Anatomy of AV Junction

Dr.Ritesh Ramachandran

Anatomically, the atrioventricular junction comprises

of right and left parietal junctions with a small septal

component.

The right parietal junction is relatively circular.

The left parietal junction surrounds the orifice of the

mitral valve and the area of fibrous continuity

between mitral and aortic valves.

The true septal component is limited to the area of

the central fibrous body and immediate

surroundings.

The Atrioventricular Junctions Anatomy :

At the atrioventricular junctions there is NO

myocardial continuity between the Atrium and

Ventricle except at the site of the penetrating

bundle of the atrioventricular conduction tissues.

The AV conduction bundle penetrates through central

fibrous body.

The Atrioventricular Junctions Anatomy

The AV ring ..

  The coronary sinus, which is present between

the orifice of the IVC and the AV opening, is

protected by a valve of Thebesius.

The triangle of Koch is delineated posteriorly by

the tendon of Todaro, anteriorly by the septal

leaflet of the tricuspid valve, and inferiorly by the

coronary sinus.

The apex of the triangle is marked by the central

fibrous body through which the atrioventricular

conduction bundle penetrates.

Right Atrium Anatomy :

The wall of the right atrium containing the specialised

tissues is known as the Triangle Of Koch. Borders are,

Posteriorly, a fibrous extension from the eustachian

valve called the Tendon Of Todaro and,

Anteriorly, the line of attachment of the septal leaflet of

the Tricuspid Valve.

Inferiorly by the Coronary sinus

The apex of the triangle is the membranous part of the

septum, which is the site of penetration of the

conduction axis.

Triangle of Koch

AV node

The orientation of the AV node

Anomalous muscular AV connections at the AV

junctions produce the Wolff-Parkinson-White

variant of ventricular preexcitation.

AV BTs connect the atria to the ventricle and

can cross the AV groove anywhere along the

mitral and tricuspid annulus, except between

the left and right fibrous trigones.

The triangle of Koch :

1. The apex of the triangle is marked by the central fibrous body through which the atrioventricular conduction bundle penetrates

2. The so-called fast pathway corresponds to the area of musculature close to the apex of the triangle of Koch.

Right Atrium Anatomy - Importance:

The area between The Inferior

Caval Vein and the Tricuspid

Valve is known as the cavo-

tricuspid isthmus.

The posterior component is

mainly fibrous, whereas the

anterior component is the

musculature of the atrial

vestibule and has a smooth

endocardial surface.

Right Atrium Anatomy :

Cavotricuspid Isthmus

Paraseptal Isthmus

Inferior/Centralisthmus

InferolateralIsthmus

Within this area are marked three isthmuses:

Paraseptal Isthmus

Inferior Or Central Flutter Isthmus ,and

Inferolateral Isthmus.

The Inferior Isthmus passes through the Sinus

Of Keith (triangle),the atrial wall inferior to the

orifice of the coronary sinus.

Cavotricuspid Isthmus

ISTHMUS :

1. Area between the IVC and the

TV corresponds to the isthmus

of slow conduction in the

circuit of common atrial flutter

2. The Inferior Isthmus is the

most appropriate target to

ablate.

3. Paraseptal isthmus is the area

often targeted for ablation of

the slow pathway in AVNRT .

Right Atrium Anatomy - Importance:

Structure of av node

The atrioventricular node (AVN), was initially

characterized by Sunao Tawara in 1906,. Tawara's original monograph, Das

Reizleitungssystem des Saugetierherzens (The Stimulus Conducting System of Mammalian Hearts).

AV Node is also known as “Tawara’s Node”.

The normal AV junctional area can be divided into distinct regions:

1. The transitional cell zone

2. The compact portion, or the AV node itself

3. The penetrating part of the AV bundle (his bundle)

4. Inferior nodal extension,

5. Atrial and ventricular muscle,

6. Central fibrous body

7. Tendon of todaro, and

8. Valves

Atrioventricular Junctional Area

The transitional cells differ histologically from atrial myocardium and connect the latter with the compact portion of the AV node.

The compact portion of the AV node is a superficial structure lying just beneath the right atrial endocardium at the apex of triangle of Koch , 5 mm long and wide.

In triangle of Koch, the Tendon Of Todaro, which forms one side of the triangle of Koch, is absent in about two thirds of hearts.

