Читать книгу Myocardial Torsion - Jorge C. Trainini - Страница 12

As previously expressed, the opposite orientation and rotational movement of the left ventricular fibers, both at the basal and distal apical segment levels, explain Torrent Guasp’s myocardial band model. The author considered that the ventricular myocardium is formed by an assembly of muscle fibers coiled unto themselves as a rope (rope theory) (Figure 1.4) and flattened laterally to form a band, which by giving two spiral turns describes a helix limiting the two ventricles and defining their function. In turn, the phylogenetic study allows to theoretically understand, through the 600 million-year evolution of the circulatory system, that the ends of the band are located at the root of the great vessels, as it is formed from a loop of the primitive circulatory duct. (109)

Figure 1.14. Segment sequence from the myocardial band histological analysis.

The muscle shaping the right ventricle corresponds to the origin of the myocardial band (right segment), which begins both in the cardiac fulcrum as in the fibrous structures related with the pulmonary artery and the tricuspid annulus (pulmo-tricuspid cord) (Figures 1.15 to 1.17). The ascending segment, which is part of the autochthonous muscle bundles constituting the left ventricle, ends at the base of the aorta, and the cardiac fulcrum constitutes a solid point of attachment of the end of the myocardial band.

Figure 1.9 demonstrates that the aortic annulus is not continuous. Its circumference is interrupted between the ends of the trigones, in the region where the mitral valve anterior cusp is inserted. Our research has demonstrated that in the course of the aortic annulus septal segment, extending from the left to the right trigone, there is a thickening we have called cardiac fulcrum (below and in front of the right trigone) where most of the myocardial band is attached, since as any muscle needs a supporting point to develop the leverage required to fulfill its function.

The insertion of the myocardial fibers in the fibrous skeleton of the heart has been considered for three centuries. The development of the myocardial band reported in 1970 by Torrent Guasp (106-108) indicates in the anatomical research that it originates and ends at the root of the great vessels, but that the fibers do not insert in the atrioventricular annuli. The myocardium attaches to these annuli but does not insert in them. In our investigations we have not found insertion of cardiomyociytes in the collagen matrix of the trigones (Figure 1.39).

However, Torrent Guasp considered that the myocardial band lacked a fixed supporting point as those present in the muscular system to develop force. In this sense, he assumed that it behaved as the circular muscle of the arteries, finding support in its own chamber blood content (hemoskeleton). In our research, we have always considered that the myocardial band should have a fixed point of attachment to allow its helical rotation in order to achieve its motions and the essential muscle force of shortening-twisting and lengthening-untwisting. The study of a supporting point in the myocardial band finds correlation with an organic engine, such as the heart, which without a firm attachment to a resistant nucleus would lack the necessary mechanical faculties for its considerable power.

Figure 1.15. Detail of the pulmo-tricuspid cord between the tricuspid orifice and the pulmonary artery where the myocardial band begins. The circle shows the site of the cardiac fulcrum, which is visualized when the pulmonary artery and the pulmo-tricuspid cord are removed from their insertion at the beginning of the myocardial band unfolding (observe figure 1.27). The right coronary artery has been cut to reveal the cord trajectory (bovine heart).

This point of attachment implies, as in any muscle, its ability to achieve the necessary leverage and also to act as a bearing or pad, preventing the force of ventricular rotation, either by torque or torsion strain from transferring to the aorta, thus dissipating the energy produced by the helical muscle motion, and avoiding aortic constriction or bending during systolic ejection. (134)

Young bovine and human hearts (from spontaneous abortion fetuses, explanted adult hearts and cadaveric hearts retrieved from the morgue) were used to study the myocardial band insertion. The dissection was performed as described in Section 4: “Anatomy of the cardiac band. Dissection”, of this same chapter.

In anatomical investigations we have found in all the bovine and human hearts studied a nucleus underlying the right trigone, whose osseus, chondroid or tendinous histological structure depends on the specimen analyzed, and is oriented towards the muscle fibers of the ascending segment which intimately penetrate its structure to attach themselves. This point of attachment would serve to support both the origin and end of the myocardial band, as the fibers that initiate the right segment, origin of the myocardial band, attach to the anterior part of this nucleus as well as to the pulmo-tricuspid cord (Figure 1.16).

Figure 1.16. Descriptive photograph of the muscle bundles emerging from the cardiac fulcrum, which belong to the right segment forming the right ventricle (transverse section of a bovine heart).

