Antitachycardia pacing: which patients and which methods?

The physics of heart rhythm disorders

The global burden caused by cardiovascular disease is substantial, with heart disease representing the most common cause of death around the world. There remains a need to develop better mechanistic models of cardiac function in order to combat this health concern. Heart rhythm disorders, or arrhythmias, are one particular type of disease which has been amenable to quantitative investigation. Here we review the application of quantitative methodologies to explore dynamical questions pertaining to arrhythmias. We begin by describing single-cell models of cardiac myocytes, from which two and three dimensional models can be constructed. Special focus is placed on results relating to pattern formation across these spatially-distributed systems, especially the formation of spiral waves of activation. Next, we discuss mechanisms which can lead to the initiation of arrhythmias, focusing on the dynamical state of spatially discordant alternans, and outline proposed mechanisms perpetuating arrhythmias such as fibrillation. We then review experimental and clinical results related to the spatio-temporal mapping of heart rhythm disorders. Finally, we describe treatment options for heart rhythm disorders and demonstrate how statistical physics tools can provide insights into the dynamics of heart rhythm disorders.

What factors lead to the acceleration of ventricular tachycardia during antitachycardia pacing?-Results from over 1000 episodes

Introduction

Ventricular tachycardia (VT) acceleration due to antitachycardia pacing (ATP) therapy could be often observed in patients with implantable cardioverter defibrillator (ICD), which usually results in additional shock. However, few studies focused on the risk factors for VT acceleration caused by ATP therapy. The purpose of this study was to investigate risk factors for VT acceleration due to ATP delivery.

Methods

We retrospectively reviewed 1056 ATP episodes in 33 patients with structural heart diseases, of whom clinical characteristics and episodes details were evaluated.

Results

At individual patient level, number of VT morphologies recorded in electrograms during follow‐up was a risk factor with cutoff point of 1 (AUC 0.79, sensitivity 72.7%, specificity 77.3%, P < .001) to predict ATP acceleration (OR 3.50, P = .008). From episode‐based analysis, VT cycle length (VTCL) and mean variation in VTCL were risk factors to predict ATP acceleration (OR 0.98, P < 0.001 vs OR 1.06, P < .001, respectively), with cutoff points of 347 ms (AUC 0.67, sensitivity 82.5%, specificity 47.6%, P < .001) and 7.3 ms (AUC 0.66, sensitivity 77.5%, specificity 56.7%, P < .001), respectively. In addition, VTs with cycle length less than 347 ms were more likely to be accelerated by burst stimulation with more pulse numbers (OR 3.31, P < .001).

Conclusions

Number of VT morphologies, VTCL, and mean variation in VTCL are risk factors predicting ATP acceleration. Burst stimulation with less pulse numbers should be performed in VTs with cycle length less than 347 ms.

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.

A physical approach to remove anatomical reentries: a bidomain study

Controlling cardiac chaos is often achieved by applying a large damaging electric shock-defibrillation. It removes all waves, without differentiating reentries and normal waves, anatomical and functional reentries. Anatomical reentries can be removed by anti-tachycardia pacing (ATP) as well. But ATP requires the knowledge of the position of the reentry, and an access to it with an invasive stimulating electrode. We show that the physics of electric field distribution between cardiac cells permits one to deliver an electric pulse exactly to the core of an anatomical reentry, without knowing its position and even to locations where access with a stimulating electrode is not possible. The energy needed is two orders of magnitude less than defibrillation energy. The results are insensitive to both a detailed ionic model and to the geometry of the fibers.

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.

Current status of the implantable cardioverter-defibrillator

Clinical trials have established the superiority of the implantable cardioverter-defibrillator (ICD) over antiarrhythmic drug therapy in survivors of sudden cardiac death and in high-risk patients with coronary artery disease. The ICD has evolved to overcome the limitation of earlier devices that required thoracotomy for implantation and were fraught with inappropriate shock delivery. Current ICDs are implanted in a similar manner to cardiac pacemakers and incorporate sophisticated rhythm-discrimination algorithms to prevent inappropriate therapy. Managing the patient with an ICD requires an understanding of the multiprogrammable features of modern devices. Drug interactions and potential sources of electromagnetic interference may adversely affect ICD function. Driving restrictions may be necessary under certain conditions. The cost-effectiveness of ICD therapy appears favorable, given the marked survival benefit seen in randomized trials relative to antiarrhythmic drug treatment. The growing number of ICD recipients necessitates an understanding of the specialized features of the modern ICD and the role of device therapy in clinical practice.

Relation of induced to spontaneous ventricular tachycardia from analysis of stored far-field implantable defibrillator electrograms

Predischarge testing of implantable cardioverter-defibrillators is often used to tailor antitachycardia pacing algorithms based on the response of induced ventricular tachycardia (VT) to pacing. Despite this practice, little is known about the relation between VT induced at predischarge study and VT that occurs spontaneously. To clarify this relation, we identified 19 patients with VT induced at predischarge study and compared the characteristics of the induced VT with the first episode of spontaneous VT. VT morphology, tachycardia cycle length, and response to antitachycardia pacing were measured from far-field electrograms stored by the implantable cardioverter-defibrillator. All subjects had coronary artery disease and previous myocardial infarction. The mean time from baseline study until a spontaneous VT episode was 162+/-121 days. Analysis of far-field electrograms revealed that spontaneous VT was morphologically different from predischarge-induced VT in 13 of 19 cases (68%). The cycle length of induced VT was significantly shorter than spontaneous VT when VT morphologies were different but not when spontaneous and induced VT had an identical morphology. Antitachycardia pacing was effective in terminating 18 of 19 (95%) induced VTs and 14 of 18 (78%) spontaneous VTs. Antitachycardia pacing was effective in terminating 9 of 12 episodes of morphologically different spontaneous VTs and 5 of 6 episodes of morphologically identical spontaneous VTs (p = NS). Thus, the characteristics of VT induced at predischarge study correlate poorly with those of subsequent spontaneous VT episodes due to the induction of faster "nonclinical" VTs at predischarge testing. This may limit the applicability of predischarge testing in tailoring antitachycardia pacing algorithms.

On the necessity of the invasive predischarge test after implantation of a cardioverter-defibrillator

The necessity of routine invasive implantable cardioverter-defibrillator (ICD) testing before hospital discharge was analyzed in 268 patients. In 98% of the patients, invasive ICD testing was not necessary if a preimplant electrophysiologic test was performed; most of the observed problems can be solved by noninvasive control.

[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.