Automatic Implantable Cardioverter/ Defibrillator (AICD) and Antitachycardia Pacemaker (Tachylog): Combined use in Ventricular Tachyarrhythmias

Antitachycardia pacing in ventricular tachyarrhythmias (VT) is associated with potential acceleration of VT: frequency of VT and discomfort of the patient (pt) can limit treatment with the AICD. We therefore evaluated the combined use of antitachycardia pacing and AICD in 6 of 14 patients (age 50–70. mean 60 years) with AICD implantation because of VT, which could he terminated by temporary overdrive pacing. With the interactive mode of the Tachylog, termination of VT by the pacemaker as well as by the AICD was assessed after implantation. In the automatic mode, the Tachylog functioned as a bipolar VVI device with antitachycardia burst stimulation: 2–5 stimuli, interval 260–300 ms. 1–2 interventions. During follow-up of 12±5 months, the Tachylog terminated VT reliably 20 to 327 times per patient. In three patients, burst stimulation accelerated VT, which was terminated then by the AICD discharge. Conclusion: Drug resistant ventricular tachyarrhythmias can be terminated by the Tachylog pacemaker avoiding patients' discomfort. In case of acceleration. ventricular tachyarrhythmias can be controlled by the automatic implantable cardioverter/defibrillator. A universal pacemaker should combine antitachycardia pacing with back-up defibrillation mode.     

The Impact of Antitachycardia Pacing with Defibrillation

Chronic recurrent ventricular tachycardia (VT) can be reproducibly terminated by programmed endocardiaJ right ventricular stimulation. However, antitachycardia pacing can be associated with possible acceleration of VT, while frequent episodes of VT and patient discomfort can limit treatment by an implantable cardioverter defibrillator (ICD). The combined use of antitachycardia pacing and the AICD (automatic implantable cardioverier defibrillator) was evaluated in 6 out of 51 patients (age 57 ± 11 years) in whom the AICD had been implanted because of recurrent VT. In each instance VT could be terminated by temporary overdrive pacing. The interactive mode of VT termination by a pacemaker (Tachylog) as well as by the AICD was assessed after implantation. In the automatic mode, the Tachylog functioned as a bipolar, ventricular inhibited (VVI) device with antitachycardia burst stimulation capability, allowing two to five stimuli at intervals of 260–300 ms and one or two interventions. During follow-up of 47 ± 24 months, the Tachylog terminated VT reliably 50–505 times per patient. When burst stimulation accelerated VT, termination was achieved by AICD discharge. Thus, drug resistant VT can be terminated by antitachycardia pacing to avoid patient discomfort. In the event of tachycardia acceleration, VT was terminated by the AICD. A universal pacemaker-defibrillafor should combine antibradycardia and antitachycardia pacing with back-up cardioversion defibrillation.     

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.     

Device Interaction—Antitachycardia Pacemakers and Defibrillators for Sustained Ventricular Tachycardia

We evaluated the combined use of permanent automatic antitachycardia pacemakers and implanted defibrillators in fen patients with recurrent monomorphic sustained ventricular tachycardia (VT). Pacemaker programming was VVI-T automatic burst in eight patients, VVI-T magnet mode in one patient, and VVI in one patient. Device interactions occurred in four patients, requiring changes in pacemaker programming. These included defibrillator multiple counting during pacing, in-appropriate pacemaker bursts initiating VT, inappropriate reset of the pacemaker antitachycardia mode by defibrillation, defibrillator discharge after pacemaker VT termination, and defibrillator VT reinitiation. Two patients required pacemaker programming out of the antitachycardia mode, and two required a change in antitachycardia pacing parameters. Seven patients remain in automatic VVI-T and three in VVI modes. Mean follow-up is 13 months and all patients are alive. Thus, although pacemaker/ defibrillafor combinations function well for patients with more than one VT rate, device interactions occur frequently and may require pacemaker reprogramming or elimination of the overdrive mode. Combined use of these devices should be cautiously considered when single device therapy is unsatisfactory. Devices that combine both pacing and defibrillation features may reduce adverse interaction.     

