Electrocardiographic Manifestation of Modulated Parasystole With Variable Escape Timing

A 65-year-old woman with a history of surgical aortic valve replacement presented with shortness of breath. The electrocardiogram demonstrated bradycardia with ventricular rhythm exhibiting 2 distinct QRS morphologies and variable escape intervals. The most likely mechanism was a dominant fascicular escape focus modulated by a second, closely coupled ventricular ectopic beat, which predictably altered subsequent escape timing depending on the coupling interval. This interaction exemplifies the classic behavior of a modulated parasystolic focus. TAKE-HOME MESSAGES: In modulated parasystole, entrance block to the ectopic focus is incomplete, allowing surrounding myocardial activity to electrotonically influence the timing of the parasystolic pacemaker without true resetting. Phase-dependent modulation explains variability in escape cycle length: depolarizing wavefronts arriving early in the pacemaker cycle may delay spontaneous discharge, whereas wavefronts arriving late may trigger premature firing.