Fig. 1. Examples of antibradycardia pacing modes. (A ) Atrial-inhibited (AAI) pacing for sinus arrest or bradycardia. The pulse generator is shown with atrial leads only. The atrium is paced, unless pacing is inhibited by sensed spontaneous atrial depolarizations. (B ) Ventricular-inhibited (VVI) pacing for atrioventricular (AV) heart block (AVHB) with atrial fibrillation. The pulse generator is shown with ventricular leads only. The ventricle is paced, unless pacing is inhibited by sensed spontaneous ventricular depolarizations. (C ) Ventricular-inhibited, atrial-triggered (VDD) pacing for AVHB with normal sinoatrial (SA) node and atrial function. The pulse generator is attached to atrial leads for sensing only and to ventricular leads for pacing and sensing. If a spontaneous atrial depolarization is sensed, the ventricle is paced after an appropriate atrioventricular interval to permit ventricular filling. This is the atrial-triggered ventricular pacing (VAT) component of the VDD mode, which also includes capabilities of the VVI mode. (D ) Dual-chamber sequential or atrioventricular universal (DDD) pacing for sinus bradycardia and AVHB. The pulse generator is shown attached to atrial and ventricular leads for dual-chamber sensing and pacing. This mode incorporates AAI, VVI, and VAT pacing capabilities. Reprinted with permission from Bernstein AD, Parsonnet V: Fundamentals of antibradycardia-pacemaker timing. ACC Educational Highlights 1995; 11:5–9; copyright American College of Cardiology.