Aortic valve

The aortic valve is considered the most studied, since it has been described long ago, beginning with Leonardo da Vinci (1513) and Valsalva (1740), and repeatedly, especially during the second half of the 20th century. At the same time, studies of past years were mainly descriptive or, more rarely, comparative. Starting with the work of J Zimmerman (1969), in which the author proposed to consider "the function of the valve as a continuation of its structure," most of the research became morphologically functional. This approach to the study of the function of the aortic valve through the study of its structure was, to a certain extent, due to the methodological difficulties of direct investigation of the aortic valve biomechanics in general. Functional anatomy studies allowed to determine the morphofunctional boundaries of the aortic valve, clarify the terminology, and to study its function to a large extent.

Due to these studies, the aortic valve has been broadly understood as a single anatomical and functional structure related to both the aorta and the left ventricle.

According to modern concepts, the aortic valve is a volumetric funnel-shaped or cylindrical structure consisting of three sinuses, three Henlel interlacial triangles, three semilunar wings and a fibrous ring whose proximal and distal boundaries are respectively ventriculoaortal and sinotubular junctions.

The term "valve-aortic complex" is used less commonly. In the narrow sense, aortic valve is sometimes understood as a blocking element consisting of three valves, three commissures and a fibrous ring.

From the point of view of general mechanics, the aortic valve is considered as a composite structure consisting of a strong fibrous (force) skeleton and relatively thin shell elements (sinus and sash walls) placed on it. The deformations and displacements of this skeleton occur under the action of internal forces arising in the shells fixed on it. The framework, in turn, determines the deformations and movements of the shell elements. The framework consists mainly of tightly packed collagen fibers. This design of the aortic valve determines the longevity of its function.

The sinuses of the Valsalva are an enlarged part of the initial aorta, bounded proximally by the corresponding segment of the fibrous ring and the valve, and distally by the sinotubular junction. Sinuses are named according to the departing coronary arteries right coronary, left coronary and non-coronary. The wall of the sinuses is thinner than the aortic wall and consists only of intima and media, somewhat thickened by collagen fibers. At the same time, the amount of elastin fibers decreases in the sine wall, and the collagenous increases in the direction from the sinotubular to the ventriculoaortal junction. Dense collagen fibers are located predominantly on the outer surface of the sinuses and are oriented in the circumferential direction, and in the subcommissioned space they take part in the formation of interstitial triangles supporting the shape of the valve. The main role of sinuses is to redistribute the tension between the valves and sinuses in the diastole and to establish the equilibrium position of the valves to the systole. Sinuses are divided at the level of their base by interstitial triangles.

The fibrous framework that forms the aortic valve is a single spatial structure of the strong fibrous elements of the root of the aorta, the fibrous ring of the base of the valves, the commissural rods (posts) and the sinotubular junction. Sinotubular junction (an arched ring, or an arched comb) is a wavy-shaped anatomical connection between the sinuses and the ascending aorta.

Ventriculoaortic joint (valve base ring) is a rounded anatomical connection between the output section of the left ventricle and the aorta, which is a fibrous and muscular structure. In foreign literature on surgery, the ventriculoortic joint is often referred to as the "aortic ring". Ventriculoaortal compound is formed, on average, by 45-47% from the myocardium of the arterial cone of the left ventricle.

The commissure is a line connecting (connecting) adjacent flaps with its peripheral proximal margins on the inner surface of the distal segment of the root of the aorta and extending its distal end to the sino-tubular junction. The commissural rods (posts) are the places of commissure fixation on the inner surface of the root of the aorta. The commissural columns are the distal extension of the three segments of the fibrous ring.

The intersecting triangles of Henle are fibrous or fibro-muscular components of the aortic root and are located proximal to the commissure between adjacent segments of the fibrous ring and the corresponding valves. Anatomically interstitial triangles are part of the aorta, but functionally they provide exit paths from the left ventricle and are affected by the ventricular hemodynamics, and not the aorta. Interstitial triangles play an important role in the biomechanical function of the valve, allowing the sinuses to function relatively independently, unite them and support a single geometry of the root of the aorta. If the triangles are small or asymmetric, then a narrow fibrous ring or the distortion of the valve develops with subsequent disruption of the function of the valves. This situation can be observed with the bicuspid valve of the aorta.

Valve is the valve closure element, its proximal margin extending from the semilunular part of the fibrous ring, which is a dense collagen structure. The valve consists of the body (the main part being loaded), the surface of coaptation (closing) and the base. The free edges of adjacent flaps in the closed position form a coaptation zone extending from the commissure to the center of the flap. Thickened triangular shape of the central part of the coaptation zone of the valve was called the node of Aranzi.

