Randomized controlled comparison of antitachycardia pacing algorithms for termination of ventricular tachycardia

Case report: successful termination of ventricular tachycardia by intrinsic anti-tachycardia pacing beyond conventional anti-tachycardia pacing

Background

Anti-tachycardia pacing (ATP) is a pain-free alternative to defibrillation shock for monomorphic ventricular tachycardia (VT). Intrinsic ATP (iATP) is a novel algorithm of auto-programmed ATP. However, the advantage of iATP over conventional ATP in clinical cases is still unknown.

Case summary

A 49-year-old man with no significant past medical history was transferred to our institution with sudden-onset fatigue from working on a farm. A 12-lead electrocardiogram showed monomorphic sustained wide QRS tachycardia with a right bundle branch block pattern and superior axis deviation with a cycle length (CL) of 300 ms. Sustained monomorphic VT originating from the left ventricle due to underlying vasospastic angina was diagnosed by contrast-enhanced cardiac magnetic resonance imaging, coronary angiography, and the acetylcholine stress test, and implantable cardioverter defibrillator implantation was performed. Nine months later, a clinical VT episode with a CL of 300 ms was observed, which could not be terminated by three sequences of conventional burst pacing. Ventricular tachycardia was finally terminated by a third iATP sequence without any acceleration.

Discussion

Although standard burst pacing by conventional ATP reached the VT circuit, it failed to terminate the VT. Using the post-pacing interval, iATP automatically calculated the appropriate number of S1 pulses required to reach the VT circuit. In iATP, the S2 pulses are delivered with a calculated coupling interval based on the estimated effective refractory period during tachycardia. In this case, iATP might have led to less aggressive S1 stimulation, followed by aggressive S2 stimulation, which probably helped terminate the VT without any acceleration.

Arrhythmogenicity of anti-tachycardia pacing in patients with implantable cardioverter defibrillator

BACKGROUND

Anti-tachycardia pacing therapy (ATP) has shown comparable efficacy to shock therapy in ventricular tachycardia (VT) termination with better quality of life. However, some ATPs may lead to VT acceleration or degeneration to ventricular fibrillation (VF), which will result in more ICD shocks. The aim of this study was to investigate the predictors of VT acceleration by ATP therapy in a real-life patient cohort.

RESULTS

We retrospectively reviewed 448 monomorphic VT episodes that required ATP therapy in 60 patients with structural heart diseases implanted with ICD or CRTD. The clinical data of the patients and the episodes' details were evaluated. We found that patients with a higher ejection fraction (EF) were more likely to be cardioverted by ATP therapy (P: 0.024). VT acceleration was more frequent in patients with lower EF (mean 31.24 ± 4.08) compared with the non-accelerated patients with higher EF (mean 37.00 ± 9.4, P: 0.016). The percentage of accelerated episodes was 8.5%. VT episodes with a mean cycle length (CL) < 310 ms are more likely to accelerate (sensitivity 76.3%, specificity 67.7%, PPV value 45%, NPV 86%, and AUC 0.790). There was a statistically significant difference in the accelerated VT episodes as compared to non-accelerated episodes regarding the number of ATP bursts (mean 3.66 ± 2.22 vs. 1.76 ± 1.35, P: < 0.001), ramp (23.7% vs. 4.2%, P: < 0.001), scanning (55.3% vs. 31.3%, P: 0.003) and burst adaptive cycle length (mean 83.55 ± 2.92 vs. 84.64 ± 2.61, P: 0.016). In a multivariate analysis, the VT CL, number of ATP bursts and ramp pacing predicted VT acceleration by ATP therapy.

CONCLUSIONS

Ventricular tachycardia in patients with low LV EF and fast VTs with a CL less than 310 ms were more likely to accelerate with ATP therapy. The number of ATP bursts and the use of ramp had a significant effect on VT acceleration. To avoid VT acceleration by ATP therapy, ramp pacing better be avoided, especially in fast VTs, and lesser number of bursts should be delivered.

