Sustained inhibition of a DDD pacemaker at rates below the programmed lower rate during automatic PVARP extension

A patient with a Pacesetter Paragon III DDD pacemaker exhibited sustained pacemaker inhibition at rates below the programmed lower rate during sinus rhythm with marked first degree AV block. In this device, a pacemaker defined ventricular extrasystole initiates automatic extension of the postventricular atrial refractory period to 480 ms and the atrial escape interval to 830 ms regardless of the programmed lower rate. Sustained pacemaker inhibition at rates below the programmed lower rate occurred because the P wave fell continually in the extended postventricular atrial refractory period, and the conducted QRS complex initiated a pacemaker ventricular extrasystole response with atrial escape interval extension. This process continued as long as the P wave stayed within the extended postventricular atrial refractory period, no ventricular extrasystole disrupted the sequence, and the R-R interval of spontaneous beats was shorter than the extended atrial escape interval. Such a pacemaker response should not be misinterpreted as device malfunction.