| CASE REPORTA 57-year-old man, patient with an old anterior wall myocardial infarction and left ventricular ejection fraction of 0.3, developed episodes of paroxysmal palpitations over the last 6 months. These selflimited episodes were often accompanied by presyncope and severe light-headedness. A 24-hour Holter monitoring showed four salvos of short-lived broad QRS tachycardia. He was referred for an electrophysiologic study.
| Electrophysiologic studyThe baseline tracing during sinus rhythm showed intervals within the normal range (PA = 32, AH = 78, HV = 55, QRS = 92 ms).During programmed ventricular stimulation, a wide complex nonsustained tachycardia (cycle length of 220 ms) was reproducibly induced with two extra stimuli delivered at the right ventricular apex. Five to ten seconds later, the broad QRS tachycardia transitioned to a slower narrow QRS complex tachycardia, with a cycle length of 335 ms (Figures 1 and 2A). During narrow QRS tachycardia, the sixth beat was a premature beat with left bundle branch-like morphology. What are the mechanisms of both tachycardias? 2 | COMMENTARY The narrow complex tachycardia cycle length diminished over the first 10 seconds, making it difficult to make any assumption regarding advancement of activation by the ventricular premature beat. The QRS complex during narrow QRS tachycardia shows left axis deviation.There was only intermittent ventriculoatrial conduction during the broad QRS tachycardia (Figure 2A), as evidenced by the initial 2 HA times being much shorter than the relatively constant HA time during narrow QRS tachycardia. The narrow QRS tachycardia showed a clear simultaneous atrial and ventricular activation, consistent with slow-fast atrioventricular nodal reentrant tachycardia (AVNRT); the unlikely differential diagnosis of an atrial tachycardia with a long atrioventricular interval.The broad QRS tachycardia without preceding His bundle potential and AV dissociation would invariably be ventricular tachycardia (VT). Very rarely, this could also be seen with a nodoventricular pathway reentry. Antidromic nodoventricular tachycardia circuit does not include the atrium; however, the retrograde arm is the AV node and His-Purkinje system. As shown in Figure 2B, there is ventricle-His bundle potential dissociation after the first two ventricular electrograms (constant VH interval), which makes antidromic nodoventricular pathway reentry untenable.