Comparison of burst versus ramp antitachycardia pacing therapy for ventricular tachycardia: A meta-analysis

Current guidelines recommend at least one attempt of defibrillator antitachycardia pacing (ATP) therapy, showing preference for burst therapy. The objective of this study is to compare ramp versus burst ATP therapy proportion of success and acceleration in treating spontaneous or induced ventricular tachycardia (VT). The review protocol was previously published in PROSPERO. Data synthesis and measures of heterogeneity (I² ) was performed by CMA® software v.3 comparing proportions in both groups. Sensitivity analysis was performed as subgroup or meta-regression according to quality, clinical characteristics, and differences in design. Thirteen studies including 30,117 VT episodes in 1672 patients were analyzed. There was no significant difference in the proportion of success between burst and ramp therapy in spontaneous VT (odds ratio = 1.116; 95% confidence interval [CI] = 0.788-1.579; I²  = 89%). There was no significant difference in the proportion of success between burst and ramp therapy in induced VT (odds ratio = 0.820; 95% CI = 0.468-1.437; I²  = 93%). No significant difference was found in the proportion of acceleration between burst and ramp in spontaneous VT (odds ratio = 0.792; 95% CI = 0.476-1.317; I²  = 83%). No significant difference was found in the proportion of acceleration between burst and ramp in induced VT (odds ratio = 1.234; 95% CI = 0.802-1.898; I²  = 55%). Sensitivity analysis did not change main results. There is no difference in success or in acceleration proportion between burst or ramp ATP therapy irrespective if the VT was spontaneous or induced. Future implantable cardioverter defibrillator programming guidelines should offer both ATP therapies without preference in one of them.

Shock Reduction With Antitachycardia Pacing Before and During Charging for Fast Ventricular Tachycardias in Patients With Implantable Defibrillators

INTRODUCTION AND OBJECTIVES

Fast ventricular tachycardias in the ventricular fibrillation zone in patients with an implantable cardioverter-defibrillator are susceptible to antitachycardia pacing (ATP) termination. Some manufacturers allow programming 2 ATP bursts: before charging (BC) and during (DC) charging. The aim of this study was to describe the safety and effectiveness of ATP BC and DC for fast ventricular tachycardias in the ventricular fibrillation zone in patients with an implantable cardioverter-defibrillator in daily clinical practice.

METHODS

Data proceeded from the multicenter UMBRELLA trial, including implantable cardioverter-defibrillator patients followed up by the CareLink monitoring system. Fast ventricular tachycardias in the ventricular fibrillation zone until a cycle length of 200ms with ATP BC and/or ATP DC were included.

RESULTS

We reviewed 542 episodes in 240 patients. Two ATP bursts (BC/DC) were programmed in 291 episodes (53.7%, 87 patients), while 251 episodes (46.3%, 153 patients) had 1 ATP burst only DC. The number of episodes terminated by 1 ATP DC was 139, representing 55.4% effectiveness (generalized estimating equation-adjusted 60.4%). There were 256 episodes terminated by 1 or 2 ATP (BC/DC), representing 88% effectiveness (generalized estimating equation-adjusted 79.3%); the OR for ATP effectiveness BC/DC vs DC was 2.5, 95%CI, 1.5-4.1; P <.001. Shocked episodes were 112 (45%) for ATP DC vs 35 (12%) for ATP BC/DC, representing an absolute reduction of 73%. The mean shocked episode duration was 16seconds for ATP DC vs 19seconds for ATP BC/DC (P=.07).

CONCLUSIONS

The ATP DC in the ventricular fibrillation zone for fast ventricular tachycardia is moderately effective. Adding an ATP burst BC increases the overall effectiveness, reduces the need for shocks, and does not prolong episode duration.

Potential proarrhythmic effects of implantable cardioverter-defibrillators

Implantable cardioverter-defibrillator (ICD) interventions have the potential to be proarrhythmogenic. New arrhythmias can occur in the setting of clinically appropriate therapies, as well as during a cardiac rhythm for which therapy is not intended. Cardioversion/defibrillation therapies, antitachycardia pacing, and antibradycardia pacing are potential triggers for the development of new arrhythmias. Newer ICDs allow better recognition and interpretation of the arrhythmias that are induced by delivered therapies. Two cases of ICD-induced proarrhythmias are described. Based on the course of these patients and review of previous reports, proarrhythmic effects of ICD interventions along with prevention and management strategies are discussed.

