Atrial Refractory Period and Ventricular Sensing

The top strip shows atrial pacing at a rate of 65/min but with a steadily increasing PR interval and then a pause. The pacer is working well and the irregularity is due to the atrial lead sensing the QRS when the QRS lies outside of the 350 ms atrial refractory period.

It is clear that it is the QRS that is sensed, as the interval from the 4th QRS to the next atrial pacing spike, is the pacing interval of the pacemaker.

The second strip shows one way of dealing with this problem. In this case the pacing rate was decreased to 60/min. which prevented the PR interval from lengthening enough to take the QRS past the atrial refractory period.

Other methods of avoiding this problem include:

• switching the pacer to a bipolar sensing mode to avoid sensing the ventricle.
• increasing the atrial refractory period (this will make spontaneous p waves more difficult to sense)
• switching to AOO.R mode (this will not sense spontaneous p waves)
• change the pacing mode to DDD

This abnormal prolongation of the PR interval can be a problem with an AAI.R pacer as this will limit the ventricular rate response. It is therefore important to check the anterograde conduction between the atrium and ventricle of each patient before an AAI.R or for that matter an AAI pacer is implanted. This can be done during the implant procedure and should be greater than 130/min.

Dr. M. Rosengarten