Myocardial Torsion. Jorge C. Trainini

narrowing, shortening-twisting, lengthening-untwisting and expansion take place depending on the propagation of the electrical stimulus along its muscle pathways (chapter 1). (125)

      The myocardial fibers forming the myocardium cannot be considered as absolutely independent entities within a defined space. Despite the intricacy of fiber bundles with polygonal shape, which in addition receive and give off collateral fibers, a predominant course of central fibers is defined with sliding planes, which together form the myocardial muscle band. It should be recalled that the myocardium constitutes a spiraling continuum in its fibers responding to the helical pattern in its muscle bundles. This arrangement indicates the need of generating a mechanical work that dissipates little energy. Therefore, the fiber layers very gradually shift their orientation, with more or less acute angles, to avoid that abrupt changes in the spatial organization dissipate the necessary work for cardiac function. The fan of fibers that is formed reduces the stress among them.

      This situation generates a tangle of fibers that allows the band to behave as a continuous transmission chain with the epicardial fibers taking an oblique direction, the intermediate fibers a transverse course and the endocardial fibers also an oblique direction, but contrary to that of the epicardial plane. The endocardial and epicardial plane access angle is approximately 60 degrees in relation to the transverse fibers. Fiber orientation defines function and thus the ejection fraction is 60% when the normal helical fibers contract and falls to nearly 30% if only the transverse fibers shorten. This occurs when the left ventricle dilates in cardiac remodeling and the fibers miss their oblique orientation, loosing muscular and mechanical efficiency.

      It should therefore be acknowledged that a gradual change in orientation is generated from the superficial to the deep fibers that form the different segments of the muscle band. In the progression from the ventricular base to the apex, the number of horizontal fibers decreases in relation to the oblique fibers, showing that the heart is organized as a continuous muscle helix. The ventricular mechanical activity must be heterogeneous during diastole with subendocardial-subepicardial relaxation gradients. During systole, the muscle layers of the myocardial band evidence pronounced and opposite torsion in the subendocardium in relation to the subepicardium, whereas in the apex the subepicardial fiber rotation acquires more relevance.

      Beyond this complexity it is necessary to establish the concept of linear and laminar trajectories. Myocardial muscle bundles and bands, which derive from phylogenetic development, essentially shape a master axis of precise dynamic requirement. The spatial muscle structure adopted by the myocardial muscle band has a double function: a) to limit the ventricular chambers and b) to fulfill the suction and driving action in its role of cardiac pump.

2. Myocardial architecture

      Left ventricle. The entire apex belongs to the left ventricle. In the distal part of the left ventricle, called apical, a muscle layer with spiral trajectory extends from the surface to the center and undergoes a rotation that turns the subepicardial fibers into subendocardial fibers, overlapping like the tiles of a roof. Consequently, the left ventricular distal end, the apex, surrounds a virtual conduit with no muscular plane at its ultimate end, lined internally by the endocardium and externally by the epicardium, with no intermediary muscle. It is essential to consider that in the apex the fibers undergo a helical rotational motion with sphincter-like arrangement as they transform from subepicardial to subendocardial fibers, following a clockwise trajectory (apical view of the diaphragmatic surface of the heart in the anatomical position) (Figures 1.5 and 1.6). (105)

      Figure 1.5. Apical view of the left and right ventricles.

      Figure 1.6. Spiral arrangement of apical muscle layers.

      In the left ventricular basal half, at the level of its free wall (Figure 1.7), the fibers are arranged similarly to the apical half. A muscle layer with spiral trajectory extends from the surface to the center displaying its fibers from outside to inside (from paraepicardial to paraendocardial regions). At this level their orientation is opposite to that of the apex, following a counter-clockwise trajectory (apical view of the diaphragmatic surface of the heart in anatomical position). This arrangement of the muscle layer in its twist limits a cavity which at the base of the heart is real and not virtual as in the apex.

      Figure 1.7. Basal third of the left ventricle.

       It depicts the muscle layers of the free wall.

      The apex should be considered as a tunnel with a muscular rim surrounding its entire ring, while at the ventricular base this ring has two parts. One part corresponds to the left ventricular free wall and the other to the interventricular septum. In addition, the most superficial basal fibers make contact with the fibrous mitral annulus, a setting that is absent at the apical level. Nevertheless, the essential functional difference between basal and apical regions is their opposite fiber motion. This characteristic determines myocardial torsion to achieve cardiac blood ejection and the untwisting that generates suction and diastolic filling.

      Right ventricle. Theoretically, two types of fibers can be identified in its distal half according to their orientation: paraendocardial and paraepicardial fibers. The former extend from the pulmonary base backwards and downwards to the apical region, whereas the others extend from the anterior interventricular sulcus to the back approaching the base of the heart. This X-shaped crossover arrangement allows the fibers in the distal end of the right ventricle to adopt a helical arrangement, turning from subepicardial to subendocardial (Figure 1.8).

      Figure 1.8. Right ventricular free wall.

      Three segments can be identified at the basal half of the right ventricle (tricuspid orifice perimeter): free wall, supraventricular crest and interventricular septum. The free wall presents the same general configuration of spiraling fibers that extend from subepicardial to subendocardial positions. Similarly to the left ventricle, there is a difference in the rotating sense of the basal fibers with respect to the distal ones. They follow a counterclockwise trajectory at the basal end and a clockwise trajectory at the distal end (apical view of the diaphragmatic surface of the heart in anatomical position).

      The comparison of the rope model (Figure 1.4) with the ventricular apical and basal regions (Figure 1.1) shows the analogy existing between the rope configuration and the myocardial fibers.

3. Segmentation of the myocardial band

      In its course, the myocardial band adopts a helical configuration that defines the two ventricular chambers. The myocardial band is characterized by a descending and an ascending band. The former includes the right, left and descending segments, while the latter is formed by the remaining ascending segment (Figure 1.4). The figure in 8 outlined by this trajectory defines two loops: a basal and an apical loop. An interesting article by R. F. Shaner in 1923 (84) expresses that the myocardium is formed by “two flattened muscles arranged into a figure of 8. These muscles shorten in opposite directions in systole, emptying the blood content.”

      The myocardial band describes two spiral turns with the insertion of its initial end along the line extending from the pulmonary artery to the orifice of the tricuspid valve, called the pulmo-tricuspid cord, in front of the aorta, while its final end attaches below the aortic root. Both ends are fixed by an osseous, chondroid or tendinous nucleus, depending on the different species (animal or human) used in the studies. This nucleus, which has been termed cardiac fulcrum, is the only perceptible edge where the muscle band fibers originate and end, and will be extensively analyzed in this chapter (Figures 1.16 and 1.19).

      The basal loop extends from the base of the pulmonary artery to the central