Choosing pacemakers appropriately

The range of implantable cardiac pacing devices has expanded, with the advances in available technology. Indications for cardiac pacing devices, that is pacemakers, implantable cardioverter defibrillators (ICDs) and cardiac resynchronisation therapy devices (CRTs), have expanded for the treatment, diagnosis and monitoring of bradycardia, tachycardia and heart failure. While the need for pacemakers is increasing, not all patients who require pacemakers are receiving them, especially in the Asia-Pacific region. There is a need to be more critical in advising the use of more expensive devices like ICDs and CRT/CRT-D devices, since most patients in the Asia-Pacific region pay out of pocket for these therapies. The AHA-ACC guidelines need not be blindly followed, since they are too wide-sweeping and are often based on the intention-to-treat basis of trials rather than on the parameters of the patients actually enrolled.

Implantable cardioverter defibrillator shock reduction using new antitachycardia pacing therapies

The main purpose of ICDs is to abort sudden death by delivering therapy at the moment of tachycardia. Shocks accomplish this goal but are painful. Alternatively antitachycardia pacing is painless and if deemed safe may be reasonable substitute. Multiple trials show a high efficacy rate by ATP (78-94%) for treating VTs below 200 bpm. ATP has had less efficacy for faster VTs (41-79%) and have higher probability of accelerating tachycardia (5-55%). The PainFREE trials address these issues. The first pilot study PainFREE Rx applied standardized VT detection and ATP regimen to 220 patients with 1100 spontaneous episodes of VT. ATP success for slow VT success was 92% and fast VT > 188 bpm raw success rate was 89%. None of these trials randomize shock versus ATP so comparative safety data was missing. Thus, the PainFREE Rx II trial was designed to make direct safety comparison between shock and ATP therapies for fast VT > 188 bpm. It included 634 patients with either ischemic or nonischemic cardiomyopathy followed for 1 year yielding 1760 episodes of slow VT, fast VT plus VF. The results of the PainFREE Rx II trial showed that a single regimen of ATP, burst pace 8 pulses at 88% VT cycle length could safely terminate 77% of fast VT and 90% of slow VT. Consequently, shocks were reduced by 70% compared to the shock group. Furthermore, ATP was proven safe because there was no increase in sudden death, syncope or even arrhythmia acceleration compared to shock. The quality of life of the ATP group was found to be superior to the shock group validating ATP's intent. Secondary yet important findings also included the fact that by programming the ICD to wait for 18 beats in PainFREE Rx II before treating an episode reduced markedly the number of episodes treated when compared to 12 beat detection as done in PainFREE Rx I. Since syncope occurred in only 1% of episodes, the authors suggested that a longer wait for ICD detection needs to be evaluated.

The Magnitude of Sinus Cycle Length Change During Ventricular Tachycardia is Predictive of Successful Antitachycardia Pacing

BACKGROUND

Despite the wide use of antitachycardia pacing (ATP) in patients with implantable cardioverter defibrillators (ICDs), predictors of ATP success remain poorly understood. We hypothesize that the degree of sympathoexcitation, as measured by the sinus cycle length (SCL) shortening during ventricular tachycardia (VT), is a predictor of ATP success.

METHODS AND RESULTS

The charts of 462 patients with dual-chamber ICDs were reviewed. A total of 88 events in 26 patients met the inclusion criteria and were analyzed. The mean SCL during the 4 seconds preceding the VT onset (SCL-baseline), and during the 4 seconds prior to ATP delivery (SCL-VT) was measured. The percent shortening in SCL was calculated as ((SCL-baseline) - (SCL-VT))/(SCL-baseline) x 100. Patients were classified into the ATP-success and ATP-failure groups depending on the VT(s) response to ATP. Using a t-test analogue for clustered data, patients in the ATP-success group exhibited a greater shortening in SCL when compared with the ATP-Failure group (5.8% compared to 4.7%, P = 0.007). The successful ATP events displayed an average SCL shortening of 6.0% compared to 1.8% in the unsuccessful ATP events (P = 0.029). When the events were analyzed, the sensitivity and specificity of a shortening in SCL of >10% in predicting ATP success were 0.29 and 1.

CONCLUSION

We have shown that the SCL change during VT, a marker of the autonomic changes that accompany a tachycardia, is useful in predicting ATP success. Our findings suggest that analysis of the SCL during VT might play a role in future programming of ATP in patients with ICDs.

