Atrioventricular Node Anatomy
The atrioventricular node is a critical cardiac structure that coordinates heart rhythm by conducting electrical impulses from the atria to the ventricles.
Atrioventricular Node Anatomy refers to the detailed structural organization, position, spatial relationships, cellular composition, and vascular supply of the atrioventricular (AV) node—an essential component of the cardiac conduction system responsible for electrical signal transmission between the atria and ventricles. Understanding the anatomy of the AV node is fundamental for comprehending its physiological function and its role in cardiac rhythm disorders.
Definition and General Description
The atrioventricular node is a compact, specialized cluster of cardiac myocytes located in the right atrium of the heart. It serves as the critical electrical gateway between the atria and the ventricles, ensuring that the electrical impulses generated by the sinoatrial (SA) node are appropriately delayed and relayed to the ventricles via the His bundle.
Anatomical Position and Boundaries
Koch Triangle Nodal Region
The AV node resides within a region known as the triangle of Koch. This anatomical triangle is bounded by three key landmarks:
- The tendon of Todaro (a fibrous extension from the Eustachian valve)
- The ostium of the coronary sinus
- The septal leaflet of the tricuspid valve
Inferior Pyramidal Space Nodal Location
The AV node is embedded inferiorly in the right atrial septum, within the so-called inferior pyramidal space—a wedge-shaped area at the base of the interatrial septum, closely related to the membranous portion of the interventricular septum.
Microanatomy and Cellular Structure
Compact AV Node
The core of the AV node, known as the compact node, is composed of densely packed, small, specialized nodal cells. These cells have fewer myofibrils and abundant gap junctions, facilitating slow but coordinated conduction.
Inferior Nodal Extensions
From the compact node, strands of transitional cells extend inferiorly and laterally, known as the inferior nodal extensions. These transitional cells blend the characteristics of atrial myocytes and nodal cells, creating a gradual interface for signal propagation.
Atrial Transitional Cell Zone
Above and around the compact node, there is a zone of transitional cells connecting atrial myocardium to the nodal tissue. This zone modulates the incoming electrical impulse and provides a buffer zone for conduction.
Spatial and Structural Relationships
Relation to the Tricuspid Valve Septal Leaflet
The AV node is situated just superior and posterior to the septal leaflet of the tricuspid valve, making this valve an important surgical landmark.
Relation to the Tendon of Todaro
The tendon of Todaro forms the superior boundary of the triangle of Koch and runs immediately above the AV node, providing a guide for locating the node during procedures.
Relation to the Coronary Sinus Ostium
The ostium of the coronary sinus lies posterior and inferior to the AV node, forming the base of the triangle of Koch and marking the inferior nodal boundary.
Vascular Supply
AV Nodal Artery
The AV node receives its blood supply primarily from the atrioventricular nodal artery. In the majority of individuals, this artery is a branch of the right coronary artery, though in some cases it arises from the left circumflex artery.
| Arterial Source | Percentage of Cases |
|---|---|
| Right Coronary Artery | 80% |
| Left Circumflex Artery | 20% |
The artery enters the inferior pyramidal space and reaches the AV node at its base.
Nodal-Bundle Transition
The AV node is continuous inferiorly with the bundle of His (AV bundle). This transition zone marks the point where the nodal tissue becomes insulated by fibrous tissue and continues as the fast-conducting His bundle, which transmits impulses to the ventricular conduction system.
AV Node Spatial Map
A spatial map of the AV node in relation to nearby cardiac structures is critical in clinical electrophysiology and surgical procedures.
Functional Relevance
The unique anatomical organization of the AV node underlies its physiological role in:
- Delaying impulse conduction to allow atrial contraction to complete before ventricular contraction
- Protecting the ventricles from excessively rapid atrial impulses
- Serving as a site for various arrhythmias (e.g., AV nodal reentrant tachycardia)
Summary Table: Key Features
| Feature | Description |
|---|---|
| Location | Triangle of Koch, right atrial septum |
| Boundaries | Tendon of Todaro, tricuspid septal leaflet, coronary sinus ostium |
| Main Structure | Compact node, transitional zone, nodal extensions |
| Blood Supply | AV nodal artery (right coronary or left circumflex) |
| Clinical Relevance | Conduction delay, arrhythmia substrate, surgical landmark |
Conclusion
Atrioventricular Node Anatomy encompasses the precise localization, microstructure, and relational mapping of the AV node within the right atrium. Its unique placement at the center of the triangle of Koch, complex cellular architecture, and critical vascular supply enable its role as a central regulator of electrical conduction between atria and ventricles, making its anatomy essential knowledge for understanding normal cardiac physiology and arrhythmogenic mechanisms.