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(Circulation. 1998;97:66-74.)
© 1998 American Heart Association, Inc.

Clinical Investigation and Reports

Empirical Versus Tested Antitachycardia Pacing in Implantable Cardioverter Defibrillators

A Prospective Study Including 200 Patients

Anselm Schaumann, MD; Friederike von zur Mühlen, MD; Bernhard Herse, MD; Bernd-Dieter Gonska, MD; ; Heinrich Kreuzer, MD

From the Departments of Cardiology and Thoracic Surgery, University Hospital Göttingen, Germany.

Correspondence to Anselm Schaumann, MD, Department of Cardiology, University Hospital Göttingen, Robert-Koch-Str 40, 37075 Göttingen, Germany. E-mail: aschaum{at}gwdg.de

	Abstract

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Background—Implantable cardioverter-defibrillators (ICDs) reduce the risk of sudden cardiac death. The objective of this study was to evaluate whether testing of antitachycardia pacing (ATP) for induced ventricular tachycardias (VTs) at predischarge examination can predict ATP success during follow-up.

Methods and Results—The study covers 200 consecutive patients who received ICD implants from June 1991 through December 1995. All underwent electrophysiological testing. In 54 patients (ATP tested, group T), ATP terminated induced VTs successfully. In 146 patients (empirically programmed ATP, group E), only ventricular fibrillation could be induced, including 18 with unsuccessful ATP attempts for induced VTs. Disregarding the results of ATP testing, the same ATP scheme was programmed in all patients: three attempts of autodecremental ramp with 81% of the VT cycle length, with 8 to 10 pulses. During a follow-up of 20.4±10 months, 95% of 3819 spontaneous VTs were successfully terminated with ATP in 42 patients of group T. In group E, 90% of 1346 spontaneous VTs in 81 patients were terminated with ATP. Acceleration after ATP occurred in 2% in group T versus 5% in group E. The success for all episodes in individual patients was 90% in >60% of the ATP tested and empirically programmed patients.

Conclusions—The results of this 200-patient prospective study comparing tested versus empirical ATP show high success (95% versus 90%) for VT termination, with low rates of acceleration. ATP is safe and very effective and should be programmed "on" in all patients regardless of the predischarge EP inducibility.

Key Words: arrhythmia • tachycardia • heart assist device • pacing

	Introduction

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Implantable cardioverter-defibrillators are an accepted therapy to reduce sudden cardiac death.¹ Third-generation devices provide not only high-energy discharges for cardioversion and defibrillation but also ATP and antibradycardia pacing for complete rhythm management.² ATP reduces the number of defibrillation shocks³ and is well tolerated, because it is seldom noticed by the patients.^{4 5} Some were concerned about arrhythmia acceleration due to ATP.⁶ Published experiences to date on ATP use are limited to small numbers of patients and relatively few episodes of spontaneous VT.⁷ Generally, ATP was found to terminate a high percentage of spontaneous VTs, ranging from 88% of 424 VT episodes in 20 patients⁸ to 92.4% of 840 VT episodes in 22 patients.² The ATP scheme used in these and other studies (43 patients,⁹ 35 patients,¹⁰ and 15 patients¹¹ with spontaneous VT) was generally not specified and varied from patient to patient. In one report, ATP parameters were changed during the course of the study.¹⁰ The objective of our study was to evaluate the correlation between the results of the EP study and ATP testing at the predischarge examination and the efficacy of ATP in terminating spontaneous mVT. In terms of important clinical consequences, the aim of the study was to demonstrate whether ATP can be used safely even for those patients in whom ATP testing is not possible.

	Methods

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Patient Selection
Between June 1991 and December 1995, 200 consecutive patients underwent implantation of a third-generation device with ATP, antibradycardia pacing, and cardioversion and defibrillation options. All devices were implanted with transvenous endocardial leads (Endotak, CPI). All implantations were performed at our cardiovascular surgery department. Indications for ICD therapy included (1) 84 patients with out-of-hospital cardiac arrest due to VF, (2) 101 patients with poorly tolerated sustained mVT, and (3) 15 patients with unexplained syncope with persistent inducibility of sustained mVT (lasting >30 seconds) despite pharmacological therapy. In all patients, reversible causes of VT/VF, such as acute ischemia or metabolic imbalance, were ruled out.

