Abstract
A 28-year-old male patient with a 3-year history of re-current tachycardia was referred for electrophysiologic eval-uation and catheter ablation. The patient had no other co-morbidities. Propafenone was discontinued 7 days beforeadmission.Electrophysiologic study: normal baseline intervals dur-ing sinus rhythm (PA = 20, AH = 68, HV = 50 ms). Anarrow QRS tachycardia was repeatedly induced by a singleatrial premature beat following a pacing drive of 400 ms, orfollowing 2 premature beats delivered during sinus rhythm.In contrast, a wide QRS tachycardia was always inducedby 2 right ventricular premature beats delivered during sinusrhythm.Thisleftbundlebranchblock(LBBB)-liketachycar-dia with a cycle length of 330–340 ms would spontaneouslychange into a slower, narrow QRS tachycardia with a cyclelengthof370msafterstabilizationoftheRRinterval(Fig.1).Transitionfromwidetonarrowtachycardiawassudden.Thecycle length of the first RR interval after disappearance oftheLBBBpatternwas310msbutincreasedthereafter.Whatwas the mechanism of change in cycle length?
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