The Right Ventricular Moderator Band

Figures

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  Figure 1. Leonardo da Vinci and the Right Ventricular Moderator Band (A) The 3 largest drawings made by Leonardo da Vinci of the ox heart (upper and middle drawings) show the right ventricle from below with the moderator band, which he called the “catena of the right ventricle,” and 2 of the 3 papillary muscles and the chordae tendineae (1511-1513). The moderator band is highlighted in the schematic view of the heart (bottom drawing) and extends between the interventricular septum and the base of the anterior papillary muscle. Reproduced with permission from the Royal Collection Trust/©His Majesty King Charles III 2024. (B) Endocardial view of the human right ventricular cavity by removing the anterior superior wall in an anatomical orientation simulated to da Vinci’s drawings. Note the moderator band, the anterior papillary muscles, and the tricuspid valve. (C) Photon-counting detector/computed tomography images and endoscopic view showing the origin of the moderator band from the right ventricular septal wall extending at the base of the anterior papillary muscle on the right free wall.

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  Figure 2. The Trabeculated Right Ventricle (A) Dissection of the right side of the heart to visualize the right atrium and right ventricular endocardium with its 3 components: inlet (tricuspid valve, leaflets, and its papillary muscles), apical trabeculations, and outlet or infundibulum/muscular cone. Although these segments do not possess clear-cut anatomical boundaries, in the analysis of hearts with congenital defects, this tripartite concept is more useful than the traditional bipartite division into the sinus and conus parts. (B) The septoparietal trabeculations and the more prominent septomarginal trabecula (SMT), continuing with the moderator band toward the base of the anterior papillary muscle, are presented. The SMT or septal band extends superiorly toward the infundibulum, supraventricular crest, and membranous septum. On the interventricular body, it extends through an inferior limb; here it divides into smaller trabeculations, including the moderator band, which forms the apical trabeculations of the right ventricle.

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  Figure 3. Moderator Band in Heart Development (A and D [magnification of A]): Four-chamber histologic sections of an 11-week-old human embryonic heart. During development, some of the trabeculations have coalesced to form the papillary muscles of the tricuspid valve, along with the extensive strap known as the septomarginal trabeculation. Note one of the septoparietal trabeculations retains its continuity with the anterior papillary muscle of the tricuspid valve and becomes the moderator band. (B and E [magnification of B]): Dissection of the right heart in a 24-week-old human heart in which the parietal wall has been displaced outward. The ventriculo-infundibular fold or supraventricular crest separates the leaflets of the tricuspid and pulmonary valves (PVs), and the pulmonary infundibulum (conus) supports the leaflets of the PV. Note how the septomarginal trabecula and the moderator band are clearly observed (dashed lines in E). (C and F [magnification of C]): Sagittal section of a 30-week-old human embryo shows the view of the ventricular cavity after removal of its parietal wall. The tricuspid valve is seen guarding its inlet, with the PV guarding its outlet. The ventricle wraps…

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  Figure 4. Subendocardial Muscular Arrangement of the Right Ventricle (A) Left image: Endocardial view of the human right ventricular cavity by removing the anterior superior wall. Note the inlet (tricuspid valve [TV]), part of the apical trabecular component, and the right ventricular outflow tract. Right image: Same orientation now showing the arrangement of the myocytes along the parietal walls, right ventricular outflow tract, and pulmonary valve. The architecture of the myocardium shows aggregates of subendocardial myocytes arranged longitudinally to the septoparietal trabeculations, septomarginal trabeculation, and medial papillary muscle, with those fibers from the septomarginal trabeculation continuing through the moderator band (red arrow) toward the antero-superior papillary muscle of the right ventricle. (B) Endocardial view of the right ventricular cavity in a human heart (left image) and dissection of the myocyte aggregates that make up the architecture of the walls of the ventricle (right image). In this specimen, the moderator band (red arrow) is thick and long, but its myocytes have the same arrangement as in the previous heart. (C) The right ventricular cavity has been…

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  Figure 5. Schematic Types of Moderator Band Morphology The graphic shows several patterns with different morphologic dimensions of the moderator band.1 (A) Short length and thick structure, making it the most common (42%). (B) Short length and thin structure (24%). (C) Greater length but maintains a thick structure (12%). (D) Long and thin structure (14%). In around 5% to 15% of cases, it is not possible to identify this muscular band.