The leaf that forms the aortic valve consists of three layers (aortic, ventricular and spongy) and is covered externally with a thin endothelial layer. Layers facing the aorta (fibrosa), mainly contains collagen fibers oriented in the circumferential direction in the form of bundles and strands, and a small amount of elastin fibers. In the coaptation zone of the free edge of the leaf, this layer is present as separate bundles. Collagen beams in this zone are "suspended" between commissural columns at an angle of approximately 125 ° relative to the aortic wall. In the body of the bundle, these bundles move at an angle of about 45 ° from the fibrous ring in the form of a half-ellipse and terminate on its opposite side. This orientation of the "power" beams and the edges of the leaf in the form of a "suspension bridge" is designed to transfer the pressure load in the diastole from the valve to the sinuses and the fibrous scaffold that forms the aortic valve.

In the unloaded flap, the fibrous beams are in a contracted state in the form of wavy lines arranged in a circumferential direction at a distance of about 1 mm from each other. The collagen fibers constituting the bundles in the relaxed leaf also have a wavy structure with a wave period of about 20 μm. When the load is applied, these waves straighten, allowing the tissue to stretch. Completely straightened fibers become inextensible. The folds of collagen beams easily straighten out with a slight loading of the leaf. These beams are clearly visible in the loaded state and transmitted light.

The constancy of the geometrical proportions of the elements of the root of the aorta has been studied by the method of functional anatomy. In particular, it was found that the ratio of the diameters of the sinotubular joint and the valve base is constant and is 0.8-0.9. This is true for valve-aortic complexes of young and middle-aged people.

With age, qualitative processes of abnormal aortic wall structure occur, accompanied by a decrease in its elasticity and the development of calcification. This leads, on the one hand, to its gradual expansion, and on the other hand, to a decrease in elasticity. Changes in geometric proportions and a decrease in the dilatability of the aortic valve occur at the age of over 50-60 years, which is accompanied by a decrease in the opening area of the valves and a deterioration in the functional characteristics of the valve as a whole. Age-related anatomical and functional features of the aortic root of patients should be taken into account when implanting frameless biological substitutes in the aortic position.

A comparison of the structure of such an education as the aortic valve of man and mammals was performed in the late 60s of the XX century. In these studies, the similarity of a number of anatomic parameters of the porcine and human valves was shown, unlike other xenogeneic aortic roots. In particular, it was shown that the human non-coronary and left coronary sinus valves were, respectively, the largest and smallest. At the same time, the right coronary sinus in the pork valve was the largest, and the non-coronary sinus was the smallest. At the same time, differences in the anatomical structure of the right coronary sinus of the porcine and human aortic valve were described for the first time. In connection with the development of reconstructive plastic surgery and aortic valve replacement with biological frameless substitutes, anatomical studies of the aortic valve have resumed in recent years.

[1], [2], [3], [4], [5], [6], [7]

Human aortic valve and aortic pork valve

A comparative study of the structure of the human aortic valve and the pork aortic valve as a potential xenograft has been carried out. It was shown that xenogeneic valves have a relatively low profile and in most cases (80%) are asymmetric due to the smaller size of their non-coronary sinus. Moderate asymmetry of the human aortic valve is due to the smaller size of its left coronary sinus and is not so pronounced.

The pork aortic valve, unlike the human, does not have a fibrous ring and its sinuses do not directly border the base of the valves. Pig wings are attached by their semilunar base directly to the base of the valve, since there is no true fibrous ring in the pork valves. The bases of xenogeneic sinuses and valves are attached to the fibrous and / or fibrous-muscular parts of the valve base. For example, the base of the non-coronary and left coronary valves of the pork valve in the form of diverging leaves (fibrosa and ventnculans) are attached to the fibrous base of the valve. In other words, the valves that form the pork aortic valve do not directly adhere to the sinuses, as in the allogeneic aortic roots. Between them is located the distal part of the valve base, which in the longitudinal direction (along the axis of the valve) at the level of the proximal point of the left coronary and non-coronary sinuses is, on average, 4.6 ± 2.2 mm, and the right coronary sinus - 8.1 ± 2.8 mm. This is an important and significant difference between the pork valve and the human valve.

Muscular insertion of the aortic cone of the left ventricle along the axis in the porcine root of the aorta is much more significant than in the allogeneic root. In porcine valves, this implantation formed the base of the right coronary valve and the sinus of the same name, and to a lesser extent the base of the adjacent segments of the left coronary and non-coronary valves. In allogeneic valves, this injection creates only support to the base, mainly, the right coronary sinus and, to a lesser extent, the left coronary sinus.

Analysis of the size and geometric proportions of individual elements of the aortic valve, depending on intra-aortic pressure, has been used frequently in functional anatomy. To do this, the aortic root was poured with various hardening agents (rubber, paraffin, silicone rubber, plastics, etc.), and also made its structural stabilization by chemical or cryogenic method under different pressures. The resulting impressions or structured aortic roots were studied by the morphometric method. This approach to the study of the aortic valve made it possible to establish certain patterns of its functioning.

In vitro and in vivo experiments, it was shown that the root of the aorta is a dynamic structure and most of its geometric parameters change during the cardiac cycle, depending on the pressure in the aorta and the left ventricle. In other studies, it was shown that the function of the valves is largely determined by the elasticity and extensibility of the root of the aorta. Vortex blood movements in the sinuses were assigned an important role in the opening and closing of the valves.

