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Figure 1 Diagnostic algorithm for bradycardic events. In addition to a backup rate (basic), a watch rate (50 beats/min) and a pause rate (30 beats/min) are defined. The spontaneous ventricular rate (Vs) may fall below the backup rate to the pause rate, at which point ventricular pacing (Vp) is initiated. (A) A "pause" is confirmed after two consecutive ventricular cycles paced at the pause rate of 30 beats/min, triggering the memorization of a marker chain. (B) A marker chain of "bradycardia" is stored after 100 consecutive ventricular cycles between the watch and pause rates. Thereafter, rescue ventricular pacing begins, gradually rising to the backup rate. Open circle = Vs; Closed circle = Vp. Vs = ventricular sensed event; Vp = ventricular paced event.