimulation in vasospastic angina pectoris

This study was designed to examine the ventricular vulnerability of patients with vasospastic angina. Fourteen patients (mean age 57 ± 9 years) with vasospastic angina underwent electrophysiologic testing during the asymptomatic phase (baseline) and after the relief of acetylcholine-induced spasm with isosorbide dinitrates. Twenty patients without structural heart disease served as a control group. By programmed ventricular stimulation, polymorphic ventricular tachycardia (VT) was induced at baseline in 6 of 14 patients, with 1 patient developing ventricular fibrillation and 7 of 14 patients developing repetitive ventricular responses. After isosorbide dinitrate, polymorphic VT was induced in only 1 patient who had ventricular fibrillation at baseline. Repetitive ventricular responses were induced in 3 of 5 patients who had VT at baseline and in 4 of the 7 patients with repetitive ventricular responses at baseline. There was a significant difference in the incidences and severity of induced ventricular arrhythmias between the 2 phases (p <0.01). Among 20 control subjects, repetitive ventricular responses were induced only in 6 patients, but no VT was induced. There was a significant difference in the incidence of induced ventricular arrhythmias and VT at baseline between the vasospastic angina and control groups (p <0.001 and <0.01, respectively). Thus, patients with vasospastic angina had increased ventricular vulnerability, even during the symptom-free period without ischemic events, which could predispose to the development of life-threatening arrhythmias aggravated by vasospastic attacks.

Ventricular arrhythmias induced by programmed ventricular stimulation after uncomplicated myocardial infarction.

The aim of this study was to correlate the occurrence of ventricular dysrhythmias induced by programmed ventricular stimulation and sudden cardiac death (SCD) after a first episode of acute myocardial infarction (AMI). Twenty-seven consecutive male patients aged fifty-four +/- six (forty-seven to seventy) years were studied prospectively. Thirty days after AMI, patients were submitted to coronary arteriography and programmed ventricular stimulation with the S2-S3-S4 protocol. Noninvasive assessments, including Holter monitoring, ECG stress test, and radionuclide ejection fraction, were also repeated six and twelve months after AMI. Ventricular dysrhythmias were induced in all patients. According to such response, patients were divided into three groups: (1) repetitive ventricular response (n = 9); (2) nonsustained ventricular tachycardia (n = 8); and (3) sustained ventricular tachycardia (n = 10). All patients consistently developed complex ventricular dysrhythmias at Holter monitoring and ECG stress test. One patient from group 2 suffered SCD and another presented a syncope. Similarly, in group 3, 2 patients suffered SCD, 1 during a documented episode of recurrent AMI. Except for 1 patient, radionuclide ejection fraction remained unchanged throughout the study in all cases. SCD was also unrelated to the presence and type of dysrhythmias at noninvasive evaluation. Therefore, the type of ventricular dysrhythmia induced by the S2-S3-S4 protocol has no correlation with late SCD in patients with a first AMI and preserved ejection fraction.

Day to day reproducibility of electrically inducible ventricular arrhythmias in survivors of acute myocardial infarction

Day to day reproducibility of the response to programmed ventricular stimulation has not been evaluated in survivors of acute myocardial infarction. Programmed ventricular stimulation was performed prospectively on 2 consecutive days in 56 patients on an average of 12 ± 5 days (range 7 to 29) after an acute myocardial infarction. No patient had a history of documented or suspected sustained ventricular tachycardia or fibrillation occurring >48 h after infarction. During initial programmed ventricular stimulation, 21 patients had induction of sustained ventricular tachycardia or fibrillation (Group I), and 35 patients had induction of either nonsustained ventricular tachycardia or no ventricular tachycardia (Group II).Repeat programmed ventricular stimulation in Group I patients induced sustained ventricular tachycardia or fibrillation in 16 of 21 patients (reproducibility 76%); the maximal induced response in the other 5 patients was nonsustained ventricular tachycardia in 2 patients and fewer than six repetitive ventricular responses in 3 patients. The day to day reproducibility was significantly higher for inducible sustained ventricular tachycardia of cycle length ≥240 ms compared with rapid sustained ventricular tachycardia of cycle length <240 ms (100% versus 44%, p < 0.009) or ventricular fibrillation (100% versus 43%, p < 0.009). Repeat programmed ventricular stimulation in Group II patients did not induce sustained ventricular arrhythmias in 31 of 35 patients (reproducibility 89%).Thus, in survivors of acute myocardial infarction, inducible slow sustained ventricular tachycardia was a highly reproducible finding, whereas inducibility of rapid sustain ventricular tachycardia and ventricular fibrillation showed a significant day to day variability. These findings should be taken into account when evaluating the prognostic significance of programmed ventricular stimulation in postmyocardial infarction patients.