The arterial supply to the AV node is a branch from

the RCA in 85 to 90 percent of human hearts, A branch of the LCX provides the AV nodal artery in the remaining hearts

Fibers in the lower part of the AV node may exhibit automatic impulse formation

The compact portion of the AV node is divided from and becomes the penetrating portion of the his bundle at the point where it enters the central fibrous body

Morphologically the AV node can be further divided into:- the Lower Nodal Bundle (LNB), the cells are

longer and arranged more parallel to one another. Extending proximally from the LNB toward the CS is the Inferior Nodal Extension (INE), or Rightward Nodal Extension.

Compact Node (CN). The cells are small and spindle-shaped with no clear orientation. The Second nodal extension (or Leftward Nodal Extension), extends from the CN toward the CS, and is usually shorter than the rightward extension (RE).

The structure of the AV Node

3D structure of the AVJ based on expression patterns of Cx43 which delineate two discrete structures.Green denotes the His bundle, yellow denotes the LNB and RE

(a Cx43-positive region), and blue denotes the CN and LE (a Cx43-negative region).

The identification of rightward and leftward

nodal extensions provided a basis for an anatomical correlate of SP conduction.

the leftward extension and CN (LE/CN) expressing virtually no Cx43, and

the RE and LNB (RE/LNB) staining positive for Cx43.

there is also evidence of Cx43 expression extending from the AVJ into the proximal His bundle.

Three main types of AV node cells are present, based on action potential morphology:-

Nodal cells have a low resting potential, a small amplitude

action potential with a slow upstroke, and pacemaker activity.

Atrionodal cells are transitional cells .The resting potential is

higher and the action potential upstroke is larger and faster

than in nodal cells

Nodo-His cells are also transitional cells. The resting potential

of nodo-His cells is higher and the action potential upstroke is

larger and faster than in nodal cells.

Atrial Muscle Action Potential

Transitional TissueMuscle Action Potential

Bundle of His Action Potential

Penetrating Bundle Action Potential

During normal anterograde AV conduction, the action potential and enters the tract of nodal tissue at two points:-

The first point is at the end of the inferior nodal extension

(next to the penetrating bundle) via the transitional tissue.

This conduction pathway most likely corresponds to the fast

pathway route.

Second, the action potential enters toward the beginning

of the inferior nodal extension.

This conduction pathway likely constitutes the slow pathway

route.

PenetratingBundle

Tendon ofTodaro

Transitionaltissue

Inferior NodalExtension

Coronary sinus

Because nodal and atrial tissues are isolated from each other by a vein along its length, the action potential cannot enter the nodal tissue at other tissue points .

From the two entry points, the action potentials propagate both anterogradely and retrogradely along the inferior nodal extension and eventually annihilate each other.

The action potential entering the nodal tract via the transitional zone propagates into the compact node and then reaches the His bundle and propagates down the left and right bundle branches.

Clinically, the AV Node plays important roles:- in coordinating and maintaining appropriate

AV conduction, protecting the ventricles from atrial

tachyarrhythmias, and functioning as a backup pacemaker in the

setting of sinoatrial (SA) node dysfunction.

Anatomically, there are two pathways consisting of

the RE/LNB and the LE/CN that can be identified on a histological and molecular basis.

Functionally, the AVJ can be described as having two pathways, the SP and the FP.

the anatomical substrate for the SP involves structures embedded within an isthmus of myocardium located along the tricuspid annulus below the CS.

Evidence exists involving the area of the RE as the anatomical substrate of the SP.

Correlating Structure and Function

The FP is less well defined from an anatomic

and structural standpoint.

The probable anatomical substrate of this

pathway is the transitional cell layers located

around the CN at the interface between the CN

and transitional cells, which express Cx43.

The anatomical bases of the FP and SP are present

in the majority of hearts despite the relatively low

frequency of AVNRT diagnoses.

However, 84% of patients undergoing

radiofrequency ablation of an accessory pathway

with no history of AVNRT functionally demonstrated

the existence of dual pathways.

AVNRT in infants is rare; however, the incidence

increases during childhood from 13% in school age children to 50% in older teenagers, representing a developmental change that

occurs within the AVJ likely driven by an increase in size of nodal extensions and by an alteration in gene expression of connexins, various ion channel isoforms, and receptors responsible for conduction during the ageing process possibly increasing the likelihood of reentry based on gene expression patterns.

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