Cardiac Resynchronization Therapy in Patients Without Typical Left Bundle Branch Block: A Systematic Review of Patient Selection Strategies and Emerging Conduction System Pacing Alternative

Cardiac resynchronization therapy (CRT) improves outcomes in heart-failure patients with left bundle branch block (LBBB), but benefit in non-LBBB patients (right bundle branch block [RBBB] and non-specific intraventricular conduction delay [IVCD]) is inconsistent and poorly predicted by QRS duration or morphology alone. To synthesize evidence through 2026 on strategies for identifying non-LBBB patients likely to benefit from resynchronization, including electrocardiographic/vectorcardiographic and echocardiographic dyssynchrony markers, and the emerging role of conduction system pacing (CSP) — particularly left bundle branch area pacing (LBBAP) — as an alternative to biventricular pacing (BiVP)A systematic review per PRISMA 2020 methodology, searching MEDLINE, Embase, and Cochrane CENTRAL from inception to 2026 for studies reporting selection criteria or comparative outcomes of resynchronization strategies in adult non-LBBB heart-failure populations. A subset of non-LBBB patients shows correctable left ventricular activation delay; vectorcardiographic QRS area outperforms QRS duration and LBBB criteria in discriminating responders (AUC 0.78 vs 0.62). CSP, particularly LBBAP, produces greater improvements in ejection fraction, reverse remodeling, and heart-failure hospitalization than BiVP in non-LBBB cohorts, though this evidence base remains predominantly observational. Non-LBBB should not be treated as a uniform contraindication to resynchronization. Refined selection tools and conduction system pacing are shifting practice toward individualized selection rather than blanket exclusion. Randomized comparative data remain the principal evidence gap.