Abstract
A 64-year-old female underwent catheter ablation of long R-P' tachycardia. Ventricular pacing exhibited retrograde conduction with an identical atrial activation sequence as during tachycardia because of an accessory pathway (AP) with a long VA conduction. A radiofrequency application within the posterior coronary sinus (CS) could achieve a change of activation pattern from distal-to-proximal to proximal-to-distal within CS proximal to the ablation site, caused by conduction block of CS musculature (CSM) at the proximal site. This phenomenon could explain that this AP was connected between the CSM and the left ventricle, in site far away from the discrete connection between the left atrium and CSM.