Aortic valve

The aortic valve is considered to be the most studied, as it was described long ago, starting with Leonardo da Vinci (1513) and Valsalva (1740), and repeatedly, especially during the second half of the 20th century. At the same time, the studies of past years were mainly descriptive or, less often, comparative in nature. Beginning with the work of J Zimmerman (1969), in which the author proposed to consider "the valve function as a continuation of its structure", most studies began to be of a morphofunctional nature. 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 study of the biomechanics of the aortic valve as a whole. Studies of functional anatomy made it possible to determine the morphofunctional boundaries of the aortic valve, clarify the terminology, and also study its function to a large extent.

Thanks to these studies, the aortic valve in a broad sense began to be considered 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 structure of a funnel-shaped or cylindrical shape, consisting of three sinuses, three intercuspid triangles of Henle, three semilunar cusps and a fibrous ring, the proximal and distal boundaries of which are, respectively, the ventriculoaortic and sinotubular junctions.

Less commonly used is the term "valvular-aortic complex". In a narrow sense, the aortic valve is sometimes understood as a locking element consisting of three cusps, 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 (power) frame and relatively thin shell elements (sinus walls and cusps) placed on it. Deformations and movements of this frame occur under the action of internal forces arising in the shells attached to it. The frame, in turn, determines the deformations and movements of the shell elements. The frame consists mainly of tightly packed collagen fibers. This design of the aortic valve determines the durability of its function.

The sinuses of Valsalva are the expanded portion of the initial section of the aorta, limited proximally by the corresponding segment of the fibrous ring and the cusp, and distally by the sinotubular junction. The sinuses are named according to the coronary arteries from which they depart: right coronary, left coronary, and noncoronary. The wall of the sinuses is thinner than the wall of the aorta and consists only of the intima and media, somewhat thickened by collagen fibers. In this case, the number of elastin fibers in the wall of the sinuses decreases, and collagen fibers increase in the direction from the sinotubular to the ventriculoaortic junction. Dense collagen fibers are located mainly along the outer surface of the sinuses and are oriented in the circumferential direction, and in the subcommissural space they participate in the formation of intercusp triangles that support the shape of the valve. The main role of the sinuses is to redistribute the tension between the cusps and sinuses during diastole and to establish an equilibrium position of the cusps during systole. The sinuses are divided at the level of their base by intercusp triangles.

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

The ventriculo-aortic junction (valve base ring) is a round anatomical connection between the left ventricular outlet and the aorta, which is a fibrous and muscular structure. In foreign surgical literature, the ventriculo-aortic junction is often called the "aortic ring". The ventriculo-aortic junction is formed, on average, by 45-47% of the myocardium of the arterial conus of the left ventricle.

Commissure is the line of connection (contact) of adjacent cusps with their peripheral proximal edges on the inner surface of the distal segment of the aortic root and its distal end is located to the sinotubular junction. Commissural rods (columns) are the places of fixation of commissures on the inner surface of the aortic root. Commissural columns are the distal continuation of three segments of the fibrous ring.

The intercuspid triangles of Henle are fibrous or fibromuscular components of the aortic root and are located proximal to the commissures between adjacent segments of the fibrous ring and their respective cusps. Anatomically, the intercuspid triangles are part of the aorta, but functionally they provide outflow tracts from the left ventricle and are affected by ventricular rather than aortic hemodynamics. The intercuspid triangles play an important role in the biomechanical function of the valve by allowing the sinuses to function relatively independently, by uniting them, and by maintaining a uniform aortic root geometry. If the triangles are small or asymmetric, a narrow fibrous ring or valve distortion with subsequent dysfunction of the cusps develops. This situation can be seen in bicuspid aortic valves.

The cusp is the valve's locking element, with its proximal edge extending from the semilunar portion of the fibrous ring, which is a dense collagen structure. The cusp consists of a body (the main loaded portion), a coaptation (closure) surface, and a base. The free edges of adjacent cusps in the closed position form a coaptation zone extending from the commissures to the center of the cusp. The thickened triangular-shaped central part of the cusp coaptation zone is called the Aranzi node.

The leaflet that forms the aortic valve consists of three layers (aortic, ventricular and spongy) and is covered on the outside with a thin endothelial layer. The layer 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 leaflet, this layer is present in the form of individual bundles. The collagen bundles in this zone are "suspended" between the commissural columns at an angle of approximately 125 ° relative to the aortic wall. In the body of the leaflet, these bundles depart at an angle of about 45 ° from the fibrous ring in the form of a semi-ellipse and end on its opposite side. This orientation of the "power" bundles and the edges of the leaflet in the form of a "suspension bridge" is intended to transfer the pressure load during diastole from the leaflet to the sinuses and the fibrous framework that forms the aortic valve.

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

The constancy of the geometric proportions of the aortic root elements was studied using the method of functional anatomy. In particular, it was found that the ratio of the diameters of the sinotubular junction and the valve base is constant and amounts to 0.8-0.9. This is true for the valve-aortic complexes of young and middle-aged individuals.