This osseus, chondroid or tendinous attachment point is found in the vicinity of the tricuspid valve (right), the aorta (posterior) and the pulmo-tricuspid cord (anterior) (Figure 1.20). To find it, it is necessary to unfold the myocardial band and move the pulmonary artery, the pulmo-tricuspid cord and the right segment to the left of the observer, stripping the root of the aorta (as a scarf that separates from the neck). Next, the muscular plane of the descending segment is separated from that of the ascending segment to follow the latter up to its insertion in the cardiac fulcrum. This maneuver reveals the fulcrum in front of the aorta and below the right trigone and the origin of the right coronary artery (Figures 1.18 and 1.19), detached from the aortic continuity and inserted as a complementary element between the aorta and the myocardium. This structure, origin and end of the myocardial band, has a more rigid consistency than the trajectory between the trigones.

Figure 1.17. Panoramic view of the pulmo-tricuspid cord.

An osseus formation, called os cordis, in bovine and sheep hearts, is reported in the veterinary literature in the same location where we have studied this structure both in bovids and humans. (69, 134) Beyond its mere mention in bovids, no reason for its presence or function was ever assigned to this structure, and it has not been described in humans. In bovids, its consistency is osseus at palpation, which has been confirmed by histological studies, and its size, according to our studies, is of approximately 45 mm × 15 mm, being its triangular shape (Figure 1.20).

Figure 1.18. The figure shows the ascending segment, which is going to insert in the cardiac fulcrum (bovine heart) (see Figure 1.19).

Figure 1.19. Cardiac fulcrum below the right trigone (bovine heart). The insert shows the resected piece.

Figure 1.20. Resected cardiac fulcrum

(bovine heart).

The findings in the human hearts are surprising from the point of view of interpretation, since it is logic to consider its presence in all the evolutionary mammalian chain. It should be assumed that this structure, analyzed in different species, has the common function of providing support to the myocardial band to generate the power needed by any muscle, which is different in diverse mammals. Therefore, its presence is constant in all the hearts studied, both in bovids and humans, but its structural characteristic is different, and this diversity in the intimal analysis of the cardiac fulcrum is undoubtedly associated with the resistance it must oppose to the activity of the myocardial band in hearts of different sizes.

The analysis of a 10-year-old human heart showed in the same place a myxoid-cartilaginous formation with approximately 2 cm diameter (Figure 1.21). A similar finding, both in structure and location, occurred in the heart of a 23-week gestation human fetus (Figures 1.22 and 1.23).

A fact defying logic is having found in the adult human heart a formation presenting consistent characteristics, both to observation and palpation, in the same location and with similar triangular morphology (Figures 1.24 to 1.27). However, the histological analysis revealed a matrix similar to that of a tendon.

The macroscopic and microscopic observation reveal the muscle fiber attachment to this solid, homogeneous nucleus, which is closely related with the aortic wall on the side of the tricuspid valve. Its configuration has been confirmed histologically. We have called his structure, origin and end of the myocardial band, cardiac fulcrum, as a parallelism and tribute to the definition of the supporting point of a lever expressed by Archimedes of Syracuse (Greek, 288 B.C. – 212 B.C.). It is located anterior to the central fibrous ridge (right trigone) and it clearly shows that the myocardial fibers of the right segment originate in its structure (Figure 1.16) same as the ascending segment courses to meet it in order to attach (Figures 1.18 and 1.19). It should be noted that to visualize the cardiac fulcrum it is essential to unfold the myocardial band. This osseous, cartilaginous or tendinous structure was always present and with the same morphology in all the hearts analyzed by us, albeit with different histological texture. No description of its characteristics or function has been reported in the literature, except the mere mention of its presence as os cordis in bovids.

Figure 1.21. Cardiac fulcrum in a 10-year-old human heart (explant).

Figure 1.22. Human embryo heart (23-week gestation) showing sectioning planes.

Figure 1.23. Cardiac fulcrum in a 23-week gestation human embryo hear.

Figure 1.24. Fulcrum in the adult human heart.

Figure 1.25. Fulcrum in

the adult human heart.

Figure 1.26. Fulcrum resected from

an adult human heart.

Figure 1.27. The drawing shows the myocardial band arrangement at the beginning of its unfolding. The pulmonary artery and the right segment have been separated from the fulcrum to show its intermediate location between the right segment (anterior location) and the ascending segment (posterior location). A: macroscopic image of the fulcrum in the adult human heart. B: microscopic image of the human fulcrum. Notice the myocardial fibers inserting into the tendinous fulcrum matrix

Histology. The microscopic analysis of the bovine cardiac fulcrum shows a trabecular osteochondral matrix with segmental lines. Its general structure resembles the metaphyseal growth of the long bones (Figures 1.28 and 1.29) and increased magnification reveals bone trabeculae with osteoblasts and segmental lines secondary to bone apposition (Figure 1.30).