Implantable Cardioverter Defibrillator Therapy

Sudden cardiac death (SCD) is a major healthcare problem worldwide. The majority of SCD events occur in patients with clinically recognized heart disease and most episodes result from ventricular tachyarrhythmias. Implantable cardioverter defibrillator (ICD) therapy prevents SCD in specific patient populations. Significant progress in the design and technology has been made since the Food and Drug Administration first approved the ICD in 1985. First-generation ICDs were large, were implanted in the abdomen, required a thoracotomy for placing epicardial defibrillation patches, and were nonprogrammable. Contemporary ICDs have been substantially downsized, are implanted via a transvenous approach, and are multiprogrammable. Device implantation has been simplified to be similar to that of a permanent pacemaker. In addition to treating life-threatening ventricular arrhythmias, ICDs now treat bradyarrhythmias, atrial arrhythmias, and congestive heart failure. The purpose of this article is to describe the evidence supporting the use of ICD therapy and to explain the current devices used in clinical practice.

Rationale and patient selection for "hybrid" drug and device therapy in atrial and ventricular arrhythmias

Three quite different forms of direct antiarrhythmic therapy are available for the treatment of cardiac arrhythmias: antiarrhythmic drugs, cardiac ablation and implantable devices (pacemakers and defibrillators). None of these therapies is fully effective and consequently they are increasingly combined. This combination therapy is often described as "hybrid" a term that implies fundamental different qualities of treatment which together provide some form of synergism. The mechanisms for the initiation and perpetuation of most cardiac arrhythmias are complex and multiple. It is therefore not surprising that single therapies are not completely effective. Theoretically the use of multiple different therapies allows more specific mechanisms of arrhythmia to be directly addressed. However, this is largely a theoretical concept that has only been strictly evaluated in a small number of studies. Studies of multiple therapies are difficult to perform unless the combination therapy is regarded as a strategy which can be compared to baseline, conventional treatment or one or more single constituent therapies from the combination. Despite the lack of formal studies there is a very substantial clinical experience which testifies to the value of hybrid therapy for the management of both atrial fibrillation and ventricular tachycardia/fibrillation.

Effect of elevated heart rate preceding the onset of ventricular tachycardia on antitachycardia pacing effectiveness in patients with implantable cardioverter defibrillators

The incorporation of antitachycardia pacing (ATP) into implantable cardioverter defibrillators (ICDs) has provided a better tolerated alternative to shocks. ATP has been shown to be effective in terminating approximately 80% to 90% of spontaneous ventricular tachycardia (VT) episodes. Although ATP is routinely used, little is known about predictors of ATP failure. Based on the evaluation of stored electrograms, we aimed to prospectively follow patients with ICDs, and to analyze parameters affecting ATP effectiveness. One hundred eighteen consecutive patients received ICDs for standard indications. Before discharge, empirical, standardized ATP therapy was programmed in all patients within VT zones. A total of 1,218 spontaneous tachycardia episodes occurred in 51 patients during a mean follow-up of 24.5 +/- 12 months. Among these, 888 VTs were diagnosed. One hundred four fast VTs were detected in the ventricular fibrillation zone and treated with primary shock delivery. ATP was attempted 881 times in the remaining 784 VT episodes. ATP terminated 640 VTs successfully, ATP failed in 55 VTs finally reverted by shocks, and 89 VTs converted to a slower VT outside the VT zone. Fifty-one of these slower VTs reverted spontaneously, and 38 were redetected and treated. Finally, in primary intention-to-treat basis, ATP was successful in 691 VTs (88%) and unsuccessful in 93 VTs (12%). There was no influence of VT cycle length on ATP success rate. Furthermore, ATP efficacy was similar between patients with left ventricular ejection fraction < or =35% or >35%, between daytime and nighttime, as well as between patients with ischemic or nonischemic cardiomyopathy. A faster heart rate immediately preceding the onset of VT (103 +/- 19 vs 78 +/- 14 beats/min, respectively, hazard ratio 4.08, 95% confidence interval 2.11 to 7.89, p <0.001), and absence of beta-blocker therapy (82% vs 93%, respectively, hazard ratio 2.71, 95% confidence interval 1.72 to 4.29, p = 0.02) were found, by Cox proportional-hazard analysis, to be the sole independent predictors of ATP ineffectiveness in ICD recipients. Thus, the present study identified both preceding sinus tachycardia (reflecting an increased sympathetic tone) and lack of beta-blocker use as independent risk factors for reduced success of ATP therapy in terminating VT. Therefore, modification of sympathetic tone may be beneficial for patients with ICDs.

