Pacing modes specific for Boston Scientific

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The RythmIQ algorithm has been designed to promote intrinsic conduction in Boston Scientific devices.

  • the device provides AAI(R) at the lower rate limit and/or sensor indicated rate
  • the device provides backup VVI pacing at a rate 15 bpm slower than the programmed lower rate limit
  • the backup VVI pacing rate is limited to no slower than 30 bpm and no faster than 60 bpm
  • for a lower rate limit < 45 bpm, the VVI backup is 30 bpm; for a lower rate limit > 75 bpm, the VVI back-up is 60 bpm
  • if 3 slow ventricular beats are detected in a window of 11 beats, the device automatically switches to DDD(R) mode
  • nominally, the Rythmiq algorithm is OFF

The AV Search+ algorithm is used to periodically check for a return of intrinsic conduction and to switch back to AAI(R) pacing/VVI backup.



Switch from AAI/VVI backup to DDD

During AAI(R) pacing/VVI backup, the device automatically switches to DDD(R) if three slow ventricular beats are detected in a window of 11 beats.

A slow beat is defined as a ventricular paced or ventricular sensed event that is at least 150 ms slower than the AAI(R) pacing rate. The 150 ms value is used to allow for normal variations between cardiac cycles.

Slow ventricular beats are any of the following:

  • V paced event
  • V sensed event > AAI lower rate limit + 150 ms
  • V sensed event > AAI(R) sensor indicated rate + 150 ms


AV Search +

If Rythmiq is programmed ON and AV Search + is programmed OFF, when three slow ventricular beats are detected in a window of 11 beats, the device switches permanently to DDD(R) mode without possibility to switch back to AAI(R).

The AV Search + algorithm promotes intrinsic ventricular activation by periodically extending the AV delay.

  • the search begins every 32-1024 ventricular cycles (programmed Search Interval)
  • when the AV Search Interval is reached, the AV delay is extended to the programmed AV Search + value
  • failed Search: if either of the following occur, the device maintains DDD(R) pacing and the AV Search + interval is restarted: no intrinsic conduction is detected within the initial search period (8 cycles) or 2 out of the last 10 ventricular events are paced (rolling window)
  • successful Search: if at least one V-sense is detected within the initial search period (8 cycles), hysteresis continues: each ventricular sensed event is marked as VS-Hy (Hysteresis), AV Conduction Counter is incremented with each VS-Hy beat; when the AV Conduction Counter is ≥ 25, sustained conduction is detected and the device switches back to AAI(R) mode with VVI back-up


Programmable parameters

  • AV Search +: OFF/ON
  • Search Interval: 32-1024 cycles
  • Search AV delay: 30-400 ms
  • Search AV Delay is applied to both the programmed Sensed AV and Paced AV delays


Arrhythmia Logbook

An event is stored in the Arrhythmia Logbook when the device switches to DDD(R) mode. Up to three events (minimum of one) with EGM details may be stored (20 seconds of EGM).