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Figure 4 Holter monitor tracings recorded from three patients in whom atrial fibrillation (AF) appeared to be initiated by the pacemaker. The top tracing is a bipolar recording from the right atrial epicardial wires showing the atrial pacing spikes, and the bottom tracing is a recording from surface lead I. In A, a failure of pacemaker sensing caused the atrial pacing spike to fall progressively later after atrial depolarization until AF was initiated when a pacing spike (open arrow) occurred during a critical period of atrial repolarization. In B and C an atrial premature beat (filled arrow) occurs which is not sensed appropriately by the pacemaker. The next pacing spike (open arrow) occurs during atrial repolarization and the rhythm converts to AF. A ventricular spike also occurs due to safety pacing.