Pacemaker Syndrome During Managed Ventricular Pacing Mode: What is the Mechanism?

A 51-year-old woman underwent implantation of a dual chamber pacemaker 10 years ago, for symptomatic intermittent second-degree type II AV block. She remained free of symptoms during follow-up. Recently, she underwent pacemaker replacement because of battery depletion (Adapta ADDR01, Medtronic Inc., Minneapolis, MN, USA). Given intact AV conduction at the time of intervention, the Managed Ventricular Pacing mode algorithm (MVP, Medtronic), which provides AAI(R) pacing with ventricular monitoring and backup DDD/R pacing as needed, was programmed with a lower rate limit of 60 ppm and an upper rate limit of 130 ppm. Ten days after the procedure, the patient complained of asthenia, dyspnea, and presyncope. ECG at pacemaker interrogation revealed sinus tachycardia with second-degree type II AV block (Fig. 1). Atrial pacing artifacts with atrial capture are seen dissociated from the native QRS complexes. Pacemaker interrogation revealed pacing and sensing thresholds as well as impedances within normal range. In our patient, the repetitive atrial pacing during ventricular systole led to pacemaker syndrome and symptoms resolved by reprogramming the device to AV sequential pacing without MVP. What is the mechanism of atrial stimulation dissociated from the native QRS complexes?