Right-Ventricular Muscle Bands and the Conduction Axis

1) Macroscopic landmarks

Septomarginal trabeculation (SMT, “septal band”).

The SMT is the dominant septal bar of the RV. In its classic configuration it is Y-shaped, bifurcating into a supero-anterior limb that courses toward the pulmonary valve and an infero-posterior limb directed toward the membranous septum; together with adjacent structures, it helps sculpt the supraventricular crest and outline the infundibulum (RVOT) [1,2]. These features are consistent across morphologic surveys and imaging-anatomic correlations, which routinely depict the SMT’s Y configuration as the key organizer of RV inflow–outflow geometry.

Moderator band.

A robust trabecular bridge from septum to the RV free wall—typically inserting into or near the anterior papillary muscle. It is functionally continuous with the SMT and contains Purkinje tissue from the distal right bundle branch (RBB); modern anatomic-electrophysiology reviews emphasize its role as both a mechanical strut and a conduit for specialized conduction tissue [2,3].

Ventriculo-infundibular fold (VIF) and supraventricular crest.

The VIF is the muscular “sling” interposed between tricuspid inflow and pulmonary outflow. Together with the muscular outlet septum and the anterior limb of the SMT, it forms the crista (supraventricular crest) that separates inflow and outflow within the RV—an arrangement particularly evident in congenital RVOT lesions and crucial when enlarging the RVOT or resecting obstructing muscle (TOF/DCRV) [1,4].

2) Membranous septum and tricuspid relationships

The membranous septum (MS) has two recognized parts: an atrioventricular (RA–LV) component and an interventricular (RV–LV) component, both adjacent to the septal leaflet of the tricuspid valve. These relationships explain why sutures placed along the posteroinferior rim of perimembranous VSDs risk the conduction tissue (see §3) [4,5]. In TOF and some DORV phenotypes, the muscular outlet septum inserts into or just in front of the SMT’s supero-anterior limb, altering the local geometry at the RVOT and the proximity of conduction tissue to planned resections [6].

Muscle of Lancisi.

Arising near the bifurcation of the SMT (trabecula septomarginalis), this slip reinforces the anterior–septal commissure of the tricuspid valve and marks the corridor where the RBB emerges on the RV septal surface before entering the moderator band (see §3) [4,6].

3) Conduction system: from Koch’s triangle to the RV free wall

AV node and Koch’s triangle.

The AV node lies within Koch’s triangle, bounded by the coronary sinus orifice, the tendon of Todaro, and the septal leaflet of the tricuspid valve—a perennial caution zone for septal incisions and annular sutures [5]. Quantitative anatomic studies confirm the constancy of these boundaries and reinforce their value for avoiding nodal injury during atrial and AV-junction procedures [5].

His bundle and bundle branches.

After penetrating the MS, the His bundle divides into left and right bundle branches. The RBB travels along the septal surface, emerges on the RV surface just posterior to the muscle of Lancisi, then courses within the moderator band toward the anterior papillary muscle, arborizing across the RV free wall [4]. In perimembranous and outlet VSDs, the RBB’s course shifts relative to the defect rims; recent clinicopathologic work underscores that relying on the medial papillary muscle alone is unsafe—attention to the membranous flap and posteroinferior rim more accurately anticipates bundle location during patching and resection [6].

4) Putting the map to work — surgical pearls

• RVOT work (TOF/DCRV). Respect the VIF–SMT–outlet septum complex; resect obstructing muscle while preserving the RBB corridor as it emerges posterior to the muscle of Lancisi and enters the moderator band. Understanding whether the outlet septum inserts into or anterior to the SMT helps anticipate where fibromuscular narrowing and conduction tissue will converge [1,4,6].
• Perimembranous VSD closure. Keep stitches superficial and short along the posteroinferior rim near the tricuspid septal leaflet to avoid the penetrating bundle/AV node vicinity around Koch’s triangle; avoid traction that might distort the septal leaflet and expose the conduction axis [4,5,6].
• Tricuspid repair. The muscle of Lancisi at the anterior–septal commissure is a dependable anchor; excessive division may destabilize leaflet support and encroach upon the RBB’s emergence zone [4,6].

5) Terminology cross-walk (for trainees)

• Septomarginal trabeculation (SMT) = septal band; limbs create a “Y” toward the pulmonary valve (supero-anterior) and membranous septum (infero-posterior) [2].
• Moderator band = septo–free-wall bridge carrying distal RBB fibers [3,4].
• VIF + outlet septum + anterior SMT limb = supraventricular crest dividing inflow/outflow [1,4].

References

[1] James TN. Anatomy of the crista supraventricularis: its importance for understanding right ventricular function. J Am Coll Cardiol. 1985;6(5):1083-1095.

[2] Wang JMH, Li W, Lu M. An anatomical review of the right ventricle. Int J Cardiol Heart Vasc. 2019;22:199-207.

[3] González-Casal D, Seoane L, Fernández-García J, et al. The Right Ventricular Moderator Band: From Leonardo da Vinci to the 21st Century. JACC Case Rep. 2024;6(9):102647.

[4] Cabrera JÁ, Ho SY, Sánchez-Quintana D. The atrioventricular conduction axis and its implications for ablation procedures. Arrhythm Electrophysiol Rev. 2021;10(4):230-240.

[5] Klimek-Piotrowska W, Holda MK, Koziej M, et al. Geometry of Koch’s triangle. Europace. 2017;19(3):452-457.

[6] Yoneyama F, Brenes JC, Bista B, et al. Right bundle branch in ventricular septal defects. J Thorac Cardiovasc Surg Tech. 2025; (Epub ahead of print). PubMed PMID: 40128147.

[7] Bashore TM, Freed MD. Right Ventricular Outflow Tract Lesions. Circulation. 2007;115(14):1933-1947.

[8] Suzuki A, Ho SY, Anderson RH, Becker AE. Further morphologic studies on tetralogy of Fallot, with particular emphasis on the aortic-tricuspid fibrous continuity. J Thorac Cardiovasc Surg. 1990;99(3):528-537.