Myocardial Torsion. Jorge C. Trainini
band twist. On the other hand, the apical loop courses from this point of inflexion to the base of the aorta. In turn, each loop consists of two segments. The basal loop consists of the right and left segments and the apical loop of the descending and ascending segments (Figure 1.4). In the general loop configuration, the basal loop envelops the apical loop, so that the right ventricular chamber presents as an open slit in the muscle mass thickness forming both ventricles (Figure 1.1). The segments, in turn, are defined by anatomical structures.
Basal loop. The posterior interventricular sulcus presents a trough that establishes the limit between the right and left segments of the basal loop. The right segment constitutes the right ventricular free wall and surrounds the tricuspid valve orifice on the outside. The left segment situated in the left ventricular free wall defines the mitral valve orifice externally. The fibers run from the subepicardium to the subendocardium following a counterclockwise helical direction (apical view of the diaphragmatic surface of the heart in the anatomical position).
Apical loop. The descending apical segment extends from the twist in the band to the apex, where it becomes the ascending segment to end mainly below the base of the aorta in the cardiac fulcrum, a structure to which the myocardial band fibers attach. Both segments mainly form the interventricular septum. Similarly to the basal loop, the fibers run from the subepicardium to the subendocardium, but in this case following a helical clockwise direction (apical view of the diaphragmatic surface of the heart in the anatomical position).
These concepts indicate that the right ventricular free wall is formed by one loop (basal) and the left ventricular free wall by both loops (basal and apical). The fundamental point for cardiac mechanics is that the base and apical muscle fibers course in different directions. This disparity finds correlation with the fiber trajectories and the helical pattern of the muscle band limiting the ventricles.
4. Anatomy of the cardiac band. Dissection
To dissect the heart it must be boiled in water with acetic acid (15 cc per liter) for approximately two hours and a half, except that a pressure cooker is used, in which case the time is reduced by half. Prior work before unfolding the muscle band consists in separating the atria from the ventricles in a very simple maneuver that demonstrates the different evolutionary origins between both types of chambers. Then, the aorta and the pulmonary artery are cut three centimeters from their origin, separating the attachment between them, and finally, a longitudinal incision is done on the superficial fibers (interband or aberrant fibers) (Figure 1.58) (105, 123, 128) extending transversally along the anterior wall of the ventricles. Prior boiling of the anatomical piece allows the easy execution of all these steps. Between the atrial and the ventricular walls there is only connective tissue, allowing the easy separation of these chambers due to the denaturation produced by heat.
The key maneuver to unfold the myocardial band consists in entering the anterior interventricular sulcus with a blunt instrument, leaving on the left side of the operator the end of the band corresponding to the pulmonary artery and its continuity with the right ventricular free wall (right segment). Next, traction is applied towards the same left side (Figure 1.2), completely releasing the pulmonary artery from the rest of the myocardial band (Figure 1.27). Below the aorta we find the cardiac fulcrum, where the origin and end of the band attach (Figures 1.9 and 1.10). (128)
Figure 1.9. Cardiac fulcrum. Septal part of the aortic annulus between the right and left trigones.
Figure 1.10. Cardiac fulcrum in the bovine heart.
It should be understood that as the myocardial band is unfolded, separating the pulmonary artery and the pulmo-tricuspid cord (anterior) from the ascending segment (posterior), the vision of the homogeneous anatomical reality is lost. This concurrence of the beginning and end of the muscle band in the cardiac fulcrum constitutes a meeting point between the right segment and the ascending segment, origin and end of the myocardial band. Thus, both band ends are situated in the same point, with the origin placed anteriorly to the end of the band. Once the initial end of the band is separated from this meeting point, of great stiffness and certain resistance to the maneuver, the heart loses its functional anatomy (Figures 1.3, 1.11 and 1.27).
The progression of the myocardial band dissection implies finding the whole extent of the right segment, the beginning of the left segment and, at the posterior margin of the right ventricular chamber, the dihedral angle formed by the interventricular septum and the right ventricular free wall (right segment).
The next step (the most delicate one) consists in entering the dihedral angle between the right ventricular and intraseptal fibers. This separation from the right ventricle allows entering a cleavage between the anterior septal band and the intraseptal band (final segment of the myocardial muscle band), at the ventral part of the septum (Figure 1.11 C). Then, the dorsal part of the septum is dissected between the posterior septal band and the left descending segment to remove and separate the aorta.
Finally, the trajectories of the muscle planes belonging to the descending segment are separated in blunt fashion from those of the ascending segment leading to the cardiac fulcrum, contiguously with the aorta at the opposite end of the muscle band, to the right of the operator, allowing the band to be unfolded in all its length (Figure 1.11 D). Figures 1.11 and 1.12 show the sequence of myocardial unfolding until it becomes a muscle band. Being able to unfold the myocardium with a similar thickness in all its extension proves that the band is real and not a “heuristic” construction.
Figure 1.11. Unfolding of the myocardial band.
The diagram in Figure 1.13 details the different segments comprising the band, as well as the final insertions constituting the origin and end of the band, very close to each other in the intact heart.
Histological analysis of the myocardial band. The histological analysis sequence of the unfolded myocardial band demonstrates its linear orientation according to the segmental continuity of its spatial organization when the band is coiled, both in its internal and external surfaces (Figure 1.14). These orientations are identical in both surfaces (internal and external). And if two things are identical it means that they present a unique orientation.
Figure 1.12. Myocardial band unfolded in all its extension.
Figure 1.13. Descriptive diagram of the muscle band segments.
It can be seen that the myocardial structure is not a lattice but a band. The lattice concept used was developed due to the band folding resulting in overlapping segments, which are functionally independent and with friction between their surfaces. The sliding motion between segments implies that they have a lubricating system that facilitates autonomous motion with lower energy expenditure despite being overlapped, as will be analyzed in this chapter. This arrangement is essential to achieve myocardial torsion, a fundamental action of cardiac mechanics that would be impossible with the lattice structure (crisscross of myocardial fibers).
No segment of the band histological sequence explored in our investigations presents a lattice organization. As the external surface of the distal descending segment (Figure 1.14, lower panel) twists to become the ascending segment, the cardiomyocytes generate in the planimetric histological sections a different architecture in their orientation from that of the internal surface, only site (cardiac apex) where this situations occurs. The rest of the orientation is always parallel. In the apex, the spiral course of the myocardial fibers, which shift from the periphery towards the center, determine a torsion where the subepicardial fibers become subendocardial, overlapping like the tiles of a roof,