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?