All patients were informed of the therapy modalities, and written consent was obtained for device implantation and the subsequent follow-up studies.

Description of the Devices
All implanted devices (11 PRx I, 79 PRx II, 81 PRx III, and 29 MINI [CPI]) have defibrillation, cardioversion, ATP, and VVI pacing capabilities. Arrhythmia detection is based on heart rate and duration of the arrhythmia, and discrimination from supraventricular rhythm can be enhanced via programmable arrhythmia stability and onset criteria.¹²

Device Programming
All devices were programmed with two or three tachyarrhythmia detection zones, with three zones used for detection rates <170 bpm (61 of 200 patients). The cutoff rates for VT detection are listed in Table 1. The highest-rate zone was set to detect VF or fast VTs with rates >240 bpm and lasting between 1 and 2.5 seconds. Therapy in this zone was programmed to five maximum energy shocks of 29 J (MINI) or 34 J (PRx). Lower zones were programmed with ATP as the first attempt followed by five shocks at maximum output.

View this table: [in this window] [in a new window]   Table 1. Programmed Cutoff Rate for VT Detection in 200 Patients

The same ATP scheme was programmed in all patients: three attempts of an autodecremental ramp with 8 to 10 pulses, 8-ms decrement within bursts, and a cycle length of 81% of the detected tachycardia. The minimal pacing interval was set at 200 ms. We did not program the decrement of the burst cycle length between bursts. The "ATP time out" (programmable maximum time duration during which ATP was permitted to continue) was set at 1.5 minutes.

Detection enhancements¹² were used to inhibit therapy during atrial fibrillation or sinus tachycardia. The "stability" criterion was programmed at 22 to 30 ms in 74% of all patients but in 100% of the patients with documented atrial fibrillation and VT detection zones <170 bpm. The "sudden onset" criterion was programmed "on" at values ranging from 9% to 16% in 35% of all patients and in 100% of patients with VT detection zones between 140 and 160 bpm.

Predischarge Testing
All patients underwent a noninvasive EP study (via their implanted devices) at the predischarge examination (5 to 10 days after implantation), and VF was induced and terminated. The EP study, at different cycle lengths down to 300 ms, using up to four premature extrastimuli was performed with the EP test feature of the implanted devices and was aimed at inducing mVT. If mVT could be induced, therapy delivery was enabled, and ATP (81% autodecremental ramp) was delivered (Fig 1). After termination of the mVT, the arrhythmia was induced again, and ATP success was confirmed. There were 54 patients (group T) with induced and successful ATP-terminated mVTs. Group E included 146 patients with empirically programmed ATP. In 128 patients, only VF was inducible. In the remaining 18 patients, ATP was unsuccessful: 6 with no change in cycle length, 12 with VT acceleration.

View larger version (48K): [in this window] [in a new window]   Figure 1. Comparison of a noninvasively induced VT terminated by ATP (8 pulses of ramp mode) at predischarge testing and the ECG of a spontaneous arrhythmia (bottom right) with same rate and QRS morphology documented 5 weeks before ICD implantation.

Follow-up Procedures
Patients were followed up 1 month after implantation and every 3 months thereafter. Patients were examined and devices interrogated to determine spontaneous episodes with RR intervals and stored electrograms (Fig 2) and success of delivered therapy. Pacing thresholds and lead impedance were measured.

View larger version (35K): [in this window] [in a new window]   Figure 2. RR intervals (top) and ECG (bottom) of a spontaneous VT (188 bpm) terminated by an attempt of ATP with 8 pulses of ramp mode. Printout of stored episode of ICD.

