Simultaneous Wide and Narrow QRS Complex Tachycardias in a Child: What Is the Mechanism?

Case Presentation An 8-year-old boy was referred for recurrent wide complex tachycardia. At 4 years of age, his initial presentation to another facility was notable for acute heart failure due to a wide complex tachycardia (260 bpm) with right bundle block (RBBB) morphology and left axis deviation resistant to adenosine and to DC shocks and finally converted to sinus rhythm (SR) after i.v. amiodarone. Cardiac work-up ruled out structural heart disease. On the present admission the heart rate was 215 beats/minute and the QRS morphology was identical to the previous episode (Fig. 1). Adenosine rapid bolus was given