The conduction systems of the hearts of large mammals are very similar (1). The main structures of the conduction system include the sinoatrial node (SA node), the atrioventriular node (AV node), the bundle of His, the right and left main bundle branches and the Purkinje fibers; all of these structures are present in humans, canine, swine and ovine (1). Pacemaking is performed by the SA node, located high on the right atrial wall near the junction of the superior vena cava and the right atrium (3, 10, 11). Beginning in the SA node, the depolarization of cells, which triggers contraction, travels through the atria to the AV node, located subendocardially in the area between the coronary sinus ostium, membranous septum and the septal/posterior commissure of the tricuspid valve know as the "triangle of Koch". The signal then spreads to the bundle of His, also located in the triangle of Koch, which penetrates through the central fibrous body separating the atria and ventricles. The bundle of His then bifurcates into right and left main bundle branches, which branch further to become Purkinje fibers that spread conduction to the ventricles. Differences in the conduction system primarily reside in the arrangement of the transitional and compact components of the AV node and in the length and route of the bundle of His.

Human
The AV node of the human heart is located at the base of the atrial septum, anterior to the coronary sinus and just above the tricuspid valve, a location that is similar to that seen in dogs (27). Unlike in dogs and sheep, the transition point, where the AV node meets the bundle of His, is difficult to distinguish. The bundle of His is located just below the membranous septum at the crest of the interventricular septum (27). The unbranched portion of the bundle of His extends 2 to 3 millimeters before penetrating the central fibrous body for a length of 0.25 to 0.75 millimeters. The bundle then bifurcates immediately after emerging from the central fibrous body (27).

Canine
The AV node of the canine heart is located at the base of the atrial septum, anterior to the coronary sinus and just above the tricuspid valve, in a position similar to that seen in humans (30). The junction joining the AV node and the His bundle consists of internodal tracts of myocardial fibers. At least three different His bundle branches extend from the AV node via a proximal branch of the His bundle (30). The penetrating bundle of the bundle of His is 1 to 1.5 mm long, significantly longer than that of humans, and runs forward and downward, just beneath the endocardium, through the fibrous base of the heart (30).

Ovine
The AV node of the ovine heart is located at the base of the atrial septum, anterior to the coronary sinus and just above the tricuspid valve (29). This position is also at the junction of the middle and posterior one-third of the os cordis. The junction between the AV node and His bundle is clearly defined as finger-like projects where the two tissues overlap. These two tissue types are easily identifiable by both size and histological examinations (29). The unbranched portion of the His bundle passes beneath the os cordis to reach the right side of the ventricular septum and remains relatively deep thereafter. The portion passing through the central fibrous body is ~1 mm long and extend 4 to 6 mm beyond the central fibrous body before bifurcating, a much more anterior bifurcation than that seen in humans (29).

Porcine
The AV node of the porcine heart lies on the right side of the crest of the ventricular septum, more inferiorly on the septum than in humans (28). From the bundle of His, the conduction path climbs to the right side of the summit of the ventricular septum and enters the central fibrous body. Notably, the bundle of His bifurcates much earlier along the conduction pathway compared to the conduction system of humans (28).