Evaluation of antifibrillatory effects of drugs

Several electrical stimulation techniques have been used to measure ventricular fibrillation (VF) threshold such as single pulse stimulation, train of pulses stimulation, continuous 50-Hz stimulation and sequential pulse stimulation. This study was undertaken to clarify the difference in VF threshold values obtained by each method. The development of VF was classified into 4 stages. Stage 1 is the phase in which fragmented activities are generated at the stimulated area of the ventricular muscle. In stage 2, the local activities propagate to surrounding ventricular muscle, causing ventricular excitation. Stage 3 is characterized by repetitive ventricular excitations. In stage 4, repetitive ventricular responses in a form of accelerating tachycardia induce disorganized excitations that finally degenerate into fibrillation. Based on this classification of stages between local excitation and fibrillation, single pulse stimulation is considered to measure VF threshold for stages 1 to 4, continuous SO-Hz stimulation for stages 2 to 4 and sequential pulse stimulation for stages 3 and 4. Thus, the antifibrillatory effect of drugs can be interpreted with some significance by using different stimulation methods. The sequential pulse stimulation method revealed that some antiarrhythmic agents caused a dissociation between repetitive nonstimulated excitation (RE) threshold and VF threshold. Procainamide and disopyramide increased RE threshold while decreasing VF threshold. Mexiletine and lidocaine elevated both RE and VF thresholds. Local infusion of procainamide at the site of test stimulation reduced both thresholds. However, after the application of procainamide to the whole heart, RE threshold remained decreased but VF threshold was elevated. It is likely that the antifibrillatory effect of the drug includes 2 different processes: antireentrant and antimultiple reentrant effects.

Differentiation and mechanisms of prevention and termination of verapamil-sensitive sustained ventricular tachycardia

The purpose of this study was to differentiate by means of electrophysiologic study, a drug's ability to terminate or to prevent ventricular tachycardia (VT). Differences between the 2 effects were examined in patients with VT and the underlying mechanisms were studied in verapamil-responsive idiopathic sustained VT. The clinical significance of the distinction for chronic oral drug therapy is discussed. Thirty-five cases of inducible sustained VT were studied. A drug was considered “preventative” if it prevented VT induction and repetitive ventricular response, and “terminating” if it stopped induced VT within 15 complexes or could stop VT after its intravenous administration. Prevention and termination occurred together in 13 of 19 cases (68%) with disopyramide, in 10 of 19 cases (53%) with procainamide, in 8 of 12 cases (67%) with lidocaine, in 11 of 15 cases (73%) with mexiledne, and in 10 of 16 cases (63%) with verapamil. In the 16 in which verapamil terminated VT, VT rate immediately before termination slowed markedly from 167 ± 33 to 134 ± 28 beats/min. In the 6 cases without preventative effects, minimal and maximal premature intervals for VT induction increased significantly, from 291 ± 70 to 335 ± 85 ms and 323 ± 68 to 423 ± 109 ms, respectively, after verapamil administration. In 2 cases in which verapamil had a terminating effect, 5 mg of verapamil restored sinus rhythm but 10 mg caused premature beats resembling VT complexes. In another 2, 5 mg of verapamil lengthened the minimal premature interval; 10 mg increased both minimal and maximal premature intervals and lengthened the VT cycle. The chronic oral aukninistration of verapamil resulted in disappearance of VT attacks in 2 of 4 patients and a decrease in both their number and duration in 2 of 4 patients in whom both termination and prevention were seen in the electrophysiologic study. However, in 4 patients who had only termination, the VT episodes remained unchanged in 2 and even increased in the other 2.

