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Figure 7 (A) Shows the electrocardiogram of an atrial tachycardia with apparent start-stop episodes. (B, top panel) Shows the intracardiac electrograms with the high-density mapping catheter. In this region of atrial electrical silence, two spines record electrical activity. During the pause on the electrocardiogram, activity persists at the origin of the tachycardia on spine-E while there is intra-atrial conduction block that gives rise to the misleading appearance of tachycardia cessation. However, note that the P-wave morphology after the pause is not typical of sinus rhythm. (B, bottom panel) This is also from the same patient and was observed earlier in the mapping process. This recording was observed in the lateral right atrium, again in the midst of an area of electrical silence; spine-D alone demonstrates high-frequency activity. Although surprising that the atria could demonstrate activity at such a rate (mean cycle length 83 ms), this was indeed a sequestrated island of activity that was isolated from the remaining atria. Ablation eliminated the local activity but did not change tachycardia. CS = coronary sinus.