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  • br Conclusions br Conflict of interest br Introduction


    Conflict of interest
    Introduction Endless loop tachycardia (ELT) is a well-recognized complication of dual chamber pacing systems [1,2]. Recent pacemaker systems adopt algorithms of ELT termination given that ELT is generally maintained at the programmed upper tracking rate. The rate of ELT depends on the upper tracking rate, atrioventricular delay (AVD), and ventriculoatrial (VA) conduction time. The sum of the retrograde VA conduction time and the programmed AVD is equal to the ELT nitric oxide synthase inhibitor length with a rate below the upper tracking rate [3]. In this situation, ELT does not satisfy the termination algorithm of some pacemakers, and the tachycardia may continue. We report a case of ELT below the upper tracking rate.
    Case report An 82-year-old female with hypertrophic cardiomyopathy (HCM; mid-ventricular obstruction) and sick sinus syndrome (SSS) was implanted with a DDD pacemaker 15 years prior to this admission. The pacemaker generator was replaced for consumption of its battery 7 years prior (Vertus plus II, Guidant, Indianapolis, Indiana, USA), and she was followed at our pacemaker follow-up clinic. She experienced subjective symptoms such as shortness of breath, palpitation, and general fatigue in March 2011, and visited our department. Chest X-ray revealed cardiomegaly (CTR=68%) and lung congestion. The electrocardiogram (ECG) performed upon admission revealed atrial fibrillation (AF), with a heart rate of 100bpm and a negative T-wave in the precordial leads. We diagnosed the patient with congestive heart failure because of rapid AF and HCM, and began treatment. Echocardiogram showed asymmetric septal hypertrophy, a general decline in wall motion, and an ejection fraction of 43%. Congestive heart failure was well-controlled by the administration of angiotensin-converting enzyme inhibitor and diuretics. Because the patient had low cardiac function, amiodarone was added (a loading dose of 400mg/day) to control AF. After administration of amiodarone, AF became atrial flutter (AFL). We therefore performed an electrophysiological study (EPS). EPS revealed cavotricuspid isthmus (CTI)—dependent, counterclockwise AFL. Linear ablation of the CTI was performed. Burst pacing from the coronary sinus ostium to confirm the block line of the isthmus induced rapid, regular, ventricular pacing at a rate of 110bpm (Fig. 1A). Pacemaker telemetry data demonstrated A sense—V pace sequence during the tachycardia (Fig. 2A-1). The earliest site of atrial activation was the His bundle (HBE) recording site (Fig. 1). This tachycardia continued for 6min until it was terminated by a premature atrial beat and was induced repeatedly. The differential diagnosis of this tachycardia included ectopic atrial tachycardia and pacemaker-mediated endless loop tachycardia (ELT). Tachycardia was terminated by reprogramming the pacemaker mode from DDD to VVI, after which rapid atrial activation also disappeared (Fig. 3). We diagnosed this arrhythmia as ELT, which was conducted to the ventricle via the pacemaker and retrogradely to the atrium via the AV node. ELT was also induced during a standard pacemaker lead threshold-check session after EPS and catheter ablation. Pacemaker parameters are listed in Table 1. Prior to this admission, the ECG of this patient always showed A pace—V sense sequence during routine pacemaker clinics. We have never detected an episode of AV block. The DDD pacemaker was programmed with long AVD (paced as AVD 300ms, sensed AVD 270ms), and dynamic post-ventricular atrial refractory period (PVARP) mode (maximum PVARP, 250ms; minimum PVARP, 200ms). Dynamic PVARP mode is a dynamic interval designed to provide a longer PVARP at slower rates to enhance protection against ELT, and a shorter PVARP to enhance atrial sensing at high rates. The upper tracking rate was 120bpm. Pacemaker telemetry showed 250ms of retrograde VA conduction time over programmed PVARP of approximately 208ms at 110bpm. We reprogrammed her pacemaker to a longer, fixed PVARP (280ms), and a shorter AV delay (220ms), to ensure an upper tracking rate of 120bpm. After reprogramming the pacemaker, ELT no longer occurred. Two months later, amiodarone was discontinued because of appetite loss. We changed her prescription from 200mg/day amiodarone to 20mg/day aprindine hydrochloride to control paroxysmal atrial fibrillation (PAF). Pacemaker telemetry data showed shortening of the VA interval during rapid V-pacing at 110bpm after the change in anti-arrhythmic drugs (Fig. 2A-2, B-1, B-2). ELT was not induced after rapid ventricular pacing, even with the same programming parameters, before catheter ablation.