What is the tachycardia mechanism?--II.

Case Presentation A 34-year-old man with a 10-year history of paroxysmal supraventricular tachycardia underwent an electiophysiologic procedure. A narrowQRS complex tachycardia with a cycle length of 260 to 280 msec reproducibly was induced by right atrial overdrive pacing. At times there was a 1:1 relationship between the atrium and ventricle (Fig. 1), and at other times there was Mobitz I second-degree AV block during the tachycardia (Fig. 2). The atrial electrogram recorded by the His-bundle catheter preceded the high right atrial electrogram. At a time wben the tachycardia cycle length was 280 msec and tbere was a 1:1 AV relationship, ventricular overdrive pacing was performed at a cycle length of 270 msec (Fig. 3). What is tbe mechanism of tbis tachycardia?