A regular supraventricular tachycardia with a 1:1 AV relationship and a CL of 420 ms is induced in the EP lab. Activation of the high right atrium is coincident with the QRS complex. Ventricular overdrive pacing is delivered from the right ventricular apex during tachycardia at a cycle length 30 ms less than the tachycardia CL. Figure 56.1 shows the response immediately upon cessation of ventricular pacing. What is the mechanism of tachycardia?
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