4.1. Anatomical Considerations
The first reason this pacing method does not apply widely until now is the formation and anatomical location of the AV conduction system. The AV node is located from the perspective of the right atrium within the triangle of Koch, an area defined by heterogeneous structures. On one side is the tendon of Todaro in the Eustachian ridge, just below is the ostium of the coronary sinus, and on the other side is the constantly moving septal leaflet of the tricuspid valve.9 From this position, the AV node, as it converts into the His Bundle, enters the electrically inactive fibrous cardiac skeleton, then passes to the membranous portion of the interventricular septum, where its two bundles, right and left, after their separation, advance into the myocardium of the corresponding areas of the septum.
Additionally, it is not only the complex path of the conduction system but also the dimensions of these formations: the mean length, width, and thickness of the compact AV node are 3.5 ± 1.2 mm, 4.5 ± 1.1 mm, and 1.2 ± 0.3 mm, respectively, and the tubular His Bundle has only a length of 20 mm and a diameter of 4 mm.10 But even the placement of the His Bundle within the myocardium anatomically varies and can be distinguished into three types: His bundle may run under the membranous portion of the interventricular septum, it may run within the muscular portion of the interventricular septum beyond the lower border of its membranous portion, so in this case, it is hard to achieve pure selective pacing, and also it may be located beneath the endocardium with no surrounding myocardial fibers.11 Therefore, the position and course, the minuscule size, and the anatomic variations of the conduction system are a challenge to be confronted with fluoroscopy to reveal electrical signals and give a stimulation. That is precisely where the second consideration for HBP lies.