This trial seeks to evaluate the performance of the extendable helix, stylet-driven pacing lead (SDL) compared to the fixed helix, lumenless pacing lead (LLL) during left bundle branch pacing (LBBP), with respect to enduring left bundle branch capture on follow-up, incidence of acute lead failure, pacing characteristics including QRS duration, pacing thresholds, R-wave amplitudes and lead impedance, and finally, safety profile during LBBP implantation. These data will guide future lead selection during LBBP implantation in achieving improved procedural success and optimal lead performance.

This study is a single-blind, randomised controlled trial, aimed at evaluating the performance of the extendable helix, stylet driven pacing lead (SDL) compared to the fixed helix, lumenless pacing lead (LLL) during left bundle branch pacing (LBBP). The study is a multicentre study across 3 major hospitals in Singapore, and LBBP performed by independent electrophysiologists experienced in performing LBBP.

The primary objective of the study is to determine differences in incidence of loss of left bundle capture by pacing lead design (LLL vs SDL). The secondary objectives are to determine if lead design affects frequency of lead failure during implantation, investigate differences in short- and medium-term LBBP parameters on follow-up by lead design, and evaluate differences in safety profile between LLL and SDL in LBBP.

This study aims to recruit a target of 210 patients. A previous retrospective study found a loss of LB capture rate of 9% with LLL and 25% with SDL. A sample size of 170 will be required for a type-1 error rate of 5% and power of 80%. Accounting for a dropout rate of 20%, a total of 210 patients will be recruited (105 per group). Eligible subjects will be randomised to either the LLL or the SDL arm. The randomisation list will be generated with a computer-generated sequence in permuted blocks of 4, stratified by centre, to ensure a final allocation ratio of 1:1.

Inclusion Criteria:

1. Fulfil current indications for pacemaker therapy according to international guideline recommendations:

   1. Symptomatic sinus bradycardia/sinus node dysfunction or AV block requiring pacemaker insertion.
   2. Symptomatic heart failure, left ventricular ejection fraction (LVEF) <35%, left bundle branch block and QRS duration >120ms.
   3. Symptomatic heart failure, left ventricular ejection fraction (LVEF) <35%, right bundle branch block and QRS duration >150ms.
   4. LVEF <50% with significant anticipated ventricular pacing requirement of ≥20%.

2. Age ≥21 years old

3. Able to provide informed consent

4. Planned LBBP implantation

Exclusion Criteria:

1. Severe tricuspid regurgitation or previous tricuspid valve intervention requiring implantation of left ventricular lead in the coronary sinus.
2. Unable to provide informed consent.
3. Pregnant women.
4. <21 years of age.

Subjects will be blinded to the type of pacing lead received. Operators will not be blinded as they will be performing the procedure. Post-procedure, investigators and outcomes assessors will be blinded to the treatment arm during adjudication of outcomes.