Figure 1.28. Histological section of the cardiac fulcrum showing trabecular bone tissue with osteologic segmental lines (bovine heart). Hematoxylin-eosin stain.

Figure 1.29. Mature trabecular bone forming the cardiac fulcrum tissue (bovine heart). Hematoxylin-eosin stain at low magnification (10×).

Figure 1.30. Cardiac fulcrum trabecular bone with osteoblasts and segmental lines. The structure constitutes the trabecular bone tissue scaffolding similar to the metaphyseal area of growth in long bones. The histological section also shows bone trabeculae with osteoblasts and segmental lines secondary to bone apposition (bovine heart). Hematoxylin-eosin stain at high magnification (40×).

Figure 1.31. Ten year old human heart. Central area of the fulcrum formed by chondroid tissue in a 10-year-old human heart. Hematoxylin-eosin stain.

Figure 1.32. Cardiac fulcrum in a 23-week gestation fetus showing prechondroid bluish areas in a myxoid stroma. Masson’s trichrome staining technique.

In the 10-year-old human heart, the histological findings in the cardiac fulcrum were associated with that early age. Figure 1.31 shows a central zone of the fulcrum formed by chondroid tissue. Given the 10-year-old age, it is logical that the fulcrum is smaller and characterized by more chondroid than bone tissue. This finding was repeated in a 23-week old human fetus with the characteristic prechondroid bluish areas in a myxoid stroma (Figure 1.32).

The osseous structure in the bovine os cordis and its relationship with the myxoid-chondroid cardiac fulcrum texture in human hearts, even in gestational stages, is rational for the interpretation analysis. This disparity is associated with the different age evolution from chondroid to osseous material and by the greater force developed in bovids requiring a more rigid supporting point.

Figure 1.33. Myocardial insertion in

the cardiac fulcrum. Bovine heart.

Figure 1.34. Insertion of myocardial fibers in the fulcrum

chondroid tissue of a bovine heart.

However, the histological analysis of the fulcrum in adult human hearts evidenced a tendinous collagenous matrix, needing an additional clarification. In principle, there is constancy in the detection, site and morphology of the fulcrum in all the hearts analyzed. This means that from a functional point of view, its presence is akin to myocardial band insertion, as established in the histological analysis, becoming a solid point of interpretation to achieve its biomechanical function. And we find this demonstration when the histological examination is directed to the myocardial insertion in the osseous, chondroid or tendinous fulcrum. All the hearts studied presented this myocardial attachment to the rigid structure of the fulcrum, integrating a cardiomyocyte-matrix unit, regardless of its osseous, cartilaginous or tendinous nature (Figures 1.33 to 1.38).

Figure 1.35. Festooned cardiomyocytes penetrating the fibro-colagenous matrix (adult human heart).

Figure 1.36. Fibrocolagenous matrix at higher magnification (adult human heart).

At this point, fundamental questions arise. Why does the human fulcrum have characteristics similar to a tendon, despite it fulfills the same function of attaching the myocardial band? Why does it not have the same structure of the fetal or child human heart?

Figure 1.37. Festooned colagenous fibers integrating the fibrotendinous matrix of the fulcrum (adult human heart).

Figure 1.38. Cardiomyocytes penetrating the fibrocolagenous tissue (adult human heart). The circle details the insertion site.

Our interpretation is that perhaps the osseous fulcrum is a vestigial organ specific of mammalian evolution. A vestigial structure must be understood as the preservation during the evolutionary process of genetically established attributes which have lost all or part of their ancestral function in a certain species. (45) As a result, it is found in the initial process of human gestation, but later loses its osseous character, remaining as a tendinous matrix able to achieve myocardial band insertion to attain a muscle power which is much lower than that of larger mammals. Recall that in bovids the fulcrum found in this investigation is of bone nature (Figures 1.28 to 1.30).

Figure 1.39. Collagen tissue and elastoid interstitial tissue are observed. There is no inclusion of cardiomyocytes.

A histological analysis has also been carried out on the trigones trying to find cardiomyocytes in them, as a possibility of insertion of the band in these structures. In our investigation, only collagen tissue was observed without cardiomyocytes in the trigones, confirming that the fulcrum is the support of the band, both in its beginning and in its termination (Figure 1.39).

