Rate dependent depression of subsidiary ventricular impulse formation--cause of Stokes-Adams attacks in a patient with rate modulated pacing.

Following His bundle ablation and implantation of a rate adaptive pacemaker (Vitatron TX 911) a 52-year-old gentleman experienced several presyncopal attacks while driving a car. On examination pectoralis muscle contraction caused temporary pacemaker inhibition. Incremental overdrive stimulation demonstrated progressive depression idioventricular automaticity and was associated with similar symptoms following overdrive at high pacing rates. Following appropriate pacemaker programming the patient remained symptom free.