Introduction: Cardiac resynchronization therapy with biventricular pacing is a well established method to im-prove quality of life and survival in patients with low ejection fraction and left bundle branch block (LBBB). Unfortunately, in some cases the response to this thera-py is limited by patient anatomy and the inability to de-liver the coronary sinus lead in a proper lateral branch. For these cases, conduction system pacing (with either His bundle or left bundle pacing) has been reported to significantly narrow the QRS complex in patients with bundle branch block morphology.

Methods: We present two case reports of patients with conventional permanent pacing indications, normal ejection fraction and left bundle branch block morpho-logy who underwent intrinsic conduction system pacing.

Results: The first case was an 62-year old male who was admitted in our hospital for recurrent syncope. The electrocardiographic monitoring showed LBBB with intermittent complete atrioventricular block. Per-manent His bundle pacing was decided. A lead delivery system consisting of a C315 His catheter (Medtronic) and a Select Secure 3830 69 cm lead (Medtronic) was placed in the septal atrioventricular junction. The area was mapped until a distal His bundle electrogram was obtained. Pacing at that site completely normalized the QRS complex. The second case was a 77-year old male who presented with dizziness and 2:1 atrioventricular block with a QRS morphology of LBBB. His bundle pacing was first attempted as described above but the threshold for QRS correction was too high. In this case, the catheter was moved 1.5 cm distally towards the apex and the lead was screwed deep into the septum with continuous pacing until the QRS complex normalized showing that left bundle capture was achieved distally to the site of block. In both patients, the echocardio-graphic dyssynchrony recorded before the procedure disappeared. The 3 months follow-up in both patients showed no change in pacing and sensing thresholds.

Conclusions: Permanent His bundle pacing and left bundle pacing are very effective in LBBB correction. Further randomized trials are needed to compare these techniques to standard biventricular pacing in patients with low ejection fraction and left bundle branch block.