Incidence of Ventricular Tachyarrhythmias

The application of ICD therapy for reducing sudden cardiac death has also allowed the characterization of tachyarrhythmias in a large number of subjects. Examining ICD recordings from treated arrhythmias illustrates the therapeutic role played by the device. Because the characteristics of patients receiving ICDs have undergone dramatic shifts over time, the incidence of various tachyarrhythmias cannot be captured by any one study. The initial patients who received ICDs were survivors of sudden cardiac death who were unresponsive to drug treatment. ICDs were a therapy of last resort. The current patient population comprises

Table 1: Criteria for adjudicating implantable cardioverter-defibrillator detected tachycardia episodes

Ventricular EGM only

Atrial and ventricular EGM

Ventricular cycle length < 260 mS

Ventricular EGM

morphology Same as baseline

Sudden ventricular rate

Irregular ventricular intervals

Beat-beat ventricular EGM

morphology variability

Ventricular rate < atrial rate (no undersensing)

Chamber of tachycardia onset = atrium

Atrial rate > ventricular rate with atrioventricular association

Evidence for SVT Evidence for VT/VF

EGM, electrogram; SVT, supraventricular tachycardia; VT/VF, ventricular tachyarrhythmias and ventricular fibrillation; AF, atrial fibrillation

Evidence supporting a particular classification is categorized as very strong, strong, or weak by the symbols "+++," "++," and "+", respectively. Evidence refuting a particular classification is categorized as very strong, strong, or weak by the symbols "—," "--," and "-" respectively aRapidly conducted atrial fibrillation may have ventricular cycle length < 260 mS

bBeat-beat morphology variability can result from conduction aberrancy during AF

c ~ 5% of VTs will have EGMs similar to baseline dVT with 1:1 retrograde conduction has atrioventricular association a combination of indications for ICD therapy, and a large number have no history of cardiac arrest or ventricular tachyarrhythmia. Examining the tachyarrhythmia incidence in studies that led to major changes in ICD indications provides a sense of what ICDs encounter in the most common clinical applications.

The Antiarrhythmics Versus Implantable Defibrillators (AVID) trial established ICDs as superior to antiarrhythmic drugs for improving survival in patients with documented serious arrhythmias.20 This result changed ICDs from a therapy of last resort to a first-line therapy against sudden cardiac death. The patients in the AVID trial are currently described as "secondary prevention" patients. These are patients for whom the ICD is prescribed for ventricular fibrillation (VF) or for ventricular tachycardia (VT) and syncope or for VT in the setting of depressed cardiac function and symptoms. There were 216 patients with prior VF and 276 patients with VT-related reasons for therapy who were studied for an average of 31 months.21

These two groups (VF or VT history) in the study exhibited different distributions of VT and VF, but VT rhythms were encountered in a significant portion of the VF group. Eighty-nine subjects (20%) had at least one episode of VF treated, and 230 subjects (54%) had at least one treated VT episode. Thus, monomorphic VT (MVT) was the most commonly treated rhythm both in the number of patients treated for it and in the total number of episodes observed during the study. Arrhythmia history predicted differences in arrhythmia incidence. Patients were more likely to be treated for the tachyarrhythmia that caused them to receive their ICD. Subjects from the non-VF group were more than twice as likely to have subsequent VT (74% vs. 30%), while the group with prior VF was more likely to have VF (28% vs. 18%). Surprisingly the proportion of patients in both groups that were treated for both VT and VF was not significantly different 18%).

After AVID, ICD usage was further expanded to a new class of patients determined for the purpose of "primary prevention". These patients are treated with ICD therapy because of clinical characteristics that place them at high risk of sustained tachyarrhythmias and sudden death. The role of the ICD is to stop the first occurrence of a serious tachyarrhythmia. This is in contrast to secondary prevention patients for whom a significant tachyarrhythmia has already been observed. The primary prevention patient profile evolved through a series of studies that applied ICD therapy for increasingly expanded clinical conditions. A key condition of all primary prevention indications is reduced ventricular function. This is determined by measuring the left-ventricular ejection fraction (LVEF). LVEF is the volume of blood ejected from the left ventricle by a contraction divided by the volume just prior to contraction expressed as a percentage. The LVEF that defined reduced function in primary prevention studies ranged from 30 to 40%. Reduced function in conjunction with other clinical aspects has been used to identify patients who could benefit from prophylactic ICD therapy.

Two key primary prevention trials are the Multicenter Automatic Defibrillator Implantation Trial (MADIT) and the Multicenter Unsustained Tachycardia Trial (MUSTT). There are subtle differences in the details between the patient populations, but both studies found ICD therapy significantly improved survival in their populations. These patients all had coronary artery disease, reduced ventricular function, asymptomatic, unsustained ventricular tachycardia, and had VT or VF induced during an electrophysiology study. After the success of ICD therapy in MADIT and MUSTT, an additional trial, the Multicenter Automatic Defibrillator Implantation Trial II (MADIT-II) established a further expansion of the primary prevention indication. The patients in MADIT-II received ICDs on the basis of having a prior myocardial infarction and an LVEF < 30%, removing the requirement to demonstrate that VT or VF can be induced during an electrophysiology study. MADIT-II ICD diagnostic data demonstrated that as patient populations had expanded, VT had remained the most commonly treated tachyarrhythmia.22 In the MADIT-II ICD cohort, 169 patients of 719 received an appropriate ICD therapy. The first appropriate therapy in the population was dominated by VT, with 82% of patients having the first appropriate therapy for VT. The remaining 18% had a first therapy for VF.

The PainFREE Rx II study, roughly 4 years after AVID, enrolled both primary and secondary prevention patients and provided an example of arrhythmia incidence in a combined population.23 There were 334 secondary prevention patients and 248 primary prevention patients enrolled in the study. There was no significant difference in the distribution of VT, Fast VT (FVT, VT with cycle length < 320 ms), and VF between the primary and secondary prevention groups based on episode classification from 191 patients with true ventricular arrhythmias. In the primary prevention population, 14% of the detected ventricular arrhythmias were VF, while 10% of the secondary prevention arrhythmias were VF. The FVT rhythms accounted for 35% of the ventricular arrhythmias in both populations while the remainder was VT.

The most recent expansion in primary prevention patients resulted from the Sudden Cardiac Death in Heart Failure Trial (SCD-HeFT). There were 829 subjects in the ICD therapy arm and the only criteria for ICD therapy were New York Heart Association (NYHA) class II or III; chronic, stable congestive heart failure; and an LVEF < 35%.24 There were 177 patients who received shock therapy for VT and/or VF: 109 patients had therapy for VT, and 68 patients received VF therapy.25 In this expansive new primary prevention patient population VT is still a dominant rhythm, but the gap between VT and VF rates in terms of number of patients is less than in prior studies. It remains to be seen in reports of the episode incidence if the gap also narrows between the rates of VT and VF episodes.

Consistently across the general ICD patient populations, the device is more likely to treat VT than VF. This has held up across all of the expansions of the main indications for ICD therapy. The most recent expansion of ICD indication, the SCD-HeFT population, showed a greater percentage of patients with VF than prior studies. Despite this shift, VT still represents a majority of episodes. There are other less common indications for ICD therapy, such as long QT syndrome, that do have different arrhythmia distributions. But in a general ICD population, VT remains the most common tachyarrhythmia.

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