Long-Term Efficacy of an Antitachycardia Pacemaker and Implantable Defibrillator Combination

Antitachycardia pacemakers and implantable cardioverter defibrillators (ICD) were implanted in 14 patients to control recurrent hemodynamically stable ventricular tachycardia (VT), All patients underwent extensive preimplant testing in the elecrrophysiology laboratory documenting that in each patient at least 50 episodes of VT could be reliably terminated by an external model of the antitachycardia pacemaker. The burst scanning mode of anfitachycardia pacing was used in all patients. ICDs were implanted solely as a back up should acceleration of VT occur, and all had high nonprogrammable rate cutoffs (mean 191 ± 12 beats/min). During a mean follow-up of 25 ± 6 months, 6,029 episodes of VT were treated in the 14 patients. Only 103 ICD discharges were required (approximately one discharge per 60 episodes of VT). Ten of the 14 patients received discharges from their ICDs. No deaths have occurred. All devices remain active and in the automatic mode. Thus, an antitachycardia pacemaker and ICD combination can safely and effectively terminate VT in highly selected patients who are subjected to extensive preimplant testing. In such patients, the vast majority of episodes of VT can be terminated with antitachycardia pacing, and only rarely is a discharge required from the ICD.     

Programmable External Automatic Antitachycardia Pacing as a Bridge to Definitive Therapy in Patients with Recurrent Sustained Ventricular Tachycardia

The efficacy and safety of external programmable automatic antitachycardia pacemakers (ATPs) used in the critical care setting for recurrent sustained monomorphic ventricular tachycardia (VT) was evaluated. Ten patients who had failed a mean of 4.0 +/- 1.4 antiarrhythmic medications (range 2-7) and who had previously required electrical cardioversion for VT were enrolled. Prior to ATP use, successful overdrive pacing termination of VT was demonstrated in all patients. Intertach (Intermedics, Inc.; n = 9) and Orthocor II (Cordis, Inc.; n = 1) ATPs were attached to temporary bipolar transvenous or epicardial pacing leads. Mean patient age was 66.4 +/- 11.5 years, and mean left ventricular ejection fraction was 22 +/- 7.5%. At the time of initial ATP use, mean VT cycle length was 347 +/- 88 msec (range 280-550 msec). A burst scanning antitachycardia pacing algorithm was used in each patient; one patient was also treated with a fixed rate burst adapted to VT cycle length. The duration of ATP use ranged from 2-25 days (median 5), successfully terminating greater than 3,369 VT episodes (median 3, range 0 to greater than 3,103 episodes per-patient). Two episodes of ATP induced rate acceleration occurred, each successfully terminated by the ATP. Only two patients required external cardioversion during ATP use, one for primary ventricular fibrillation and one for rapid polymorphic VT associated with antiarrhythmic drug withdrawal. ATPs also provided antibradycardia pacing and allowed for serial programmed ventricular stimulation. No complications were associated with transvenous catheter or ATP use.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Clinical Testing of a New Pacemaker Function to Monitor Ventricular Capture

Automatic beat-by-beat capture functions are designed to minimize the pacing energy delivered, while maintaining the highest safety by delivering an immediate back-up stimulus in case of loss of capture. The objective of this study was to estimate the lowering of ventricular pacing amplitude allowed by such a function, compared to amplitudes usually set manually in routine practice. An automatic ventricular pacing threshold test is launched every 6 hours to measure the automatic capture threshold (AT). From AT the function calculates: (1) the "capture amplitude" (V_c) = AT + 0.5 V at a minimum output of 1 V and (2) the "safety amplitude " (V_s) = twice AT at a minimum output of 2.5 V. The function preferentially uses V_c and verifies capture after each paced beat. In case of loss of capture, a back-up spike is delivered and V_s is implemented until the next threshold measurement. We estimated the ventricular amplitude delivered by the pacemaker from data stored in the pacemaker memory. We compared these values with the pacing amplitude typically programmed manually (MPA) by physicians at twice AT and a minimum of 2.5 V. Data from 57 recipients of Talent 3 DR pacemakers were analyzed. Complete data sets were available in 25 patients at 1 day, 28 at 1 month, and 39 between 1 day and 1 month. No loss of capture or ventricular pause was observed on 53 ambulatory electrocardiograms (ECG); and pulse amplitude automatically delivered by the device was significantly lower than the MPA at each of the three time points analyzed. This new beat-by-beat capture function allows a significant lowering of the pacing amplitude compared to manual settings, while preserving a 100% safety.     

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.     

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.     

The Relationship between Pacing Site and Induction or Termination of Sustained Monomorphic Ventricular Tachycardia by Antitachycardia Pacing