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  Figure 6. Structure of the Normal MB in the Human Heart Dissections of the right ventricular cavity showing different morphologies of the moderator band (MB) pointed by the red arrows. (A and B) Characterized by short and thick band. Note that in panel A, the MB originates from a single root and in panel B from 2 roots. In panels A and B, the MB divides into 2 or 3 branches before reaching the anterior papillary muscle. (C and D) The MB is a long and thick structure. Note that in panel A, the origin of the MB is located near the apex of the right ventricle, whereas in panel B it originates near the tricuspid valve (TV). (E) The morphology of the short, thin structure of the MB. (F) A long and thin MB. In panels E and F, the MB divides into multiple minor trabeculations before reaching the anterior papillary muscle. AMP = anterior papillary muscle; SMT = septomarginal trabeculation.

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  Figure 7. The Right Bundle Branch and the Moderator Band (A and B) Four-chamber histologic sections show the location of the right bundle branch (RBB) (red arrows). The images, taken from the heart of a 6-month-old infant, show how the right branch is located close to the endocardium following the septomarginal trabecula and in relation to the septal papillary muscle. B is an enlarged image of A showing how the RBB enters the moderator band (MB); in this case of type I morphology, toward the anterior papillary muscle. (C and D) Gross dissections made to show the approximate locations of the components of the conduction axis viewed from the right side. Note the location of the compact AV node and penetrating His bundle (green arrow). The RBB can be seen on the right side of the interventricular septum following the septomarginal trabeculation (SMT) and MB (red arrow). The MB in C is of type I morphology and in panel D is of type III morphology. AO = aorta; CS = coronary sinus orifice; IVC = inferior vena cava; LV = left ventricle; OF = oval fossa; PA = pulmonary artery; RV = right ventricle; SVC = superior vena cava; SLTV = septal leaflet tricuspid valve; SPT = septoparietal trabeculations.

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  Figure 8. Anatomical Correlation of Photon-Counting/Detector-Computed Tomography Images (A and C) Four-chamber section showing the interatrial septum and the muscular interventricular septum. Note the most common origin of the moderator band in relation to the interventricular septum in the middle zone, which occurs in almost 70% of hearts. C is an enlarged image of the zone of the moderator band exit point, showing how the band divides into a branch to the apex of the right ventricle (red dashed lines) before attaching to the base of the anterior papillary muscle. (B and D) Photon-counting detector/computed tomography images and endoscopic view showing the origin of the moderator band of the interventricular septum and its branching from the lower edge of the band toward the apex of the right ventricle (red dashed lines). LA = left atrium; RA = right atrium.

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  Figure 9. Echocardiographic and Anatomical Correlation of the Moderator Band Echocardiography images in parasternal long-axis (A), short-axis (B), and apical four-chamber (C) planes and their correspondence with the same type of sections in 3 human hearts. The moderator band (yellow arrows) is a thick, echo-dense band-like structure that traverses the right ventricular cavity and connects the interventricular septum and the anterior papillary muscle, as seen in all 3 specimens. LV = left ventricle/left ventricular; other abbreviations as in Figures 7 and 8.

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  Figure 10. Ablation of Idiopathic PVCs From the Moderator Band Multimodality imaging to guide ablation of idiopathic premature ventricular complexes (PVCs) originating in the MB of the RV. (A) Twelve-lead electrocardiogram of a patient with symptomatic frequent PVCs with a left bundle branch block morphology and discordant inferior axis (positive in II, negative in III, and isoelectric in aVF) suggestive of an intracavitary origin in the RV. (B) Three-dimensional electro-anatomical map of the RV in AP cranial view showing earliest ventricular activation (red pin) in the mid portion of the MB, with corresponding electrograms on multielectrode catheter. (C) Long-axis view of the RV and the MB on intracardiac echocardiography. (D) Electrocardiographic imaging map of PVCs locating earliest ventricular activation site at the RV MB (red dot). (E) PA cranial view of merged three-dimensional electro-anatomical map, preprocedural magnetic resonance imaging with segmented MB anatomy, and intracardiac echocardiography contours of the MB, with the earliest activation site annotated (red arrow) and successful ablation sites (red and pink dots). MB marked in all images by red arrow. Ao = aorta; other…