Eccentricities of dual chamber implantable pacemakers

The consequences of a short 160 ms postventricular atrial refractory period (PVARP) were studied in 27 patients with Medtronic Elite series of pacemak ers (programmed to the unipolar mode) by using chest wall stimulation and m yopotential interference. Because the ventricular refractory period is nonp rogrammable at 230 ms, appropriately timed signals produced As-Vr combinati ons where As is an atrial sensed event and Vr is a ventricular event detect ed in the ventricular refractory period. The As-Vr combination inhibited th e release of the atrial stimulus at the completion of the atrial escape int erval in the DDI, DDIR, DDD, and DDDR modes. A paced ventricular beat (Vp) was documented to initiate automatic PVARP extension in circumstances where Vp was preceded by a Vr event and the pacemaker interpreted the Vr-Vp sequ ence as 2 ventricular events without an intervening atrial event and Vp as a ventricular premature beat. Knowledge of these eccentricities should faci litate the interpretation of complex pacemakers responses to interference.