Classification of Spontaneous Episodes
The spontaneous arrhythmias were analyzed with the help of the stored electrograms or RR intervals. An arrhythmia was classified as VT if (1) it started with a sudden change in the heart rate, (2) the RR intervals were regular, and (3) the ECG tracings resembled a known mVT or a typical QRS morphology was observed. Clinical symptoms were included in the analysis. The diagnosis of sinus tachycardia at exercise was based on no preceding symptoms and short QRS duration during tachycardia comparable to the electrograms of baseline rhythm. The arrhythmia was classified as atrial fibrillation if it showed irregular RR intervals, a >30-ms change from beat to beat, no sudden onset, and short QRS durations in the electrogram. A sustained arrhythmia was classified as VF if the heart rate was >240 bpm.

Acceleration of mVT was defined as a heart rate increase of >10% after the ATP attempt. An accelerated or unchanged VT rate after the ATP attempt was defined as an ineffective ATP attempt. The termination of spontaneous mVT after an ATP attempt was verified with the help of the stored RR intervals and electrogram tracings and required a significant drop in the heart rate and a change in the QRS morphology. To get more detailed information on the probability of ATP succeeding in any given patient, we examined the number of patients in whom ATP was attempted during spontaneous VT and compared the individual success rate based on the percentage of terminated VTs in each patient for the two groups.

Statistical Analysis
Differences between the two groups were compared by the ² test or Fisher's exact test and unpaired Student's t test when appropriate. Values of P<.05 were considered significant.

	Results

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Patient Population
In this study, 200 consecutive patients were included. Most of the patients (clinical data in Table 2) were male (86%) and had coronary artery disease (61%). No antiarrhythmic drug was given to 61% of all patients. Because of underlying heart failure or a history of myocardial infarction, 70% received ACE inhibitors.

View this table: [in this window] [in a new window]   Table 2. Patient Characteristics of Group T (Tested ATP) and Group E (Empirically Programmed ATP)

Monomorphic VT could be induced twice and terminated with ATP during predischarge EP tests in 54 patients. In the other 146 patients, mVT was noninducible (128 patients) or we were able to induce only fast VTs without successful ATP termination (18 patients). In the latter group, therefore, ATP could not be adequately tested, and ATP was programmed empirically.

The follow-up of the patients was 20.4±11 months (Table 2). Of 200 patients, 150 (41 group T and 109 group E) reached at least 1 year follow-up, and 72 patients (20 group T and 52 group E) completed 2 years of follow-up.

Death During Follow-up
During the follow-up, 20 of 200 patients (10%) died. Fourteen patients from group E (146 patients) died of progressive heart failure (11), accident (1), and carcinoma (2) 6 to 59 months after receiving implants. Six of 54 patients from group T died of progressive heart failure (3), noncardiac death (2), and unwitnessed sudden cardiac death (1 patient) after 9 to 39 months. The causes of patient deaths according to the classification scheme as defined by Epstein et al¹³ are shown in Table 3. Because all patients except 1, who died of a carcinoma, had appropriate therapy delivery during follow-up before death occurred, there is no evidence of ICD system–related causes of death.

View this table: [in this window] [in a new window]   Table 3. Final Classification of Death1

Spontaneous Tachyarrhythmia Episodes
During a mean follow-up of 20.4 months, 123 of 200 patients had 5379 spontaneous ventricular arrhythmias (Table 4). The mean number of spontaneous arrhythmias for patients of group T was 72 (median, 10) and for patients of group E, 10 (median, 1). A higher percentage of group T, 42 of 54 patients (78%), had recurrent ventricular arrhythmias, compared with 81 of 146 patients (55%) of group E (P<.01).

View this table: [in this window] [in a new window]   Table 4. Recurrences and Termination of Spontaneous Ventricular Tachyarrhythmias

Spontaneous Episodes of VF
VF or rapid VT >240 bpm was treated with high-energy shocks 72 times in 16 of 54 patients from group T and 142 times in 53 of 146 patients of group E. All shocks were successful and restored regular rhythm. VF as the only recurrent ventricular tachyarrhythmia was seen in none of the group T patients but in 15 of 81 patients with spontaneous arrhythmias in group E (Table 4).

