Pseudohysteresis in a universal AV sequential pacemaker.

We describe pacemaker pauses and pseudohysteresis resulting from sensing of retrograde P waves in a patient with an implanted AV sequential universal pacemaker. Although retrograde P waves are usually followed by a ventricular spike and therefore create the possibility for "endless loop tachycardia," in our patient some retrograde P waves which followed premature ventricular contractions or junctional beats produced pacemaker pauses. The reason for the occurrence of this phenomenon is that the sum of the VA conduction time and the AV delay is shorter than the pacemaker's hardware rate limit. Therefore, the ventricular spike that should follow the retrograde P wave is inhibited. In the pacemaker used by us, the atrial refractory period is only initiated by an atrial event, allowing the pacemaker to sense retrograde P waves occurring after premature junctional or ventricular depolarizations. Although no therapy is needed for the phenomenon described, it is important to understand its true significance and to avoid unnecessary surgical procedures.