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?