9.4 Left Atrioventricular Fibrous Support

Left Atrioventricular Fibrous Support constitutes the dense connective tissue framework that structurally reinforces the left atrioventricular (mitral) valve apparatus. This fibrous support is an essential component of the heart’s fibrous skeleton, providing mechanical stability and electrical insulation between the atrial and ventricular myocardium. It anchors the mitral valve leaflets, ensuring proper coaptation during ventricular systole and maintaining valvular competence.

Structural Overview

The left atrioventricular fibrous support is composed of a complex network of interconnected fibrous segments forming a rigid yet flexible ring around the mitral orifice. This fibrous ring is continuous with other elements of the cardiac skeleton, including the fibrous trigones and the membranous septum, integrating the mitral valve into the heart’s overall connective tissue framework.

The main subdivisions of this fibrous support are:

• Anterior Left AV Fibrous Segment
• Posterior Left AV Fibrous Segment
• Medial Left AV Fibrous Segment
• Lateral Left AV Fibrous Segment

Each segment contributes to specific anatomical and functional roles in the support and attachment of the mitral valve and adjacent cardiac structures.

Segments of Left Atrioventricular Fibrous Support

Anterior Left AV Fibrous Segment

This segment forms the fibrous continuity between the anterior mitral leaflet and the aortic valve’s noncoronary and left coronary cusps. It is a key area where fibrous tissue bridges the left atrium, left ventricle, and the aortic root, creating the aortomitral curtain. This continuous fibrous sheet prevents direct muscular connection between the left atrium and ventricle at this junction, serving as an electrical insulator.

Posterior Left AV Fibrous Segment

The posterior segment encircles the posterior mitral leaflet and is less rigid than the anterior portion. It is characterized by a fibrous ring interspersed with adipose tissue and is intimately related to the posterior left atrial wall and the coronary sinus. This area provides attachment for the posterior mitral leaflet and contributes to the flexibility required for valve opening and closing.

Medial Left AV Fibrous Segment

Located medially, this segment is adjacent to the membranous septum and fibrous trigone area. It interfaces with the atrial and ventricular septal structures, forming part of the fibrous skeleton that supports the septal leaflet of the tricuspid valve as well. It plays a critical role in maintaining the structural integrity of the interventricular septum and the annular continuity.

Lateral Left AV Fibrous Segment

The lateral segment surrounds the lateral aspect of the mitral annulus, attaching the lateral mitral leaflet to the left atrial wall and left ventricular myocardium. It provides a firm anchorage point that allows the leaflet to maintain its position and function during the cardiac cycle, accommodating the dynamic changes between systole and diastole.

Fibrous Continuity Patterns and Interfaces

Left AV Fibrous Continuity Pattern

The left atrioventricular fibrous support exhibits a pattern of fibrous continuity that integrates the mitral valve with the aortic valve and the surrounding myocardium. The aortomitral curtain, formed chiefly by the anterior fibrous segment, is the hallmark of this continuity, ensuring mechanical cohesion between the left ventricular outflow tract and the mitral valve apparatus.

Left AV Framework-Atrial Muscle Interface

At the atrial side, the fibrous ring of the left AV support interfaces with the atrial myocardium. This interface allows the transmission of contractile forces and maintains the spatial orientation of the mitral annulus relative to the left atrium, facilitating coordinated atrioventricular mechanical function.

Left AV Framework-Ventricular Muscle Interface

The ventricular interface involves the fibrous ring blending into the ventricular myocardium, particularly the base of the left ventricle. This integration provides a strong anchor for the mitral valve leaflets and chordae tendineae, enabling effective valve closure during ventricular systole and preventing regurgitation.

Left AV Framework-Mitral Attachment Interface

The mitral valve leaflets are attached circumferentially to the fibrous ring of the left AV support. This attachment is reinforced by collagenous tissue, ensuring leaflet stability and flexibility. The fibrous support prevents annular dilation and preserves the three-dimensional shape of the mitral annulus, which is crucial for optimal valve function.

Left AV Fibrous Support Map (Conceptual Diagram)

Functional Significance

The left atrioventricular fibrous support fulfills several vital functions:

• Mechanical Stability: It maintains the shape and size of the mitral annulus, preventing dilatation during ventricular contraction and relaxation.
• Valve Competence: By securely anchoring the mitral leaflets, it ensures effective leaflet coaptation, preventing mitral regurgitation.
• Electrical Insulation: The fibrous tissue acts as an insulator preventing aberrant electrical conduction between atria and ventricles, thus preserving normal cardiac rhythm and conduction pathways.
• Integration with Cardiac Skeleton: It serves as a central hub integrating the mitral valve with other heart components, including the aortic valve and membranous septum, maintaining overall cardiac structural integrity.

Clinical Relevance

Alterations or damage to the left atrioventricular fibrous support, such as from calcification, infection (endocarditis), or degenerative disease, can lead to mitral valve dysfunction including regurgitation or stenosis. Surgical repair or replacement of the mitral valve requires careful consideration of this fibrous framework to restore normal valve function and cardiac mechanics.

This comprehensive fibrous support system is therefore indispensable for the normal anatomy and physiology of the left heart and plays a crucial role in maintaining effective cardiac function.