The influence of atrial systole on ventricular capture by failing artificial pacemakers. II. Experimental observations

Underestimation of Pacing Threshold as Determined by an Automatic Ventricular Threshold Testing Algorithm

In this case report, we describe markedly different pacing thresholds determined by a manual threshold test and the automatic Ventricular Capture Management algorithm. The discrepancy in pacing threshold values reported was due to the difference in the AV intervals used with the different testing methods. We propose that the differences in right ventricular dimensions with altered diastolic filling periods affected the threshold in this patient with a new passive fixation lead in the right ventricular apex.

Changes in morphology of the paced QRS complex related to atrial contraction

A patient with 2:1 AV block underwent temporary ventricular pacing. All the paced stimuli resulted in ventricular capture, but a marked variability in morphology of the paced QRS complexes occurred. Two different types of paced QRS complex (labeled A and B) were recognized. Type B complexes were manifest only when the pacing stimulus was preceded by a sinus P wave within a time interval ranging from 0.15 to 0.52 sec. The P wave-induced changes in morphology of the paced QRS complexes were interpreted as due to displacement of the pacing ventricular lead caused by atrial systole.

Atrial systole and ineffective pacemaker stimuli

An unusual case of pacemaker failure induced by atrial activity is presented. The pacemaker stimulus was ineffective for ventricular capture not only with low energy output (decreased pulse duration) of the generator, but also with pulse width two to three times the stimulation threshold. The two possible mechanisms of this phenomenon, one electrotonic and the other mechanical, are discussed. This report strongly supports the mechanical hypothesis.

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.