Relation of Atrial Refractoriness to Upper and Lower Limits of Vulnerability for Atrial Fibrillation/Flutter Following Implantable Ventricular Defibrillator Shocks

Abstract

Background —Implantable ventricular cardioverter defibrillator (ICD) shocks can cause atrial fibrillation/flutter (AF). This study investigated the pathogenesis of AF after ICD shocks in a canine model. Methods and Results —The study was conducted in 8 dogs. In 5 dogs (group 1), truncated exponential (8 ms, 78% tilt) monophasic and biphasic shocks were delivered through a bipolar epicardial (patch) or endocardial lead. After the last S1 of atrial pacing at a cycle length of 350 ms, shocks of 0.1 to 7.6 A (0.005 to 27.7 J) were delivered, timed to the atrial effective refractory period (AERP). Ventricular defibrillation thresholds were also determined. In 3 dogs (group 2), the effect of the open versus closed chest technique on AF induction was tested in the endocardial biphasic shock configuration. AF was induced in all 8 dogs and in all waveforms and configurations. Mean AF duration was 11.5±6 s, with a mean ventricular rate of 184±37 bpm. Ventricular shocks could induce AF only if they were timed between an AERP of −60 to 40 ms, −40 to 60 ms, −40 to 60 ms, and −20 to 60 ms in the epicardial monophasic, epicardial biphasic, endocardial monophasic, and endocardial biphasic configurations, respectively. The mean±SD of the upper limit of vulnerability (ULV) for AF induction (in J) was 5.2±0.6, 3.5±0.4, 5.2±1.2, and 2.5±0.1 for the epicardial monophasic, epicardial biphasic, endocardial monophasic, and endocardial biphasic configurations, respectively ( P <0.05). The lower limit of vulnerability (LLV) was 0.8±0.1, 0.8±0.1, 0.9±0, and 0.6±0 for the epicardial monophasic, epicardial biphasic, endocardial monophasic, and endocardial biphasic configurations, respectively ( P =NS). The ventricular defibrillation threshold (in J) for all wave forms and configurations was higher than the ULV ( P <0.05). Conclusions —(1) An atrial LLV and ULV exist for ventricular ICD shock–induced AF; (2) the shock-induced AF is related to both shock intensity and its timing to AERP; and (3) avoiding this atrial window of vulnerability may minimize the risk of post-ICD shock AF.