Fig. 1

(a) Presenting 12-lead ECG (at 25 mm/sec sweep speed) with rapid conducting atypical atrial flutter/AF (b) Episode stored in the ICD`s memory showing interpretation of fast, irregular AF as ventricular fibrillation and delivery of an internal shock (black arrow) (c) Depth of penetration of the lead into the interventricular septum (from lead tip to the proximal electrode − 10.8 mm) (d) 12-lead ECG (at 50 mm/sec sweep speed) and intracardiac electrogram recorded from the lead tip depicting a large proximal LBB potential (asterisk) (e) Pacing at that site with decremental amplitude revealed a transition from non-selective LBB pacing (with an LVAT of 95 ms– black arrow) to left septal myocardial capture (wider QRS and an LVAT of 111 ms - asterisk). ECG– electrocardiogram; AF– atrial fibrillation; LBB– left bundle branch; LVAT– left ventricular activation time