(A) Cast of characters. (B) Non-pacing associated ventricular tachycardia or fibrillation (VT/VF) onset. The general scheme applies to all pacing modes. Sudden onset of monomorphic VT (MMVT) following a ventricular premature depolarization (VPD) (arrow), which has a different morphology from the ongoing MMVT. Ventricular premature depolarizations with different morphology can also occur. AP = atrial stimulus; S-L-S = short-long-short; VP = ventricular stimulus.

(A) Pacing-associated onset of VT/VF. Normal pacing occurs within 5 cycles of the onset of the VT. Initially there is continuous VVI pacing. Ventricular tachycardia begins without an S-L-S sequence. (B) Pacing permitted onset of VT/VF. This time there is an S-L-S sequence not caused by pacing. In this example, a VPD produces a short interval (S) followed by a pause (L) followed by VT after a short (S) sequence. Other abbreviations as in Figure 1.

(A) With VVI pacing, a VPD produced as a short pause (S) followed by a long interval (L) that is interrupted by a pacing stimulus (arrow) that is followed by a short interval and the onset of VT. (B) DDD pacing, in this case with A sense-V pace. An atrial premature depolarization (arrow) produces an early ventricular paced cycle (S) followed by a longer pause (L) broken by an A sensed/V paced sequence, which initiates a short interval (S) and VT. (C) MVP mode: a VPD (arrow) is not sensed because it occurs in the blanking period after an A-paced event. The ensuing S-L-S sequence initiates VT. Other abbreviations as in Figure 1.

Ventricular pacing interrupted by conducted sinus or an atrial premature depolarization that initiates a short-long-short (S-L-S) sequence and supraventricular tachycardia.