Название: Myocardial Torsion
Автор: Jorge C. Trainini
Издательство: Bookwire
Жанр: Медицина
isbn: 9789876917971
isbn:
2 The evolutionary concept emerging from phylogeny.
3 New imaging procedures obtained with magnetic resonance imaging by diffusion tensor. (17, 24, 82, 142)
4 Echocardiography. (59, 70, 71)
5 Electrophysiological studies performed with three-dimensional electroanatomical mapping. (121-127, 131, 132)
Regarding the argued difficulty to dissect the myocardium, which is more apparent than real (128), we should consider that once the myocardial band originated as a loop in the arterial semicircle of amphibians and reptiles to adapt to the physiological demands of aerial life, the muscle bundles became firmly attached to their contact surfaces, hampering the necessary cleavage planes necessary for their anatomical dissection. The evolutionary goal was to develop a sufficiently solid hemodynamic structure with the strength to generate the suction and pumping of the blood volume that supplied the whole organism. Thus, every attempt to dissect an anatomical segment from the rest of the myocardium, avoiding the real cardiac arrangement, always turns into an obstacle due to the structural plan of the axes where the orientation of the myocardial band courses.
An explanation for this muscular homogeneity hiding the myocardial band, implies considering the required functioning in birds and mammals to achieve blood ejection at a high speed in a limited time span by an organ that must supply two circulations (systemic and pulmonary). Despite these considerations and the classical concept about myocardial anatomy, its dissection finds a structure with defined planes where the successive and related physiological heart motions of 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 СКАЧАТЬ