Fallback algorithm and pacing rate Copy Copy


  • 87-year-old woman
  • Accolade pacemaker (Boston Scientific) for complete atrioventricular block
  • palpitations
  • fallback episodes (ATR) in the device memory


  • acceleration of atrial activity with one out of three atrial intervals sensed outside the refractory periods, one interval sensed in the PVARP and one out of three intervals not sensed since occurring in the post-ventricular total atrial blanking; succession of a short interval and a long interval (due to the unsensed interval); no incrementation of the entry counter
  • all atrial signals are counted; incrementation of the entry counter (+1 for each fast atrial interval); increase in the ventricular pacing rate to the maximum tracking rate (VP-MT) for each atrial activity outside the refractory periods triggering an AV delay
  • the entry counter is filled
  • the duration begins (ATR-Dur)
  • persistence of ventricular pacing at the upper rate limit throughout the duration
  • after 8 paced ventricular cycles during which the atrial arrhythmia persists (duration programmed on 8 cycles), switching to an asynchronous mode (ATR-FB)
  • ventricular pacing at a gradually decreasing rate



  • this relatively elderly patient presented numerous relatively short episodes of atrial arrhythmias responsible for palpitations, which is paradoxical in a complete atrioventricular block patient
  • tracings revealed a delay before the occurrence of fallback with rapid ventricular pacing at the maximum tracking rate followed by a gradual slowing of the ventricular pacing rate
  • the association of loss of atrial systole and runaway ventricular pacing was probably responsible for the symptoms experienced by the patient
  • the acceleration of the pacing rate at the upper rate limit persists as long as the duration is not completed
  • to decrease this delay, it is possible to reduce the number of intervals of the entry counter and the number of ventricular cycles of the duration (programming possible up to 0)
  • the advantage is to reduce the time spent at the upper rate limit, while the risk is to favor repeated fallbacks (DDIR mode) on very short episodes
  • to avoid sudden rate variations (sudden drop in ventricular rate at the time of the fallback), the return to the lower rate limit or rate response is smoothed and gradual, which explains why the rate remains high after the fallback
  • the speed of rate decrease is also programmable to prevent rate variations without overly prolonging the fast ventricular pacing duration