The influence of atrial activity on ventricular capture by failing artificial pacemakers. I. Report of two new cases and review of the literature.

Atrial activity can influence the ability of a failing artificial pacemaker to excite the heart. An appropriately timed atrial beat may cause failure in excitation by pacemaker stimuli which are usually successful in ventricular capture. Conversely, stimuli which usually fail in excitation may be made to succeed by an appropriately timed atrial beat. Two case reports and a review of the literature are presented. Alternative mechanisms for this influence of atrial activity are electrotonic effects (Wedensky facilitation or inhibition) and mechanical effects (motion of the pacing catheter or ventricular myocardium). The authors consider the latter mechanism preferable.