Fundamental Anatomy and Its Impact on Clinical Practice: The Rightward Extension of Bachmann's Bundle (Part II-III).

The rightward extension of Bachmann's bundle (BB) represents a subepicardial myocardial continuum integrating the interatrial bridge with the sinus node region, terminal crest, and right atrial appendage. Rather than a discrete insulated tract, it consists of aligned cardiomyocytes forming a bifurcated arrangement that supports preferential longitudinal and circumferential conduction. This convergence zone near the superior cavoatrial junction enables coordinated biatrial activation but also introduces anisotropic properties that may facilitate conduction delay or arrhythmogenesis. These anatomical insights provide the basis for BB-region pacing, which aims to reproduce near-physiological atrial activation, improving interatrial synchrony and potentially reducing atrial arrhythmia burden compared with conventional right atrial appendage pacing. TAKE-HOME MESSAGES: The rightward extension of BB constitutes a subepicardial, anatomically integrated myocardial system closely related to the sinus node region and the crista terminalis, characterized by alignment parallel aggregation of its cardiomyocytes rather than structural insulation. This organized architecture provides the anatomical substrate that underlies physiological activation of the bundle and supports the rationale for targeted stimulation strategies aiming to reproduce or improve native interatrial conduction and prevent atrial desynchronization during pacing.