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 th e QRS complex. An electrophysiological study revealed antidromic atrioventr icular tachycardia involving an atrioventricular connection with decrementa l conduction as the anterograde (HRA) at the cycle length (CL) of 600 ms, t he QRS configurations were not identical to those during the wide QRS tachy cardia or constant pacing at the CL of less than 500 ms. The process by whi ch this arborized atrioventricular accessory pathway with the Mahaim fibers physiology was interrupted by radiofrequency catheter ablation is describe d. Radiofrequency energy was delivered to the site recording a Mahaim poten tial 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. Shortenin g of the conduction time over the Mahiam pathway might have resulted in cha nging of the propagation from a slow to fast conduction zone or acceleratio n in response to thermal effect in a node-like structure on the atrial inse rtion site.