Fulcrum imaging studies. Bovine hearts studied with computed tomography (Figures 1.40 to 1.42), magnetic resonance imaging (Figures 1.43 and 1.44) and X-rays (Figure 1.45) identified the osteochondral nucleus found in dissection, with the same morphology and analogous size.

Figure 1.40. Computed tomography. A hyperdense, approximately 3.7 cm long image, with 298 HU density is seen in the interventricular septum adjacent to the root of the aorta (bovine heart).

Figure 1.41. Computed tomography of the cardiac fulcrum (bovine heart). The inset shows the resected fulcrum.

Figure 1.42. Computed Tomography. In the area indicated by the arrow there is an image adjacent to the aortic root on the interventricular septum (bovine heart).

Interpretation. The anterior portion of the cardiac fulcrum is associated with the pulmo-tricuspid cord (Figures 1.15 to 1.17). It is situated between the pulmonary artery and the tricuspid valve and in front of the aortic valve at the level of the right coronary artery, giving rise to the origin of the myocardial band (right segment). Since the right segment separates into two main groups of fibers forming the paraepicardial and paraendocardial bundles, a raphe is defined between then called the pulmo-tricuspid cord.

According to histological studies, analyzed in our investigations, the osseous, chondroid or tendinous fulcrum is close to the tricuspid valve (right), the aorta (posterior) and the pulmo-tricuspid cord (anterior). In order to find it, it is necessary to unfold the myocardial band stripping the aortic root (Figure 1.27) and separating the muscle plane of the descending segment from that of the ascending segment. Then, the latter must be followed up to its insertion in the fulcrum. Before affixing to it, the ascending segment gathers into a bundle whose most external fibers form a curve to attain this attachment, while the most internal fibers enter directly without describing any deviation.

Figure 1.43. Nuclear magnetic resonance image in the bovine heart.

Figure 1.44. Nuclear magnetic resonance image in the bovine heart.

The pulmo-tricuspid cord is located in front of an area that surrounds the anterior two-thirds of the aortic annulus circumference in a U shape, and its open end (posterior) is occupied by the anterior cusp of the mitral valve. This tissue has a trigone at each end. The right fibrous trigone (central fibrous mass) is the most prominent and is placed between the tricuspid (right) and aortic (posterior) orifices and the pulmo-tricuspid cord (anterior) (Figure 1.9). The left less prominent trigone is located between the mitral valve orifice (left) and the aorta. There is no connective tissue in the continuity of the aortic orifice with the posterior cusp of the mitral valve, as laterally, the two fibrous masses extend by means of a band of connective tissue that partially surrounds the mitral valve orifice to progressively vanish. The septal cusp of the mitral valve is found as a wedge between both trigones and can be considered an extension of the atrial endocardium.

Figure 1.45. Radiologic image of the cardiac fulcrum with mammography technique in the bovine heart.

The cardiac fulcrum is adjacent and anteroinferior to the right trigone, constituting a solid, homogeneous structure to palpation with osseous, chondroid or tendinous histology. It is the site where the right segment and the ascending segment fibers attach, origin and end of the myocardial band. These insertions must be understood as the supporting point of the myocardial band to fulfill its hemodynamic function.

Regarding the study on this point of attachment which we have termed cardiac fulcrum, it must be considered as an organic structure that supports the band, allowing it to develop the necessary force for the fundamental rotational motions of the left ventricle. (131, 132) We should not forget that the heart pendulates within the thoracic cavity and its points of attachment are the origin and end of the myocardial muscle band, very close to each other at the root of the aorta and of the pulmonary artery. The origin of the band at the cardiac fulcrum is essentially a point of attachment for the right segment forming the band origin and also its end with the ascending segment. This anatomical site finds correspondence with the active motions of the cardiac cycle: left ventricular systole and suction. The site of the cardiac fulcrum in the rope model shown in Figure 1.46 simplifies the understanding of the myocardial band.

Figure 1.46. Site of the cardiac fulcrum in the myocardial band (rope model). The inset shows (in red) the schematic diagram of the rope model overlapping with the myocardium.

Conclusion. This anatomical description, both in bovid and human hearts, would put an end to the lack of support of the myocardial band to fulfill its twisting function, as the cardiac fulcrum (Figure 1.47) is the insertion point to achieve the necessary leverage, same as a muscle with its bone insertion. The observation of the tendinous cardiac fulcrum as a meeting point between the origin and end of the myocardial band would confirm its helical configuration. Moreover, by achieving energy dissipation, this nucleus would avoid the twisting motion of the ventricular helix from shifting to the aorta and hampering systolic ejection.