Background: With the development of left ventricular pacing for cardiac resynchronization, there is an interest in the possibility of improving ventricular antitachycardia pacing (ATP) efficacy by pacing from the LV electrode(s).
Objective: This study assessed the efficacy of pacing delivered from the left coronary vein (LCV) compared to that delivered from the right ventricular apex (RVA) upon ventricular tachycardia (VT) induction and termination.
Methods: Sixty patients undergoing provocative ventricular electrophysiology (EP) studies in three centers were enrolled. Multipolar EP catheters were placed in the atrium, the RVA, and LCV. VT induction was attempted from the RVA and LCV in random order. Upon detection of monomorphic VT, burst ATP for up to 10 pulses at 88% VT cycle length was delivered from the RVA or LCV, in a random order, and crossed over when possible. Identical VT morphologies were reinduced to allow paired comparison of RVA versus LCV ATP.
Results: Data from 55 patients were analyzed. Thirty-four morphologically distinct monomorphic VT types were induced in 22 patients. ATP succeeded in 18 (55%) and VTs in 13 patients. RVA ATP terminated 15 of 23 (65%) VTs, and LCV ATP terminated 10 of 23 (43%) VTs (P = 0.14). ATP delivered ipsilateral to the earliest activation site required 5.0 ± 2.6 pulses to terminate compared to 4.8 ± 1.7 pulses when delivered from the contralateral site (P = 0.90). Paired comparison was possible for 13 VT morphologies in 11 patients. Paired RVA and LCV ATP efficacy was identical (54 % vs 54%, P = 1.0).
Conclusion: ATP delivered from a LCV lead offers no efficacy advantage over pacing from the RVA. (PACE 2010; 27–32)     

Long-Term Management of Ventricular Tachycardia by Implantable Automatic Burst Tachycardia-Terminating Pacemakers

This report describes the Jong-term follow-up of two patients who received implantable automatic burst tachycardia-terminating ventricular pacemakers for the treatment of drug-refractory sustained ventricular tachycardia. After implantation, both pulse generators continued to terminate ventricular tachycardia without any major complications. In one patient, after three years, many episodes of ventricular tachycardia were slower than the tachycardia-detection criterion rate of 137 per minute; ventricular tachycardia was then terminated by chest wall stimulation that activated the burst function of the pacemaker. In this particular patient, the pulse generator was removed after four and one-half years and replaced with a DDD system because of the pacemaker syndrome and attacks of ventricular tachycardia, often at a rate of about 100/minute. In the second patient, the pacemaker continued to terminate ventricular tachycardia for over five and one-half years as determined by the repeated activation of the flag (memory) function of the pacemaker indicating detection of tachycardia by the pulse generator and resultant delivery of burst pacing.     

Termination of Ventricular Tachycardia by an Implantable Atrial Pacemaker and External Pacemaker Activator

An atrial pacemaker was implanted in a patient who had dilated cardiomyopathy, sinus node dysfunction, and drug-resistant ventricular tachycardia (VT). VT episodes were terminated by atrial overdrive pacing using an implanted pacemaker and a newly developed hand-held external programmer/transmitter. Although successful cases of termination of intractable VT by ventricular pacing have recently been reported, the ventricular method might increase the risk of accelerating VT. Atrial overdrive pacing is a safer method since it minimizes the possibility of tachycardia acceleration and, combined with antiarrhythmic drugs, it appeared to be a unique and useful approach for the treatment of drug-resistant VT.     

Automatic Implantable Defibrillator Discharge Resulting From Routine Pacemaker Programming

A middle-aged gentleman with an idiopathic dilated cardiomyopathy, drug-refractory sustained ventricular arrhythmia, and high-degree AV block was managed with an automatic implantable cardioverter-defibrillator (AICD) and a Cordis Multicor II VVI pacemaker. During a routine follow-up visit, the pacemaker threshold was determined. Seven seconds after reprogramming the Cordis pacemaker to the "stat" VVI mode, the AICD discharged. The time to discharge after reprogramming the pacemaker equalled the previously determined AICD charge time. No other rhythm disturbance was documented and the rate with double counting of pacemaker stimulus and QRS complex was less than the triggering rate for the AICD. In conclusion, the AICD can be triggered by pacemaker programming signals unrelated to subsequent pacemaker function.     

Increased Pacing Threshold After an Automatic Defibrillator Shock in Dogs: Effects of Class I and Class II Antiarrhythmic Drugs

A high energy shock delivered by an automatic defibrillator may interfere with pacemaker function. To provide insight into the changes that occur in the threshold for ventricular pacing after the shock from an automatic defibrillator, we measured the time to capture during asynchronous ventricular pacing in dogs from endocardial or epicardial sites, after a 30 joule shock was delivered via conventional automatic defibrillator (AICD) patch electrodes. After a 30 joule shock, there was a transient loss of ventricular capture. The duration of capture loss was related to current strength. During endocardial pacing at threshold current, the time to capture was 4.9 +/- 1.2 s, whereas at current values twice threshold the time to capture from endocardial pacing was 2.2 +/- 0.9 s. No difference was found between endocardial and epicardial pacing sites in the time to capture. To ascertain the mechanism of capture loss we: (1) examined the effects of converting the pacing catheter to a current sink (transiently shunting to ground); (2) altered excitability by an infusion of flecainide; (3) blocked sympathetic input (propranolol). No change in time to capture was noted by shunting the pacer to ground. After an infusion of flecainide the time to capture from endocardial pacing was significantly prolonged to 14.9 +/- 2.2 s at the threshold value (P less than .01) and 5.6 +/- 2.1 s at twice threshold (P less than .05). Conversely, intravenous propranolol had no effect on the time to capture after shock from endocardial pacing. These data indicate that there is a transient increase in pacing threshold after the shock from an automatic defibrillator.(ABSTRACT TRUNCATED AT 250 WORDS)     

