AR Interval and Pacemaker Timing

The AR State and Pacemaker Timing

Introduction to AR Interval

  • The AR Interval occurs when the Atrial Escape Interval (AEI) times out completely.

  • An atrial output is triggered at the end of the AEI.

  • The AV Delay Interval starts but is interrupted by a sensed ventricular channel event, typically an intrinsic R wave.

Pacemaker Timing Schemes

1. Ventricular Based Timing

  • In ventricular based timing, a sensed R wave restarts the Atrial Escape Interval (AEI) timer.

  • The timing of the next atrial output depends on when the R wave (or other ventricular channel event) is sensed.

  • Clinical Implication: When the AR state is present with ventricular based timing, the next atrial output occurs at a rate faster than the programmed base rate.

  • Example: If the AV Delay Interval is set to 200 ms and an R wave is sensed at 150 ms, the subsequent atrial output will occur 50 ms earlier than the calculated base rate.

  • For a base rate of 60 ppm (1000 ms), the atrial output will occur at 950 ms, resulting in a rate of approximately 63 ppm (measured from A spike to A spike).

2. Atrial Based Timing

  • In atrial based timing, the AEI timing begins with the atrial event.

  • In the presence of PVCs (Premature Ventricular Contractions), atrial based timing temporarily switches to ventricular based timing for one cycle.

  • Following the PVC, the next atrial output is scheduled at the pacing interval minus the AV Delay interval.

  • Example: With a device programmed to 60 ppm (1000 ms) and an AVD (AV Delay) of 200 ms, a PVC initiates a clock cycle of 800 ms for the next atrial output pulse (1000 ms - 200 ms = 800 ms).

Examples of A-A Intervals:

  • Ventricular Based Timing

  • Atrial Based Timing

A-A = 1000 ms
A-A = 1000 ms
A-A = 1000 ms
A-A = 930 ms