Inappropriate Therapy
We could demonstrate with stored electrograms that therapy was inappropriately delivered in only 4.5% of the patients: in 8 patients for atrial fibrillation (4 of the 8 patients did not have stability programmed on initially) and in 1 patient for sinus tachycardia.

Antitachycardia Pacing for Spontaneous mVTs
Of the 5379 spontaneous ventricular arrhythmias, 5165 tachycardias received ATP as the initial ICD therapy (Table 4). Hence, VF requiring shocks was the initial arrhythmia in only 4% of episodes. The ATP attempts successfully terminated 4845 episodes (94%) in the whole patient population. In group T, 3640 episodes (95%) were terminated by ATP versus 1205 episodes (90%) in group E (P<.01, Fig 3).

View larger version (21K): [in this window] [in a new window]   Figure 3. Success of ATP for termination of 5165 spontaneous VTs. VT termination for tested ATP (left) versus empirical programmed ATP (right). Ineffective ATP did not change VT rate (unchanged) or accelerated VT.

Acceleration into a 10% higher VT rate or VF was noted in only 2.4% of the episodes treated with ATP in group T and 5.1% in group E. Defibrillation subsequently successfully terminated all tachycardias after failed or accelerated ATP attempts. Even at rates >200 bpm, ATP was able to successfully terminate a high proportion of VTs: 185 of 224 VTs (83%) in 21 patients of group T and 293 of 369 VTs (79%) in 37 patients of group E (Table 4). All 5 patients with no ATP success for VT >200 bpm had other spontaneous VTs at rates <200 bpm that were terminated with ATP attempts.

For the probability of ATP succeeding in any given patient, we examined the percent success for all episodes in each individual patient for the two groups. The mean individual ATP success rate for each patient was 82% in group T versus 85% in group E (P=NS, Table 5). A 90% success of ATP for all episodes in individual patients was achieved in 60% of group T and 65% of group E with spontaneous recurrent VT. ATP success rates were still high for fast spontaneous VT (Table 5). In both groups, there were only 3 patients who did not benefit from ATP. Each of these 3 patients had only one spontaneous VT during follow-up in which ATP was not successful. While the results show high success for ATP, it is possible that the therapy delay associated with unsuccessful ATP attempts may have resulted in syncope with minor injuries in 2 patients (one in each group). However, no patient died or had serious complications from ATP therapy.

View this table: [in this window] [in a new window]   Table 5. Individual ATP Success Rate for the Termination of Spontaneous VT and Fast Spontaneous VT>200 bpm

ATP for Spontaneous VT in Patients With Unsuccessful Testing of ATP
During follow-up of the 6 patients with no change in the induced VT cycle length at the predischarge examination, 4 had recurrent arrhythmias. One patient had three VF episodes. Three patients had VT episodes: ATP terminated one episode each in 2 patients and three of four VTs in the third patient. Of the 12 patients with inducible VTs whose VTs were accelerated after ATP at the predischarge test, 7 had recurrences. All except 1 patient had high success rates for ATP (Table 6). All episodes not terminated via ATP attempts were successfully terminated by subsequent shock delivery from the devices.