Programmed stimulation in patients with minor forms of right ventricular dysplasia

Programmed electrical stimulation (PES) was performed in 17 patients, mean age 31 +/- 8 years, with minor forms of right ventricular dysplasia diagnosed because of (1) premature ventricular beats (PVBs) of left bundle branch block (LBBB) morphology; (2) no clinical or non-invasive evidence of cardiac abnormalities; (3) angiographic evidence of right ventricular wall motion abnormalities and bioptic findings of fibro-adipose infiltration. Fifteen patients had frequent and complex PVBs while two had sustained ventricular tachycardia (VT). During PES, sustained VT was induced in 2/2 patients with spontaneous sustained VT; ventricular repetitive responses were induced in 2/15 cases (13%) with complex and frequent PVBs. In conclusion, in minor forms of right ventricular dysplasia, PES induces VT only in patients with clinical VT; on the contrary, in patients with PVBs it is only possible to induce repetitive ventricular responses in a small proportion of cases; it is therefore not possible to select patients at high risk of developing severe ventricular arrhythmias.

The Response of Digitalized Canine Ventricle to Programmed Stimulation: A Study on Triggered Activity Arrhythmias in the Whole Heart

Triggered activity (TA) has recently received increased attention as a mechanism responsible for cardiac arrhythmias. However, few studies have shown TA in the intact heart. In an ouabain-treated dog's heart we have shown: (a) overdrive acceleration, (b) a concordant relationship between the postpacing interval (PI) and pacing cycle length (CL), and (c) a discordant relationship between the PI and number of paced beats necessary to induce TA. These findings appear to agree with the distinctive characteristics of TA arrhythmias elucidated in previous in vitro studies and suggest TA rather than a reentrant tachycardia. In addition, it is possible that this heart preparation could be considered as a suitable model for the study of TA arrhythmias. These results were obtained using a programmed stimulation protocol in this dog model: (1) Following single programmed ventricular stimulation during sinus rhythm, a repetitive ventricular response (RVR) of more than 3 beats occurred in only 20% of hearts. The relationship between PI and the coupling interval (CI) of the extrastimulus was concordant in 80% (12/15) and discordant in 13% (2/15) of all experiments. The PI-CI relationship was influenced by the mutual relationship between the stimulating, recording, and originating sites of TA. (2) RVR of more than 3 beats was induced by consecutive overdrive ventricular stimulation during sinus rhythm (78%). In addition, the PI-pacing CL relationship was concordant (100%). (3) The transient termination of sustained VT that occurred spontaneously after administration of a large dose of ouabain was seen in only 15% of the cases after a single programmed premature ventricular stimulation. The return cycle-CI relationship was biphasic in 75% (15/20) experiments and discordant in 25% (5/20) of the experiments. (4) The termination of spontaneous sustained VT by overdrive ventricular stimulation occurred in only 8% of the cases. Transient overdrive acceleration of VT occurred after overdrive pacing (53%). In contrast, overdrive suppression occurred in only 13%. Thus, the characteristics of TA arrhythmias observed in the whole heart preparations differed, in some respects, from those obtained by in vitro studies. These quantitative observations could suggest a differentiation, based on probability, between TA and the reentrant mechanism that would respond to programmed stimulation in a similar manner. The differentiation between reentrant and triggered ventricular tachycardia can be made with reasonable assurance using these programmed stimulation techniques.

