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Figure 3 Patient 24. Initiation of AV reentrant echo beats following the administration of 30 mg ATP in a patient with a concealed posteroseptal AP and no electrophysiologic evidence of dual AV node physiology. One echo beat occurs after each of four consecutive sinus beats (QRS complexes No. 2 through No. 5) with the first echo following a slight prolongation (20 ms) in PR interval. A sinus beat associated with a shorter PR interval subsequently occurs, followed by an atrial premature beat and another AV reentrant echo beat.