Evaluation of Accelerated Bachmann Bundle Area Pacing in Heart Failure With Reduced ejectIon Fraction Who Have electrocarDioGraphic Evidence of Interatrial Block and Indicated for Implantable Cardioverter Defibrillator

Inclusion Criteria: 1. Age ≥18 years at the time of screening. 2. A documented diagnosis of chronic heart failure with New York Heart Association (NYHA) class II-IV. 3. Left ventricular ejection fraction (LVEF) ≤40%, documented by an imaging study performed within 12 months prior to screening. 4. Receiving optimized guideline-directed medical therapy (GDMT) for heart failure with reduced ejection fraction (HFrEF), unless contraindicated or not tolerated: * Participants receiving ongoing treatment must be on a stable regimen for at least 1 month prior to screening (except for diuretics). * Most patients with heart failure require diuretics for volume control, and dose adjustments may be made based on clinical status, including symptoms, signs, and body weight. Each participant should receive individualized diuretic therapy to maintain optimal volume status. 5. Presence of interatrial block (IAB), defined as a P-wave duration ≥120 ms on a 12-lead electrocardiogram or ECG recording device. 6. An indication for dual-chamber implantable cardioverter-defibrillator (ICD) implantation for primary or secondary prevention. 7. NT-proBNP measured within 3 months prior to randomization meeting one of the following criteria: * NT-proBNP \>300 pg/mL Exclusion Criteria: 1. Non-paroxysmal AF. 2. Uncontrolled tachyarrhythmia. 3. Sinus bradycardia requiring continuous atrial pacing or atrioventricular block requiring ventricular pacing. 4. Acute decompensated heart failure at the time of screening or hospitalization for worsening heart failure within 4 weeks prior to enrollment. 5. Moderate to severe primary valvular heart disease (functional mitral regurgitation or tricuspid regurgitation is not an exclusion criterion). 6. Prior mechanical tricuspid valve replacement. 7. Coronary revascularization (PCI or CABG) or valve surgery/repair performed within 12 weeks prior to enrollment, or planned after randomization. 8. Obstructive hypertrophic cardiomyopathy. 9. Infiltrative cardiomyopathy, including but not limited to amyloidosis, sarcoidosis, or Fabry disease. 10. An indication for cardiac resynchronization therapy (CRT). 11. Chronic kidney disease, defined as an estimated glomerular filtration rate (eGFR) \<15 mL/min/1.73 m² calculated using the CKD-EPI equation. 12. Chronic liver disease, defined as alanine aminotransferase (ALT), aspartate aminotransferase (AST), or alkaline phosphatase \>3 times the upper limit of normal at screening. 13. Severe pulmonary disease, such as cor pulmonale or irreversible lung disease requiring inhaled therapy or long-term oxygen therapy. 14. Uncontrolled hypertension, defined as a mean blood pressure \>140/90 mmHg based on outpatient clinic measurements or home blood pressure recordings within the previous 30 days, or ongoing up-titration of antihypertensive medications. 15. Pregnant or breastfeeding women, or women planning pregnancy or breastfeeding during the study period. 16. Active malignancy requiring treatment at the time of screening. 17. Life expectancy \<12 months.