A Case of Long‐RP Tachycardia: What is the Mechanism?

A 33-year-old woman with a 12-year history of paroxys-mal palpitations that were not responding to medical treat-ment was referred for EP study. Baseline 12-lead ECG wasnormal. A 12-lead ECG of her tachycardia showed narrowQRS tachycardia with an RP interval that was longer thanthe PR interval. P waves were inverted in inferior leads.(Fig. 1).During EP study her baseline conduction intervals werenormal. Clinical tachycardia was induced by programmedstimulation from the right atrium. Tachycardia cycle lengthwas 310 ms. During tachycardia, earliest atrial activationwasrecordedinmiddlecoronarysinuselectrodes(C7and8)along the posterior left ventricular wall.Tachycardiawasentrainedfromtherightventricularapexat a pacing cycle length of 290 ms. The response of tachy-cardiauponterminationofentrainmentisshowninFigure2.A premature ventricular complex during His bundle re-fractorinessfromrightventricularapexreproduciblyelicitedthe response showed in Figure 3.What is the mechanism of tachycardia, and what is theexplanation of the response to His-bundle-refractory PVC?