1.6: Cardiac and Atrioventricular Conduction

The Cardiac Conduction System

The full complement of the histologically specialised tissues making the conduction system of the heart comprises the sinus node and the atrioventricular system (Figure 1.6.1). The latter is made up of the atrioventricular node, the penetrating atrioventricular bundle and the ventricular bundle branches. The geometry of the right atrium is such that it is made up of bands of muscle which separate the orifices of the great veins and the oval fossa. The spread of excitation from the sinus to the atrioventricular node has been shown to spread preferentially along these broad bands of ordinary atrial myocardium.

Figure 1.6.1: The cardiac conduction system. Normally, the insulating fibro-fatty tissue plane at the atrioventricular junction prevents atrial myocardium from contacting ventricular myocardium. The penetrating bundle is the only muscular bridge.

The Sinus Node

The 'ultimum moriens', the last part of the heart to stop beating when the organ is isolated from the body, first prompted Wenckebach to believe that this may also be the seat of the heart beat.[1] The discovery of the sinus node in the heart of a mole culminated in a paper in 1907 in which Keith and Flack described 'a remarkable remnant of primitive fibres persisting at the sino•auricular junction in all mammalian hearts. These fibres are in close connection with the vagus and sympathetic nerves, and have a special arterial blood supply; in them the dominating rhythm of the heart is believed to normally arise'.[2] The subsequent elegant combined anatomico-physiological studies of Lewis and the Oppenheimers in 1910 confirmed the pacemaking role of the sinus node.[3] The sinus node predominantly occupies an antero-lateral location of the superior cavo-atrial junction within the terminal groove (Figure 1.6.2A). Only occasionally it is horseshoe-shaped draping over the right atrial summit. In most adult hearts it is shaped like a tadpole measuring about 3mm in diameter at its widest part and 15 to 20mm in length. A tapering 'tail' of the node may be traced from the epicardium to pass intramyocardially toward the inferior part of the terminal crest. The sinus node is easily recognised by the light microscope at low magnification. It is made up of small cells grouped together in interconnecting fascicles set in a fibrous tissue matrix (Figure 1.6.2B). The fibrous matrix becomes more prominent with increasing age. At the margins of the node is a short transitional area where nodal cells merge into atrial myocardium. In places, discrete tongues of transitional cells are found which extend into the terminal crest and toward the myocardial sleeve of the superior caval vein. The blood supply to the node shows considerable variation. A main artery penetrating the length of the node is seen in some hearts. In others, the nodal substance is penetrated by ramifications of an artery approaching the node through one or both ends, there being variations in nodal approaches. Even the origin of the sinus node artery is diverse, arising from the right or left coronary artery at different locations. Collections of ganglion cells are usually observed in the epicardium and also in the environs of the sinus node.

Figure 1.6.2: (A) The sinus node (dotted shape) is superimposed onto the terminal groove in this picture of the right atrium viewed from the right side. The arrows indicate the sectioning plane of the histological section shown in B. (B) This section from an infant heart is stained in Masson’s trichrome stain that colours myocardium red and fibrous tissue blue. The sinus node is readily identifiable by its composition of small myocytes in a fibrous matrix.

The Atrioventricular Conduction System

Occasional reference to this as the system of His-Tawara gives credit to two of the pioneering investigators in this field. The myocardial bridge that connects atrial myocardium to ventricular myocardium across the insulating fibro-fatty tissues of the atrioventricular junction was found by His in 1893 and given the appellation ‘penetrating bundle of His’.[4] Tawara's monograph[5] accompanied by colour plates in 1906 gave a detailed description of the atrioventricular node and how it was a continuum with the bundle described by His and the ventricular fibres previously described by Purkinje.[6] This firmly estabIished the presence of an atrioventricular conduction system (Figure 1.6.1) and was subsequently confirmed by Keith and Flack in the same year.[7] Gross anatomical landmarks to the location of the atrioventricular system are invaluable guides to cardiac surgeons and interventionists who have to perform intracardiac procedures since trauma to any part of the system can produce dire complications.

The atrioventricular node is located at the apex of an angle formed by the tendinous continuation of the Eustachian valve (tendon of Todaro) and the annular insertion of the septal leaflet of the tricuspid valve (Figure 1.6.3). The coronary sinus completes the base of the triangular shape which bears the name 'triangle of Koch' in recognition of Koch's elegant descriptions.[8] The tendon of Todaro inserts into the central fibrous body. In the adult the atrioventricular node measures about 4 mm in width and 8 mm in length. In histological sections the compact part of the node is easily recognisable being composed of interconnecting fascicles of small cells, closely adherent to the central fibrous body. In cross•section the node appears like a haIf-oval lying against the fibrous body (Figure 1.6.3D). A transitional zone of attenuated myocardial cells extends into the atrial myocardium. The node becomes the penetrating bundle as the conduction system passes through the central fibrous body (Figure 1.6.3C).

Figure 1.6.3: (A) This view of the right atrium and right ventricle shows the anterior and posterior borders of the triangle of Koch (broken lines) that mark location of the atrioventricular node and bundle (orange shapes). The arrows B, C, D indicate the cuts made through the conduction system as shown on the histologic sections. B, C and D are step sections stained with Masson’s trichrome technique and displayed in similar orientation tracing the atrioventicular conduction system from the AV node (AVN) that adjoins the central fibrous body (cfb), to the penetrating His bundle (H), and the branching bundle (BB) dividing into the left (LBB) and right (RBB) bundle branches.

The penetrating bundle veers to the left as it continues into the branching bundle to emerge in the left ventricle beneath the commissure that separates the right-coronary and non-coronary aortic valve leaflets. The bifurcation into left and right bundle branches marks the beginning of the branching bundle (Figure 1.6.3B). The right bundle branch is cord-like and frequently is the continuation of the nodal-bundle axis. It turns downwards and passes intramyocardially into the substance of the septomarginal trabeculation directly beneath the medial papillary muscle complex. It then passes subendocardially towards the right ventricular apex and crosses the ventricular cavity within the moderator band before ramifying. The left bundle branch is morphologically different from the right bundle branch. It descends from the nodal-bundle axis as a sheet of cells within the subendocardial tissues of the aortic outflow tract. Tawara's original reconstructions show the bundle radiating in fan-like fashion into three major divisions which are interconnected distally by a subendocardial network that ramifies into the ventricular myocardium (Figure 1.6.4).[5] Later investigations using careful serial reconstructive techniques support the trifascicular concept seemingly in conflict with the 'hemiblock' theory which promotes a bifascicular morphology.[9]

Figure 1.6.4: This picture from Tawara’s monograph (1906) shows the tree-fascicular arrangement of the left bundle branch in man. (Tawara S 1906 Das Reizleitungssystem des Säugetierherzens. Eine Anatomisch-Histologische Studie Über das Atrioventrikularbündel und die Purkinjeschen Fäden. Gustav Fischer, Jena.)