Electrophysiologic Effects of Enoximone in Patients with

The electrophysiologic effects of enoximone were evaluated in 12 patients with idiopathic dilated cardiomyopathy undergoing routine programmed electrical stimulation (PES). Enoximone (1.5 mg/kg for 20 min followed by 0.75 mg/kg for 20 min) significantly shortened the spontaneous sinus cycle length as well as the atrial and ventricular refractory period. Neither during control PES nor during enoximone infusion were sustained ventricular arrhythmias provoked. However, in 5 of 12 patients, there was an increase in the number of repetitive ventricular responses induced by PES during enoximone administration. The observed electrophysiologic changes may be indicators for increased electrical instability of the myocardium in selected patients.

Electrophysiologic Effects of Enoximone in Patients with

The electrophysiologic effects of enoximone were evaluated in 12 patients with idiopathic dilated car-diomyopathy undergoing routine programmed electrical stimulation (PES). Enoximone (1.5 mg/kg for 20 min followed by 0.75 mg/kg for 20 min) significantly shortened the spontaneous sinus cycle length as well as the atrial and ventricular refractory period. Neither during control PES nor during enoximone infusion were sustained ventricular arrhythmias provoked. However, in 5 of 12 patients, there was an increase in the number of repetitive ventricular responses induced by PES during enoximone administration. The observed electrophysiologic changes may be indicators for increased electrical instability of the myocardium in selected patients

Predictive Value of Various Types of Ventricular Response to Programmed Ventricular Stimulation: Relation to Holter Monitoring

The study was performed to determine the predictive value of programmed stimulation for identification of pts with ventricular arrhythmias: 75 patients were studied by means of 24-hour ambulatory ECG (24 ECG) and programmed right (in some patients also left) ventricle stimulation at sinus and two or three pacing rates using two (standard) and three extrastimuli or burst stimulation (extensive protocol). Lown classes 0, 1-3 and 4a-4b were observed in 24 ECG in 35, 14, and 26 patients, respectively. In programmed stimulation 1-6 repetitive ventricular responses (RVR) were found in 56 pts, nonsustained ventricular tachycardia in 11 and sustained ventricular tachycardia in 21 pts. High incidence of induced VT was found in pts with complex ventricular arrhythmia in 24 ECG, 81% of this group, in all but six pts only standard protocol was used. The 1-6 RVR were observed in almost 40% of pts without any arrhythmia. Only VT induction is a useful index for high risk patients.

Reappraisal of criteria for assessing drug efficacy in patients with ventricular tachyarrhythmias: Complete versus partial suppression of inducible arrhythmias

To test whether increased difficulty in inducing ventricular tachycardia during antiarrhythmic therapy can be considered a sufficient criterion for predicting long-term efficacy of such therapy in patients with ventricular tachyarrhythmias, 95 patients were studied with a graded stimulation protocol (single and double premature stimuli during sinus rhythm and ventricular drives of 120, 140, 160 and 180 heals/rain). After a control study, the effects of oral antiarrhythmic drugs on the ability to induce ventricular tachycardia were assessed. The median number of drug trials was four per patient.After antiarrhythmic therapy, four subgroups of patients were identified. In 36 patients, there was no change in inducibility (group 1), whereas in 18 patients ventricular tachycardia was rendered more difficult to induce; that is, a sustained ventricular tachycardia was inducible at a basic drive at least 40 beats/min faster than during the control study (group 2). In 34 patients, ventricular tachycardia induction was suppressed (group 3) and in 7 patients with nonsustained ventricular tachycardia, only 3 to 5 repetitive ventricular responses were induced after treatment (group 4).During follow-up of 15.5 ± 11.5 months, 10 patients of group 1 had a recurrence of ventricular tachycardia and 6 died suddenly, whereas in group 2 only 1 patient died suddenly and in group 3, 2 patients had a recurrence of ventricular tachycardia (group 1 versus 2 and 3, p < 0.001, Mantel-Cox and Breslow; group 2 versus 3, no difference). Thus, increased difficulty in inducing ventricular tachycardia is a sufficient criterion for predicting long-term efficacy of an antiarrhythmic drug regimen.