View this table: [in this window] [in a new window]   Table 6. ATP Termination of Spontaneous VT in 7 Patients in Whom ATP Accelerated Induced VT

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
ICDs are very effective in reducing the risk of sudden cardiac death.¹ All patients with life-threatening ventricular arrhythmias not due to treatable acute disorders are candidates for ICD implantation. For patients with multiple episodes of VT, ATP therapy plays an important role, because the VT is often terminated by ATP before the patient becomes hemodynamically symptomatic,¹⁴ and patients generally tolerate ATP far better than electrical discharges.^{2 4 5} The battery life of devices is also extended if shock therapy is avoided.^{15 16} Because it is very difficult to predict the recurrence rate and type of spontaneous ventricular arrhythmia episodes, we prospectively assessed the efficacy of ATP for termination of spontaneous episodes of VT and whether that efficacy is predicted by testing ATP with induced VTs. Two hundred patients underwent predischarge testing aimed at inducing sustained VT after implantation of an ICD. The predischarge test was done 5 to 10 days after implantation depending on the general physical status of the patient. With smaller devices and exclusively subpectoral implantation of the device, it has become our practice to move up the predischarge test to day 3 to 5 after implantation. This shortened period has occasioned no change in ATP efficacy. Patients in group T (n=54) had inducible VTs that required demonstration of termination at least twice with ATP. Patients in group E either had noninducible VTs (n=128) or could not be successfully tested for induced VT (n=18). To compare the success of ATP for the termination of VT, the same ATP scheme was programmed in both groups: three attempts of an autodecremental ramp with a coupling interval of 81% and a cycle length of 81% of the VT cycle. The ATP scheme was not altered during follow-up. We found that ATP was highly successful in terminating spontaneous VTs during follow-up in both groups.

Study Population
The study population is a typical ICD cohort in terms of age (60 years), sex (86% male), cardiac disease (coronary artery disease, 61% and cardiomyopathy, 26%), and presenting arrhythmia, compared with other series reporting on the use of ATP.^{4 11 17 18 19 20 21} In contrast to these studies, our patients were not preselected on the basis of successful termination of induced VTs^{20 22} or on the basis of the underlying heart disease or history of spontaneous arrhythmias.^{21 22}

Implanted Devices and Episode Analysis
Our inclusion criteria required devices providing adequate history detail and a quality of stored ECG tracings to be sufficient for discriminating supraventricular tachycardia from VT. This enabled us to reevaluate the spontaneous episodes. Nearly all (5043, 94% of the 5379 spontaneous episodes) were stored in the PRx II, PRx III, and MINI devices with ECG storage and only 6% in the PRx I device without an ECG tracing. ECG tracings were sometimes not available for all episodes because some patients had so many episodes between consecutive follow-ups that the storage capacity of the implanted devices (PRx II or PRx III) was exceeded. In patients with multiple episodes, we compared the frequency, onset, stability, and RR intervals to ECG-documented VT episodes. All episodes included in the analysis were episodes in which appropriate therapy delivery could be demonstrated. Our conclusion concerning ATP should in general be applicable for all ICD devices because ATP schemes from different manufacturers provide similar programmable features.

Inappropriate Therapy Delivery
The ability to retrieve stored electrograms depicting events before therapy delivery proved to be an important tool to reevaluate spontaneous episodes and classify tachycardias as ventricular or supraventricular and therapy as appropriate or inappropriate.²³ Therapy was delivered inappropriately in only 9 patients (4.5%), a very low incidence compared with other studies.^{24 25 26} As we have reported previously,¹² we believe that the reason for this very low rate is our use of rate stability and sudden onset as detection enhancements to inhibit therapy in the case of atrial fibrillation or sinus tachycardia.

Overall ATP Success Rate and Individual Success Rate
Previously published studies involved relatively small numbers of patients and no single ATP scheme. In contrast, we used one specific ATP scheme that was not altered during follow-up. Furthermore, the large number of spontaneous episodes that could be analyzed permits us to draw conclusions and make general recommendations as to the future use of ATP. There were no patient dropouts during the study and no serious complications of ATP attempts. All results have to be viewed in light of the ATP ramp scheme programmed, even for VT rates at 240 bpm, and not changed during follow-up to allow a comparison between the two groups.

