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.