ICD therapy in the new millennium

Remarkable progress has been made in the 15 years since ICD therapy was approved for human use. The early "shock boxes" had almost no diagnostic capabilities and required thoracotomy for epicardial patch implantation with typical duration of hospitalization of about a week. Pulse-generator longevity was less than 2 years. Modern devices provide detailed information about the morphology and rate of electrocardiographic signals before, during, and after arrhythmia therapy. The down-sizing of pulse generators and improvements in lead design and shock waveforms allow the simplicity of defibrillator implantation to approach that of pacemakers, with defibrillation thresholds comparable with those initially observed with epicardial patches. Despite the marked reduction in size and increase in diagnostic capabilities, device longevity is now longer than 6 years. Routine outpatient ICD implantation is presently feasible and will increase in frequency if ongoing primary prevention trials prove beneficial. Further advances in lead technology and arrhythmia discrimination should increase the efficacy and reliability of therapy. Finally, devices have the capabilities to treat multiple problems in addition to life-threatening ventricular arrhythmias including atrial arrhythmias and congestive heart failure.

The implantable cardioverter-defibrillator

Ventricular arrhythmias remain a major cause of cardiovascular mortality. Therapy for serious ventricular arrhythmias has evolved over the past decade, from treatment primarily with antiarrhythmic drugs to implanted devices. The implantable cardioverter-defibrillator (ICD) is the best therapy for patients who have experienced an episode of ventricular fibrillation not accompanied by an acute myocardial infarction or other transient or reversible cause. It is also superior therapy in patients with sustained ventricular tachycardia (VT) causing syncope or hemodynamic compromise. Controlled clinical trials have confirmed the utility of these devices. As primary prevention, the ICD is superior to conventional antiarrhythmic drug therapy in patients who have survived a myocardial infarction and who have spontaneous, nonsustained ventricular tachycardia, a low ejection fraction, inducible VT at electrophysiologic study, and whose VT is not suppressed by procainamide. The effect of the ICD on survival of other patient populations remains to be proven. The device is costly, but its price is generally accepted to be reasonable. The ICD has been a major advance in the treatment of ventricular arrhythmias.

Ventricular tachycardia during follow-up in patients resuscitated from ventricular fibrillation: experience from stored electrograms of implantable cardioverter-defibrillators

OBJECTIVE

The purpose of this study was to use the electrogram storage capabilities of the implantable cardioverter-defibrillator (ICD) to categorize any arrhythmic event during follow-up in a group of patients who had survived an episode of ventricular fibrillation (VF) and to possibly identify clinical predictors of future arrhythmic events.

BACKGROUND

Little is known about the electrophysiologic characteristics of ventricular arrhythmias recurring during follow-up in survivors of VF as the sole documented arrhythmia at the time of resuscitation.

METHODS

Forty patients (58+/-10 years; 73% men; left ventricular ejection fraction 42+/-18%; 70% with coronary artery disease) who had survived an episode of VF and subsequently received an ICD capable of intracardiac electrogram recording and storage were followed for 23+/-11 months. In all patients, the arrhythmogenic substrate was investigated by means of programmed electrical stimulation (PES).

RESULTS

Among the 40 patients, 41 episodes of ventricular arrhythmias were documented in 13 patients (33%): 36 episodes of ventricular tachycardias (VT) were recorded in 11 patients (28%) and 5 episodes of VF were recorded in the remaining 2 patients (5%). Age, gender, cardiac disease and left ventricular ejection fraction failed to distinguish between patients with clinical recurrences and patients without. The sensitivity, specificity and positive accuracy of PES were 29%, 63% and 46%, respectively, for prediction of ventricular arrhythmia recurrence; 45%, 70% and 36%, respectively, for prediction of VT; and 50%, 98% and 50%, respectively, for prediction of VF during follow-up.

CONCLUSIONS

In survivors of VF receiving ICD therapy, VT is the most common ventricular arrhythmia recorded on device-incorporated electrograms during follow-up. This finding, associated with the relatively well-preserved ventricular function, may account for the ability of these patients to survive at time of the index arrhythmia; the use of antitachycardia pacing as a modality to treat arrhythmia recurrences may contribute to reduce the incidence of shock during follow-up in these patients.

