Exit block of focal repetitive activity in the superior vena cava masquerading as a high right atrial tachycardia

AT Originating from the SVC. An unusual case of atrial tachycardia (AT) ori ginating from the superior vena cava (SVC) is reported. A 34-year-old man w ithout structural heart disease underwent catheter ablation for drug-resist ant AT. During the tachycardia, low-amplitude spiky electrograms with a cyc le length of 120 to 175 msec were recorded in the SVC and exhibited 2:1 exi t block to the atria, masquerading as the atrial activation observed with h igh right AT. These spiky electrograms also ere observed during sinus rhyth m, but they appeared immediately after the local atrial electrograms. The s pikes were traced to a point 3 cm abo ce the junction of the right atrium. Radiofrequency ablation at the site of the earliest appearance of the spike in the SVC successfully eliminated the tachycardia. During the following 1 5 months, no clinically significant atrial arrhythmias, including atrial fi brillation, occurred. This report indicates that careful mapping, including inside the SVC, will be a requisite in patients with high right atrial tac hyarrhythmias.