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Figure 1 VVI/R

For pacing facilitated short-long-short (S-L-S), sinus rhythm is interrupted by a single ventricular premature depolarization (VPD) (coupling interval 700 ms, asterisk) ("short"). A 1,500-ms pause (bracket) ("long") is terminated by a single ventricular paced beat (VP) at the lower rate limit (40 beats/min). A VPD (coupling interval 440 ms) ("short") after the VP anticipates ventricular tachycardia (VT) that degenerated to ventricular fibrillation (VF) (not shown).