AV


 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 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.


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/Cent
isthmus
Inferolateral
Isthmus


 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:


 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 Tissue
Muscle 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.


Penetrating
Bundle
Tendon of
Todaro
Transitional
tissue
Inferior Nodal
Extension
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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