Programming the atrial arrhythmia fallback algorithm

ECG 12A: Tracing begins identical to the previous tracing followed by regularization of ventricular rhythm with reduction in pacing rate;

ECG 12B: The EGM shows the slowing down and regularization of the pacing rate following programming of the mode switching algorithm;

ECG 12C: The slowing of the pacing rate allows the reappearance of spontaneous rhythm; the prolongation of ventricular diastoles allows better visualization of the sensing of a fast and disorganized atrial arrhythmia; ventricular pacing no longer occurs except for the few long diastoles (AF with slow conduction);

ECG 12D: The EGM confirms the diagnosis of conducted AF with a small number of paced cycles when the rate reaches the minimum programmed rate;

Comments: These tracings show the normal functioning of a dual-chamber pacemaker during an episode of atrial fibrillation; programming of the switching mode allows a return to a more physiological heart rate than on the previous tracings. To avoid a sudden drop in ventricular rate, the return to the base rate or to the response rate occurs progressively.

The ideal switching mode algorithm displays the following features:

1. rapid triggering to avoid prolonged rapid ventricular pacing during the initial arrhythmia sensing phase,
2. ability to rapidly revert back to a synchronous mode at the end of the arrhythmic episode,
3. good ability to diagnose atrial arrhythmia even in the presence of atrial signals of varying amplitude and rate,
4. ability to avoid switching modes in response to cross-talk, noise or sinus tachycardia.

Take-home message: A fallback algorithm (whose characteristics vary according to the manufacturers) requires a number of rapid atrial cycles to arrive at the diagnosis of atrial arrhythmia and to switch to an asynchronous mode. It is therefore common to observe a few cycles of rapid ventricular pacing at the beginning of an atrial fibrillation episode.