Successful Ablation of a Wide Complex Tachycardia with Distinct Intra-Cardiac Electrograms

A 13-year-old boy was hospitalized after a syncopal episode that occurred during exercise. He suddenly felt chest tightness, sweating and palpitations, followed by a transient loss of conciseness. Upon emergency medical team arrival, he was awake and oriented. Baseline ECG showed sinus rhythm at a rate of 98 bpm, with narrow QRS, and no signs of long QT, Brugada, or pre-excitation. Physical examination, blood tests, 24 hours Holter monitoring, transthoracic echocardiography and stress test were all within normal limits. Eight days later he experienced a second episode of palpitations while walking to school. ECG revealed regular wide complex tachycardia (WCT) at a rate of 200 bpm, with LBBB morphology that terminated with Adenosine (Figure 1). The clinical tachycardia was easily induced by programmed electrical stimulation (Figure 2A). Diagnostic electrophysiological maneuver (Figure 2B) was followed by successful ablation, during which a unique phenomenon was noted (Figure 3). What is the diagnosis of the tachycardia and what are the unique findings noted during and after ablation?