Absence of fallback during atrial arrhythmia

ECG 11A: Pacemaker-driven tachycardia; rapid and irregular ventricular pacing (not compatible with PMT which is regular); probable atrial fibrillation;

ECG 11B: EGM shows the sensing of a rapid and irregular atrial rhythm with permanent ventricular pacing; absence of fallback following the sensing of atrial fibrillation explaining the rapid ventricular pacing;

Comments: The management of patients with pacemaker devices presenting atrial arrhythmia episodes is complex given the various mechanisms known to trigger atrial arrhythmias and the risk of rapid ventricular pacing when tracking a rapid atrial rhythm.

The device can be programmed to respond to an atrial arrhythmia by switching to an asynchronous mode enabling to avoid high rate ventricular pacing which may compromise hemodynamic stability. Mode switching is the ability of the pacemaker to automatically switch from an atrial tracking mode (DDD or VDD) to a non-atrial tracking mode (DDI or VDI). The ventricular pacing rate gradually changes from the maximum synchronous rate to the response rate or base rate. This prevents a sudden drop in ventricular rate. When the atrial tachyarrhythmia ends, the switching mode returns to the programmed synchronous pacing mode.

Mode switching is programmed by default and should not be switched off even if the patient does not have a known history of atrial arrhythmia. In this patient, the absence of fallback is associated with prolonged rapid ventricular pacing at the maximal triggering rate. The combination of lack of atrial systole and rapid ventricular pacing is responsible for the observed signs of cardiac decompensation.

Fallback programming in this patient is necessary to avoid ventricular tracking of rapid atrial activity.

Take-home message: A pacemaker-driven irregular tachycardia at the ventricular level and oscillating near the maximum triggering rate in a patient implanted with a dual-chamber pacemaker suggests the occurrence of atrial fibrillation and a disabled or a dysfunction of the fallback algorithm (mode switching).