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Figure 1 Patient 11. Initiation of AV reentrant echo beats following the administration of 20 mg ATP during sinus rhythm in a patient with a concealed left free wall AP and electrophysiologic evidence of antegrade dual AV node physiology. Note that the first two echo beats follow a sudden jump in PR interval of 70 ms (dual AV node physiology) while the prolongation in PR interval preceding the third echo beat is of 40 ms only. Also note that the sinus cycle length preceding the first echo beat is regular at 710 ms.