Saroja Bharati.
Maurice Lev Congenital Heart and Conduction System Center, The Heart Institute for Children, Hope Children’s Hospital, Christ Hospital and Medical Center, Oak Lawn, Rush Medical College, Rush University, Rush-Presbyterian-St. Luke’s Medical Center, Chicago, Finch University of Health Sciences, Chicago Medical School, North Chicago, University of Illinois at Chicago, Chicago, USA
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The anatomy of the atrioventricular septal junction is not as simple as
one would like it to be. There are several anatomic structures situated at the atrioventricular junction in very
many directions with varying histologic counterparts (Fig. 1)1-13. Therefore, one can deal with this subject in a
simplistic superficial manner or one can discuss the issue in a very profound manner. I am going to take the
middle of the road approach. By that, I mean, I will present the morphology of the atrioventricular septal junction
in a relatively simple manner that will be found useful by the electrophysiologist with particular emphasis to the
terms “triangle of Koch” and “isthmus.” This discussion, therefore, will not include the accessory by pass
atrioventricular pathways and connections that may be present in and around the atrioventricular conduction
system.
Fig. 1: Schematic representation of teh atrioventricular junction depicting the approaches to the
atrioventricular node and the atrioventricular node.
RA = right atrial myocardium - superior approaches; LA = left atrial myocardium - left atrial approaches;
CFB = central fibrous body; VS = summit of the ventricular septum; MV = approaches from the mitral
valve; TV = approaches from the tricuspid valve; RV = right ventricular side theseptum - right
ventricular approaches; LV = left ventricular side of the septum; AVN = atrioventricular node. Arrows
point tothe approaches tothe atrioventricular node from the tricuspid valve area, right atrial aspect,
right ventricular aspect, atrial septal aspect, left atrial and mitral valvular aspect. With permission from
Bharati and Lev1.
The so called “triangle of Koch” extends from the mouth of the coronary sinus to the central fibrous body and
extends to the septal leaflet of the tricuspid valve and extends from the latter to the coronary sinus.
Thus the so called “triangle of Koch” takes into the areas of the atrioventricular node and its approaches and the
penetrating atrioventricular bundle1-13. The atrioventricular node and its approaches and the penetrating
atrioventricular bundle in a broad sense comprises the entire atrioventricular septal junction (Fig. 1). Thus, the
atrioventricular septal junction lodges that part of the conduction system and the surrounding structures such as:
1.the distal atrial preferential pathways;
2.the atrioventricular node and its approaches;
3.the atrioventricular bundle, penetrating part;
4.the coronary sinus;
5.the eustachian and thebesian valves;
6.the anulus of the septal leaflet tricuspid valve;
7.the anulus of the mitral valve-posterior commissure;
8.the aortic valve base;
9.the central fibrous body;
10.the membranous part of the atrioventricular septum;
11.the tendon of Todaro;
12.the summit of the ventricular septum – the right, the left.
The distal atrial preferential pathways
The distal atrial preferential pathways include the anterior or superior, posterior or inferior and atrial septal
myocardial fibers as they merge with the posterior and anterior approaches to the atrioventricular node.
The atrioventricular node and its approaches
Likewise, the approaches to the atrioventricular node is formed by the posterior and the anterior aspects of the
right atrium including the atrial septal myocardial fibers. One must also include the atrial myocardial fibers that
fan out from the sinoatrial node superiorly or anteriorly, anteroseptally, anterolaterally and posteriorly. The
posterior approaches include the myocardial fibers from the coronary sinus, left atrium and mitral valve.
The superior or anterior approaches include the right atrial, atrial septal and left atrial fibers. It is emphasized
that the approaches from the septal leaflet of the tricuspid valve are closely related to right side of the summit of
the ventricular septum. The approaches from the tricuspid valve are those atrial myocardial fibers that extend
from the annulus of the septal leaflet of the tricuspid valve to the central fibrous body including the myocardial
fibers from the right ventricular side of the septum (Fig. 1).
Approaches related to the coronary sinus
The approaches beneath the coronary sinus area may be called the posterior or inferior approaches. The left
atrial approaches include those fibers from the left atrial myocardium and the mitral valve.
At light microscopic level, the approaches are loosely arranged atrial myocardial cells, somewhat light staining
with different morphological characteristics.
It is to be noted that dominance of one approaches over the other is seen histologically. The functional
significance of these variations are yet to be determined. It is also of interest that such dominance of one
approaches over the other as they merge with the approaches to the atrioventricular node are seen in young
sudden death victims.
The atrioventricular node and the atrioventricular bundle
The size, shape and location of the atrioventricular node and the penetrating atrioventricular bundle vary
considerably. Likewise, size and shape of the other structures such as the coronary sinus, eustachian valve, the
tendon of Todaro, the central fibrous body and the membranous part of the atrioventricular septum vary a great
deal. No two hearts look alike, either normal or abnormal.
Thus, the so called “Koch’s triangle” anatomically includes the most vital parts of the conduction system such as
the approaches to the atrioventricular node, the atrioventricular node and the penetrating atrioventricular bundle
and the other anatomic structures that are intimately related to them. It is emphasized that anatomically the
so-called “triangle of Koch” is a wide area and it varies in size and shape considerably from heart to heart and
the atrial myocardial fibers merge with the conduction fibers from different directions with different histologic
counterparts (Fig. 1).
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