The overall success of ATP for the termination of spontaneous VT was 94% (4845/5165) in 123 of 200 patients with spontaneous VT. In both patient groups, the overall termination rate with ATP was very high, 95% (3640/3819) in group T and 90% (1205/1346) in group E, and comparable to other reported results: 92% in 33 patients,¹⁵ 91% in 44 patients with tested ATP,²⁰ 96% in a patient population in which 47% of the patients underwent no formal testing,¹⁶ and 93% and 83% in 36 patients depending on left ventricular function (EF>30% versus EF<30%¹⁸ ).

Because of the large number of episodes in the present study, the difference in overall success rate (95% versus 90%) is statistically significantly higher for VTs of tested patients (P<.01), but this small difference of 5% has minimal clinical implications; furthermore, the individual success rate (a more valuable piece of information for the individual patient) was not statistically different, 82% in patients of group T compared with 85% in group E, and was closely comparable to ATP success rates for preselected patients, eg, 80% in 26 patients⁵ and 81% in 15 patients.⁴ For clinical purposes, it might be more pertinent to note how often a 90% success for ATP was seen in individual patients (Table 5). An overall individual ATP success rate for VT termination 90% in 63% of patients and for spontaneous VT >200 bpm in 52% of patients is supportive of a generalized use of ATP. This conclusion is reinforced by the fact that the individual ATP success of group E is not below that for group T.

Whether another ATP, eg, a burst sequence, would have been as successful is unknown. Unfortunately, the published studies on ATP either have no unique ATP scheme,¹⁹ do not mention which ATP scheme was programmed,⁹ have interindividual changes and reprogramming during follow-up (five different ATP modes²⁰ ), or exclude VTs with rates >200 bpm from ATP therapy by programming the device to shock therapy only.^{9 19 27 28} With regard to the safety of ATP, the subanalysis of patients with accelerated VT at the predischarge test also demonstrated a good outcome during follow-up (Table 6).

Acceleration Rate
The major argument used against ATP therapy is the potential risk of VT acceleration. Therefore, some have advised programming ATP therapy only in patients in whom the benefit of ATP can be demonstrated during EP testing.⁶ This concern has never really been substantiated. Trappe et al²⁰ reported an acceleration rate of only 3% in selected patients with recurrent inducible VTs that had been successfully terminated during a predischarge test. Conversely, one study with a small group of patients showed a higher success rate of ATP for spontaneous (94%) than for induced (66%) tachycardias.¹⁷ In our study, although ATP success could not be tested in patients in group E, the acceleration rate for spontaneous VTs was low. Furthermore, the few VT episodes not terminated or accelerated by an ATP attempt were successfully terminated with a cardioversion shock. Our study showed that the acceleration rate after ATP attempts is acceptably low (2.4% in group T, 5% in group E). In some selected patients, it may be necessary to change some programmed parameters. For example, in patients with a low ejection fraction, it may be advisable to program primary shock delivery at lower VT rates if during follow-up ATP never terminates fast VT in these patients. It is even possible that for individual patients with a higher rate of unsuccessful ATP attempts, minor changes in the ATP scheme could result in lower acceleration rates for this particular patient.

Limitations of the Study
Even though the ATP scheme we used (ramp 81%) seemed to be highly effective, we cannot exclude the possibility of equal or better ATP success with slightly different ATP schemes. Further studies are needed to answer the question of how many stimuli and attempts with decremental or nondecremental bursts are advisable for empirical ATP programming. However, we believe that it may be difficult to get beyond the 90% to 95% success rate we were able to achieve.

Conclusions
Our data clearly demonstrate that the recurrence rate of VT or VF, as well as the ATP success rate, does not depend on demonstrated ATP efficacy during the predischarge test. Our results show unambiguously that an overwhelming majority of ICD patients will profit from ATP programming; thus, we recommend application of ATP to any patient.

	Selected Abbreviations and Acronyms

 

ATP = antitachycardia pacing EP = electrophysiological ICD = implantable cardioverter-defibrillator mVT = monomorphic ventricular tachycardia VF = ventricular fibrillation VT = ventricular tachycardia

Received May 6, 1997; revision received August 25, 1997; accepted September 23, 1997.

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