[The current status of implantable automatic defibrillators]

The implantable cardioverter defibrillator has become an important therapy for patients with sustained or life threatening ventricular arrhythmias. Although the concept for the implantable cardioverter defibrillator originated in the late 1960s, the first device was implanted in humans in 1980. Since then, the technology has improved rapidly the design, function and reliability of the devices have been greatly modified. There are currently five companies dealing with defibrillators in Spain incorporating multiple options in defibrillation, pacing and sensing capabilities. New devices with atrioventricular pacing and atrial defibrillation possibilities will soon become available. The purpose of this article is to review the principal functions of implantable cardioverter defibrillators currently available.

Antitachycardia pacing in patients with implantable cardioverter defibrillators: how many attempts are useful?

The purpose of this study was to determine the termination and acceleration rates for 1 to 6 attempts of antitachycardia pacing [ATP] delivered by ICD in order to terminate spontaneously occurring VTs. Twenty-four ICD recipients with active ATP programs, including a maximum of six ATP sequences and spontaneously occurring VTs during follow-up, were investigated. During a mean follow-up of 42 +/- 15 months (range, 17-63 months) 413 spontaneous VT episodes (17 +/- 14; range, 1-49 per patient) resulting in appropriate ATP delivery by the ICD occurred. ATP successfully terminated 328 episodes (80%) with a mean number of 1.6 +/- 1.1 pacing sequences. Eighty episodes (19%) were accelerated by ATP and 5 (1%) were unresponsive to ATP. The ATP success decreased until the third ATP sequence (59%-->31%-->24%), but increased again in the fourth to sixth attempt (46%-->46%-->29%). The acceleration rate increased from sequence one to sequence three (8%-->13%-->28%), but decreased significantly in further ATP attempts (19%-->0%-->0%). The mean time delays until redetection or termination after 4, 5, and 6 attempts of ATP were 22 +/- 5 seconds, 37 +/- 2 seconds, and 41 +/- 9 seconds, respectively. Nine patients (37%) used > or = 3 ATP attempts during follow-up and all of them had a therapeutic benefit from it. Five out of 13 VTs (38%) treated with > or = 4 attempts could ultimately be terminated by ATP. The results of this study demonstrate that the first ATP sequence is the most effective and that > 4 ATP attempts may be useful in a minority of patients. There seems to be a low risk of VT acceleration by the fourth to sixth ATP sequence. Because of the associated time delay, a high number of ATP attempts should only be programmed in patients with hemodynamically well-tolerated stable VTs.

Antitachycardia pacing: which patients and which methods?

This article is a review on the value of antitachycardia pacing in patients with implantable cardiac-defibrillators (ICD). Antitachycardia pacing is highly effective in terminating monomorphic ventricular tachycardias, with a success rate of 80-90%. Which algorithm is used for termination seems to be of less importance, with respect to both efficacy and safety. Spontaneous episodes of ventricular tachycardia are slower and more easily convertible than those induced by programmed stimulation. It is thus possible that fine-tuning of the antitachycardia pacing algorithm, using induced episodes, is of limited value with respect to efficacy during follow-up. Prospective studies need to be performed to resolve this issue. Spontaneous monomorphic ventricular tachycardia can also occur in patients who are noninducible. Antitachycardia pacing should therefore also be considered for such patients. Inappropriate therapy, most often due to supraventricular arrhythmias, has been reported in up to 25% of patients. The sensitivity and specificity of algorithms developed to differentiate supraventricular from ventricular tachycardias still require validation.

Transvenous defibrillation lead systems

The use of the implantable cardioverter defibrillator has grown dramatically over the past 10 years. One of the major advances in defibrillation technology is the development of transvenous lead systems. Compared with traditional epicardial lead systems, transvenous defibrillation leads reduce perioperative mortality, hospitalization, and costs. Transvenous lead systems provide reliable sensing of ventricular tachyarrhythmias, although redetection of ventricular fibrillation can be prolonged, especially with integrated lead systems. Both ramp and burst adaptive pacing are equally effective for the termination of ventricular tachycardia and are successful in up to 90% of spontaneous events. Defibrillation thresholds are higher with transvenous leads than with epicardial patches. These thresholds are reduced with the use of multiple transvenous leads, subcutaneous patches, or with reversing shock polarity. However, the development of biphasic waveforms has made the largest impact on the efficacy of these lead systems, allowing dual coil transvenous systems to be effective in about 90% of patients. Defibrillation efficacy is further enhanced and implantation simplified by the incorporation of an active pulse generator located in the left pectoral region. Active pectoral pulse generators with biphasic waveforms will be the primary lead system for new implants.

