DDD Pacing - PR State

The PR State

The "PR" or "inhibited" state is the final state of DDD pacing.

• In the PR state:

  • The atrial escape interval (AEI) is interrupted by a sensed event on the atrial channel.

  • A PV interval begins but is stopped by a sensed ventricular event.

  • No pacing occurs; the device remains inhibited.

  • This is the normal way the PR interval occurs.

• A normal, but anomalous appearing PR interval can occur at fast sinus rates.

• Example:

  • Patient has first-degree heart block with a long PR interval.

  • Pacemaker is set up with an AV interval of $175$ ms and a maximum tracking rate (Upper Rate Limit) of $110$ ppm ($545$ ms).

  • If the sinus rate is greater than $110$ bpm, the P wave will be sensed, and the PV interval will start.

  • However, at the end of the PV interval, the ventricular output will not occur because the Upper Rate Limit of Ventricular Pacing is set for $110$ ppm.

  • The ventricular channel cannot output a spike until the upper rate interval timer completes its cycle.

  • If an intrinsic R wave occurs before this cycle is completed, the V output will remain inhibited.

  • One will see a P wave followed by an R wave at an interval GREATER THAN the programmed AV or PV delay.

• Notice how the Upper Rate Limit (URL) is interrupted by the sensed R wave.

• Since it is never allowed to complete, ventricular outputs are not permitted.

• On the ECG recording, the rhythm looks like 1st-degree heart block at a rate greater than the programmed upper rate of the device.

4 States of DDD Pacing

The 4 states of DDD pacing are: AV, PV, AR, PR