The aim of this study proposal is to compare the effectiveness of two distinct pacing modalities for cardiac resynchronization therapy. Our primary hypothesis is that left bundle branch area pacing (LBBAP) using conventional stylet driven pacemaker leads is an effective resynchronization method that yield to at least similar clinical benefits and outcomes when compare to biventricular pacing.

Recently some retrospective or uncontrolled studies were published that compared cardiac resynchronization therapy (CRT) using left bundle branch area pacing (LBBAP) and biventricular pacing (BiV), although with a limited number of patients. Moreover, only one of these studies compared LBBAP + right ventricle stimulation to conventional BiV stimulation prospectively. None of the studies used conventional stylet-driven pacemaker leads for LBBAP. Therefore, the present trial aims to fill-in the gap in the current literature regarding LBBAP for CRT and provide a first randomized head-to-head comparison against CRT.

The aim of this study proposal is to compare the effectiveness of two distinct pacing modalities for cardiac resynchronization therapy. Our primary hypothesis is that left bundle branch area pacing (LBBAP) using conventional stylet driven pacemaker leads is an effective resynchronization method that yields to at least similar clinical benefits and outcomes when compared to biventricular pacing.

The study is a multicentre, randomized trial comparing LBBAP against biventricular pacing. After characterization of the type device implanted (Pacing or Defibrillator), randomization will occur between both groups in a 1:1 ratio. The trial purpose is to demonstrate the efficacy of LBBAP to treat clinical heart failure in-patient indicated to receive a CRT.

Inclusion Criteria:

• Aged 18 or above
• Left Ventricular Ejection Fraction (LVEF) and NYHA meeting one off the previous indications documented with transthoracic echocardiography (TTE) no later than 3 months before randomization.
• New York Heart Association (NYHA) class II-IV
• Baseline left bundle branch block and QRS >130ms or QRS >150ms of any morphology or a wide paced QRS duration.

Exclusion Criteria:

• Younger than 18 years old
• Pregnancy
• Any unstable medical condition, life expectancy < 12 months
• Inability to provide consent or to undergo follow-up
• Narrow QRS duration (≤130ms) or prolonged QRS duration with typical Right Bundle Branch Block (RBBB) morphology on 12 lead ECG and sinus rhythm.
• Significant hypertrophic cardiomyopathy - any septum abnormality contra-indicating LBBAP implantation (according to physician's evaluation)
• Patients who are unable to perform a 6 minute walk test (6MWT) Note: patients should be able to perform a 6 minute walk test, but it is not a requirement to have the test performed before the procedure (e.g. due to inability of planning).