The proarrhythmic potential of implantable cardioverter-defibrillators

The implantable cardioverter-defibrillator (ICD) is remarkably effective in preventing sudden cardiac death in high-risk patients, but it also has the capacity to provoke or worsen cardiac arrhythmias. Tachyarrhythmias or bradyarrhythmias may result from the delivery of antitachycardia or antibradycardia therapies by tiered-therapy defibrillators. This proarrhythmia, although rarely fatal, increases the morbidity associated with defibrillator therapy. Proarrhythmia is related as much to suboptimal programming as to technical limitations of the device. The proarrhythmic potential of ICD therapy can be minimized by tailoring the "electrical prescription" according to characteristics of the clinical arrhythmia and individual ICD idiosyncrasies.

Ventricular arrhythmias detected after transvenous defibrillator implantation in patients with a clinical history of only ventricular fibrillation. Implications for use of implantable defibrillator

BACKGROUND

Patients with a history of ventricular fibrillation (VF) have been shown to have a clinical profile, response to electrophysiological testing (EPS), and response to antiarrhythmic therapy that distinguishes them from patients with a history of sustained monomorphic ventricular tachycardia (MVT). Despite these differences, it is not clear whether VF in these patients is triggered by MVT or occurs de novo. The incidence of MVT and VF in such patients after their index VF event has important implications for therapeutic decisions regarding implantable defibrillator selection and programming.

METHODS AND RESULTS

The records of 111 consecutive patients who had undergone transvenous cardioverter/defibrillator (ICD) implantation for malignant ventricular arrhythmias were reviewed retrospectively. For each patient, all device tachyarrhythmia detections were examined and classified as VF, MVT, rapid polymorphic VT, or other. The number of events, time to first arrhythmia detection, and cycle length of MVTs were recorded. There were 55 patients with a history of only VF and 56 with a history that included an episode of MVT. Over 14 months of follow-up, with all patients initially off of antiarrhythmic medications, MVT was detected by only 18% of patients with a history of only VF compared with 54% of those with a history that included MVT (P = .002). Among patients who did detect MVT, those with a history of only VF had fewer episodes (7 +/- 7 versus 20 +/- 31, P = .001) and a shorter mean MVT cycle length (279 versus 314 ms, P = .03) than those with a clinical history of MVT. Abrupt onset of VF not preceded by MVT was detected in 11% of patients with VF only. In addition to a history of MVT, male sex, age < 60 years, and MVT inducible on EPS were all significantly associated with an increased likelihood of MVT detection. On multivariate analysis, the inducibility of MVT was the primary independent predictor of MVT detection but was of minimal incremental predictive value in the subgroup of patients with a history of only VF. When EPS results were not considered, arrhythmia history was the primary independent predictor of MVT detection.

CONCLUSIONS

Patients with a history of only VF infrequently have MVT detected by their defibrillators. When these patients do detect MVT, it is faster than that detected in patients with a clinical history of MVT before ICD surgery. A significant percentage of VF survivors detected the abrupt onset of VF not preceded by MVT, suggesting that the deterioration of rapid MVT to VF is not the only clinically important mechanism of VF induction. These findings may have important implications for the understanding of the mechanism of VF induction and for use of an implantable defibrillator.

Mechanisms in the interruption of reentrant tachycardia by pacing

The mechanisms by which pacing interrupts reentrant tachycardia associated with a structural obstacle were investigated using a computer model of propagated excitation. The model simulated cycle length-dependent refractoriness and slow propagation during incomplete recovery of excitability. Previously established features of the mechanism consisting of collision of reentrant with paced antidromic propagation and block of orthodromic propagation were demonstrated in the model, and factors affecting the mechanism were defined. Arrival time of paced orthodromic excitation at a potential block site and the duration of refractoriness at that site were major factors. Arrival time was determined by pacing stimulus time and propagation velocity. Slow propagation of a particular response acted to prevent the required block during that response, but enhanced the likelihood of a block of a subsequent response by affects on the onset time and duration of refractoriness at the block site at fast rates. In some conditions, responses to later stimuli resulted in block and interruption of tachycardia, while earlier stimuli with slower propagation during the same cycle failed to. Tachycardia rate affected its interruption by pacing by means of the shorter refractory period of the potential block site at fast rates, so that a paced response with a particular arrival time might fail to block. A greater number of successive paced responses were then required to terminate rapid tachycardia.

