Shortening of Conduction Time Over Arborized Atrioventricular Accessory Pathway With Mahaim Fibers Physiology Just Before Interruption During Radiofrequency Ablation

A 21-year-old woman had paroxysmal wide QRS tachycardia with a left bundle branch block configuration and a retrograde conducted P wave just behind the QRS complex. An electrophysiological study revealed antidromic atrioventricular tachycardia involving an atrioventricular connection with decremental conduction as the anterograde limb and normal atrioventricular node as the retrograde limb. During constant pacing from the high right atrium (HRA) at the cycle length (CL) of 600 ms, the QRS configurations were not identical to those during the wide QRS tachycardia or constant pacing at the CL of less than 500 ms. The process by which this arborized atrioventricular accessory pathway with the Mahaim fibers physiology was interrupted by radiofrequency catheter ablation is described. Radiofrequency energy was delivered to the site recording a Mahaim potential at the tricuspid annulus during constant pacing from the HRA at the CL of 429 ms. The stimulus-QRS interval gradually shortened as it reached the power plateau without changing the preexcited QRS configuration. Shortening of the conduction time over the Mahiam pathway might have resulted in changing of the propagation from a slow to fast conduction zone or acceleration in response to thermal effect in a node-like structure on the atrial insertion site.