LAUSR

A 71‐year‐old male with repetitive palpitations underwent an electrophysical study and catheter ablation. A 12‐lead ECG during the palpitations showed a regular narrow QRS tachycardia with a short RP interval. During the electrophysiologic study, the atrial–His (AH) interval and His‐ventricular interval during sinus rhythm were normal. Right ventricular apical pacing exhibited retrograde conduction with a decremental property and the earliest atrial activation site (EAAS) was in the His‐ bundle region. Rapid pacing from the right atrial appendage (RAA) at a cycle length of 460ms exhibited 1:1 AH conduction with an AH interval of 140ms (Figure 1A). When the cycle length was shortened to 350ms, the AH interval was prolonged to 240ms, and a short RP tachycardia at a cycle length of 370ms was induced (Figure 1B). During the tachycardia, the EAAS was in the His‐bundle region, and the septal ventriculoatrial (VA) interval was 60ms. What was the mechanism of this tachycardia?