The pacer-cardioverter-defibrillator: function and clinical experience

Pacer-Cardioverter-Defibrillator. This article reviews the function of the pacer-cardioverter-defibrillator (PCD). Detection of ventricular arrhythmias occurs in two programmable zones, with onset and stability modifiers available to diminish overdetection of sinus tachycardia and atrial fibrillation, respectively. The sensing circuitry utilizes an auto-adjusting sensitivity with exponential decay to allow detection of low-amplitude ventricular fibrillation electrograms without T wave oversensing. Treatment can be accomplished by tiered therapy with two types of antitachycardia pacing, cardioversion and defibrillation. Cardioversion and defibrillation shocks are programmable between single pathway when two leads are used and simultaneous or sequential shock delivery when a three-lead system is used. A telemetered marker channel and electrogram aid in assessing device function during implantation and follow-up. Previously published literature is cited to expand on various aspects of PCD function and programming.

The role of cardioversion therapy in patients with implanted cardioverter defibrillators

Stable ventricular tachycardias can be treated with pacing or electrical countershock. Use of pacing includes several advantages, but it is not always effective; when pacing is not effective, shocks can be used for cardioversion of the arrhythmia. Use of such shocks includes advantages and disadvantages, but generally they are well tolerated and form an important part of the treatment of patients with sustained ventricular arrhythmias.

Advances in follow-up techniques for implantable defibrillators

The complexity of newer implantable defibrillators has made device follow-up increasingly more intricate. Extensive data-logging capacity provides specific information on recorded events, which facilitates more accurate determination of patient arrhythmias. This helps the clinician judge whether the device is detecting and treating arrhythmias appropriately, or whether false sensing of external signals or supraventricular rhythms is occurring. There is also a record of the efficacy of delivered therapy from the device that helps in optimizing subsequent programming. Programming itself has become much more complicated, with multiple independently programmable therapy zones, each with numerous available therapeutic modalities. In addition, defibrillator status information has been improved. Accurate battery voltage measurements give a reasonable estimate of remaining device life, and pace/sense and shock lead impedances can be measured to provide information on total system integrity. Together, these advances allow more specific programming of the device to the individual patient's condition but require increasing experience and expertise of the physician.

A prospective randomized comparison of autodecremental pacing to burst pacing in device therapy for chronic ventricular tachycardia secondary to coronary artery disease

A number of modes of antitachycardia pacing therapies are available in the newer generations of implantable cardioverter/defibrillators. The efficacy of synchronized burst overdrive pacing for the termination of induced and spontaneous monomorphic ventricular tachycardia (VT) was compared with synchronized autodecremental (ramp) pacing in 21 patients who received an implantable antitachycardia pacemaker/cardioverter/defibrillator for treatment of recurrent sustained monomorphic VT. Patients undergoing serial noninvasive VT induction studies after device implantation were prospectively randomized to receive trials of burst or ramp pacing therapies in a crossover study design. Antitachycardia pacing therapies were equally efficacious in treating induced VT (68% for ramp, 76% for burst pacing trials). The efficacy of ramp (93%) and burst (96%) pacing therapies was significantly higher in terminating spontaneously occurring episodes of VT than in terminating induced episodes (p = 0.001). The incidence of tachycardia acceleration was similar for both modes of pacing. The incidence of VT acceleration was lower for spontaneously occurring episodes of VT (0.01%) than for induced episodes of VT (6%, p < 0.01). Thus, antitachycardia pacing is an effective therapy for episodes of monomorphic VT, and the risk of accelerating VT to a hemodynamically unstable form is low. Antitachycardia pacing therapies are more effective against spontaneously occurring episodes than induced episodes of VT. Differences in tachycardia cycle length and duration may contribute to these effects.