Case 
A 28-year-old man presented with orthodromic atrioventricular reentrant tachycardia and underwent successful ablation of manifest right posterolateral atrioventricular accessory pathway (AP) [Appendix 1]. After successful ablation, the HV was still found to be short (26 ms) and a fasciculo-ventricular pathway (FVP) was suspected. The HV interval was short and   fixed and the QRS morphology remained unchanged during decremental atrial pacing (Fig 1). He also had junctional beats of identical QRS morphology (Appendix 2) confirming the diagnosis of FVP. There was an underlying right bundle branch block (RBBB) in sinus rhythm resurfacing after the ablation (Fig 1).
Para-Hisian pacing (PHP) was performed to confirm abolition of retrograde AP conduction. We observed the following responses reproducibly during a PHP. Pacing at distal His bundle (HB) region was performed @ 450 ms of pacing cycle length (PCL) with decreasing pacing output starting from 10mA @2 ms pulse width [Fig 2]. What is the mechanism behind the paradoxical response of Stimulus to A-EGM (SA) prolongation?