The study of the dynamics of the geometric parameters of the aortic valve was carried out in an animal experiment using the methods of high-speed film angiography, cinematography and cinodiography, and also in healthy individuals with the help of kinangiocardiography. These studies made it possible to accurately assess the dynamics of many elements of the root of the aorta and only presumably assess the dynamics of the shape and profile of the valve during the cardiac cycle. In particular, it was shown that systolodiastolic expansion of the sinotubular compound is 16-17% and is closely correlated with arterial pressure. The diameter of the sinotubular junction reaches its maximum values at the peak of the systolic pressure in the left ventricle, thereby facilitating the opening of the valves due to the divergence of the commissure to the outside, and then decreases after the closure of the valves. The diameter of the sinotubular junction reaches its minimum values at the end of the phase of isovolytic relaxation of the left ventricle and begins to increase in the diastole. The commissural bars and the sinotubular junction, due to their flexibility, participate in the distribution of the maximum stress in the flaps after they are closed during the period of rapid growth of the inverse transvalvular pressure gradient. Mathematical models were also developed to explain the movement of the leaflets during their opening and closing. However, the data of mathematical modeling largely did not agree with the experimental data.

The dynamics of the base of the aortic valve affects the normal operation of valve flaps or an implanted frameless bioprosthesis. The perimeter of the base of the valve (dog and sheep) was shown to reach the maximum value at the beginning of the systole, decreased during systole and was minimal at its end. During the diastole, the perimeter of the valve increased. The base of the aortic valve is also capable of cyclic asymmetric changes in its size due to a reduction in the muscular part of the ventriculoaortic joint (interstitial triangles between the right and left coronary sinuses, as well as the bases of the left and right coronary sinuses). In addition, shearing and torsion of the root of the aorta were detected. The largest twisting deformations are noted in the field of the commissural column between the non-coronary and left coronary sinuses, and the minimal ones between the coronary and right coronary sinus. Implantation of a frameless bioprosthesis with a semi-rigid base can change the compliance of the aortic root to torsional deformations, which will lead to the transfer of twisting deformations to the sinotubular connection of the composite aortic root and the formation of the distortion of the bioprosthetic flaps.

The normal aortic valve biomechanics in young people (an average of 21.6 years) was studied by transesophageal echocardiography with subsequent computer processing of the video image (up to 120 frames per second) and analysis of the dynamics of geometric characteristics of the aortic valve elements depending on the time and phases of the cardiac cycle. It was shown that the systole significantly changes the valve opening area, the radial angle of the flap inclination to the valve base, the diameter of the valve base and the radial length of the valve. The diameter of the sinotubular junction, the circumferential length of the free edge of the sash and the height of the sinus are less affected.

Thus, the radial length of the valve was maximal in the diastolic phase of the isovolytic reduction of intraventricular pressure and the minimum - in the systolic phase of the reduced exile. The radial systolodiastolic stretch of the leaf was, on average, 63.2 ± 1.3%. The valve was longer in diastole with a high diastolic gradient and shorter in the phase of the reduced blood flow, when the systolic gradient was close to zero. The circumference of the systolic and diastolic distention of the valve and sinotubular junction was 32.0 ± 2.0% and 14.1 ± 1.4%, respectively. The radial angle of the flap inclination to the base of the valve varied, on average, from 22 to diastole to 93 ° in systole.

The systolic movement of the valves that form the aortic valve was conventionally divided into five periods:

1. the preparatory period fell on the phase of the isovoluminal increase in intraventricular pressure; the valves were straightened, somewhat shorter in the radial direction, the width of the coaptation zone decreased, the angle increased, on average, from 22 ° to 60 °;
2. the period of rapid opening of the valves lasted 20-25 ms; with the beginning of the expulsion of blood at the base of the valves, an inversion wave was formed, which rapidly spread radially to the body of the valves and further to their free edges;
3. The peak of the opening of the valves was in the first phase of maximum expulsion; During this period, the free edges of the leaflets bent as far as possible towards the sines, the shape of the valve opening approached the circle, and in the profile the valve resembled the shape of a truncated inverted cone;
4. the period of relatively stable opening of the valves fell to the second phase of maximum expulsion, the free edges of the flaps straightened along the axis of the flow, the valve took the form of a cylinder, and the flaps gradually covered; By the end of this period, the shape of the valve opening became triangular;
5. The period of rapid closure of the valve coincided with the phase of reduced exile. At the base of the wings a wave of reversion formed, stretching the contracted flaps in the radial direction, which led to their closing at the ventricle edge of the coaptation zone first, and then to the complete closure of the valves.

The maximum deformations of the aortic root elements occurred during the periods of rapid opening and closing of the valve. With a rapid change in the shape of the valves that form the aortic valve, high stresses can arise in them, which can lead to degenerative changes in the tissue.

The mechanism of opening and closing the leaf with the formation, respectively, of the wave of inversion and reversion, as well as an increase in the radial angle of the flap inclination to the valve base in the phase of isovolumic pressure increase inside the ventricle can be attributed to the damping mechanisms of the aortic root, which reduces deformations and strains of valve flaps.