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.