1 INTRODUCTION
An implantable cardioverter defibrillator (ICD) is currently the most reliable therapy for preventing sudden death in patients with sustained ventricular tachycardia (VT) associated with organic heart disease.1-4 ICDs can terminate life-threatening VTs by using electrical shocks, or anti-tachycardia pacing (ATP), or both. Recent studies have shown that electrical shocks for ventricular tachyarrhythmia may lead to a worse prognosis, such as mortality or morbidity.5,6Therefore, it is essential to reduce the use of shock therapy to terminate VT. Although the effectiveness of ATP therapy has been extensively reported, 7-9 it occasionally induces VT acceleration,10,11 or progression to ventricular fibrillations (VFs). Many studies on the effectiveness of ICD therapy have reported that the cycle length defines VTs best targeted by ICD therapy. However, in addition to cycle length, the targeted VTs are characterized by RR interval variability (Figure 1A). Variation in the RR interval in tachyarrhythmia distinguishes VTs from supraventricular tachycardia, such as atrial fibrillation.12 Concerning the RR interval variability in VT, there are few reports on the clinical characteristics of VT or the effectiveness of ATP therapy.
This study aimed to evaluate the relationship between RR interval variability in VTs and clinical characteristics of VT, including the effects of ICD therapy.