Automatic implantable cardioverter-defibrillators in Chagas' heart disease patients with malignant ventricular arrhythmias

A total of 46 consecutive Chagas' disease patients had an automatic cardioverter defibrillator implanted at our institution from October 1998 to January 2004. A retrospective longitudinal study was carried out to identity type of life-threatening ventricular arrhythmias as well as type of therapy delivered. Of these, 41 (91%) had been recovered from cardiac arrest. Five (15%) of 33 patients in whom echocardiography was done had no left ventricular function. Antiarrhythmic therapy was delivered to 37 (80%) patients during postimplant follow-up. Thirty-one of 37 (84%) patients received both shock and antitachycardia pacing, five (13%) only antitachycardia pacing, and one (3%) patient only shock. Median time to first shock was 16 days, varying from 1 to 576 days. Ventricular fibrillation was the cause of first shock in 12 patients (32%), ventricular tachycardia in 11 (29%), and ventricular tachycardia not responding to antitachycardia pacing degenerating into ventricular fibrillation in nine (24%). Five patients with ventricular tachycardia were treated with antitachycardia pacing. Probability of freedom from device discharged was 47% at 90 days, 34% at 180 days, and 9% at 360 days in the postimplant follow-up. Thus, patients with chronic Chagas' heart disease recovered from cardiac arrest have a peculiar arrhythmogenic profile characterized by a high frequency of ventricular fibrillation and no left ventricular systolic dysfunction and a short period of time for first shock.     

Management of ventricular arrhythmias

Underlying causes of ventricular tachycardia (VT) or complex ventricular arrhythmias (VA) should be treated if possible. This may include beta-adrenergic blockade radiofrequency catheter ablation and automatic implantable cardioverter-defibrillators. The ACC/AHA Class I indications for an AICD are discussed. Patients with AICDs should be treated with biventricular pacing, not with dual-chamber rate-responsive pacing at a rate of 70/minute.     

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.     

Automatic Implantable Cardioverter Defibrillator/Permanent Pacemaker Interaction: Loss of Pacemaker Capture Following AICD Discharge

A 78-year-old man treated with amiodarone for recurrent ventricular tachycardia, had sequential placement of a bipolar VVI pacemaker and an automatic implantable cardioverter defibrillator (AICD). During defibrillation threshold testing, there was failure to capture of the pacer in the post-shock period. The time of failure to capture appeared energy-related: the greater the energy delivered, the longer the failure to capture. Careful attention will be necessary in constructing combined AICD/pacemaker units.     

Clinical performance of a ventricular automatic capture verification algorithm

BACKGROUND: This study was conducted to evaluate the clinical performance of the ventricular automatic capture feature as implemented in the Insignia I Ultra pacemaker system (Guidant) utilizing a variety of ventricular leads. Currently, the optimal programming of the pacemaker output considers both pacemaker efficiency (prolonging battery longevity) and patient safety (adequate safety margin). The ability of a pacemaker to automatically adjust the ventricular output above the pacing threshold while maintaining the appropriate safety margin has been explored since the early 1970s and is only available today in conjunction with a specific low polarization lead system. METHODS: One hundred and five patients were enrolled from 17 European centers utilizing 31 different types of ventricular leads were followed through their 3-month follow-up visit. There were no restrictions on the type of ventricular leads used. RESULTS: The average mean difference between the commanded autothreshold test (0.652 + 0.335 V) and the manual threshold test (0.651 + 0.335 V) was 0.001 + 0.49 (P < 0.0001). The average mean difference between the ambulatory autothreshold test (0.696 + 0.322 V) and the commanded autothreshold test (0.682 + 0.315 V) was 0.002 + 0.74 (P < 0.0001). Holter recordings confirmed that there were no loss of capture incidences without a backup pulse being delivered. In addition, the mean number of backup pulses delivered in a 24-hour period was less than 0.1% of the total number of paced beats. CONCLUSIONS: This study provided that the automatic capture feature while using a variety of leads accurately determines the ventricular stimulation threshold and safely delivers a backup pulse when required.