Case Presentation
A 16-year-old boy was referred for an electrophysiological study in view of rapid episodic palpitations with documented regular narrow complex tachycardia. He had no manifest pre-excitation. Clinical tachycardia with cycle length of 360 ms, was easily inducible by premature atrial complexes. A diagnosis of a concealed left lateral accessory pathway was made with an eccentric atrial activation sequence both during tachycardia and right ventricular (RV) pacing (Figure 1, left-hand panel). The pathway was mapped at the left posterior mitral vestibule during RV pacing, performed through the distal tip of the His bundle catheter (Figure 1, right panel). Intracardiac electrograms at the start of radiofrequency (RF) energy are depicted in the left hand panel of Figure 2. As the energy continues, shown in the right-hand panel of Figure 2, there is an alternation of QRS complex width and morphology. How is this explained?