With age, qualitative processes of disruption of the 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, to a decrease in elasticity. Changes in geometric proportions and a decrease in the extensibility 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 cusps and 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 a formation as the aortic valve of humans and mammals was performed in the late 1960s. These studies demonstrated the similarity of a number of anatomical parameters of the porcine and human valves, in contrast to other xenogenic aortic roots. In particular, it was shown that the non-coronary and left coronary sinuses of the human valve were, respectively, the largest and smallest. At the same time, the right coronary sinus of the porcine valve was the largest, and the non-coronary was the smallest. At the same time, the differences in the anatomical structure of the right coronary sinus of the porcine and human aortic valves 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 been resumed in recent years.

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Human aortic valve and porcine aortic valve

A comparative study of the structure of the human aortic valve and the porcine aortic valve as a potential xenograft was conducted. It was shown that the xenogeneic valves have a relatively low profile and are asymmetric in most cases (80%) 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 porcine aortic valve, unlike the human one, does not have a fibrous ring and its sinuses do not directly border the base of the cusps. The porcine cusps are attached with their semilunar base directly to the base of the valve, since the true fibrous ring is absent in porcine valves. The bases of xenogeneic sinuses and cusps are attached to the fibrous and/or fibromuscular parts of the valve base. For example, the base of the non-coronary and left coronary cusps of the porcine valve in the form of diverging leaflets (fibrosa and ventnculans) are attached to the fibrous base of the valve. In other words, the cusps that form the porcine aortic valve are not directly adjacent to the sinuses, as in the allogeneic aortic roots. Between them is the distal part of the valve base, which in the longitudinal direction (along the valve axis) at the level of the most proximal point of the left coronary and non-coronary sinuses is equal, on average, to 4.6 ± 2.2 mm, and of the right coronary sinus - 8.1 ± 2.8 mm. This is an important and significant difference between the porcine valve and the human valve.

The muscular insertion of the aortic cone of the left ventricle along the axis in the porcine aortic root is much more significant than in the allogeneic one. In porcine valves, this insertion formed the base of the right coronary cusp 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 cusps. In allogeneic valves, this insertion creates only support for the base, mainly, of the right coronary sinus and, to a lesser extent, of the left coronary sinus.

Analysis of the sizes and geometric proportions of individual elements of the aortic valve depending on the intra-aortic pressure was used in functional anatomy quite often. For this purpose, the aortic root was filled with various hardening substances (rubber, paraffin, silicone rubber, plastics, etc.), and its structural stabilization was carried out chemically or cryogenically under different pressures. The resulting casts or structured aortic roots were studied using the morphometric method. This approach to the study of the aortic valve made it possible to establish some patterns of its functioning.

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

The dynamics of the geometric parameters of the aortic valve were studied in an animal experiment using high-speed cineangiography, cinematography, and cineradiography, as well as in healthy individuals using cineangiocardiography. These studies allowed us to estimate the dynamics of many elements of the aortic root quite accurately and only tentatively estimate the dynamics of the shape and profile of the valve during the cardiac cycle. In particular, it was shown that the systolic-diastolic expansion of the sinotubular junction is 16-17% and closely correlates with arterial pressure. The diameter of the sinotubular junction reaches its maximum values at the peak of systolic pressure in the left ventricle, thereby facilitating the opening of the valves due to the divergence of the commissures outward, and then decreases after the closure of the valves. The diameter of the sinotubular junction reaches its minimum values at the end of the isovolumic relaxation phase of the left ventricle and begins to increase in diastole. The commissural columns and sinotubular junction, due to their flexibility, participate in the distribution of the maximum stress in the leaflets after their closure during the period of rapid increase in the reverse transvalvular pressure gradient. Mathematical models have also been developed to explain the movement of the leaflets during their opening and closing. However, the data from mathematical modeling were largely inconsistent with the experimental data.

The dynamics of the aortic valve base affects the normal operation of the valve leaflets or the implanted frameless bioprosthesis. It was shown that the perimeter of the valve base (dog and sheep) reached its maximum value at the beginning of systole, decreased during systole and was minimal at its end. During diastole, the valve perimeter increased. The aortic valve base is also capable of cyclic asymmetric changes in its size due to the contraction of the muscular part of the ventriculoaortic junction (intercuspid triangles between the right and left coronary sinuses, as well as the bases of the left and right coronary sinuses). In addition, shear and torsional deformations of the aortic root were revealed. The largest torsional deformations were noted in the area of the commissural column between the non-coronary and left coronary sinuses, and the minimum - between the non-coronary and right coronary. 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 torsional deformations to the sinotubular junction of the composite aortic root and the formation of distortion of the bioprosthesis leaflets.

A study of normal biomechanics of the aortic valve in young individuals (21.6 years on average) was conducted using transesophageal echocardiography with subsequent computer processing of video images (up to 120 frames per second) and analysis of the dynamics of the geometric characteristics of the aortic valve elements depending on the time and phases of the cardiac cycle. It was shown that during systole, the valve opening area, the radial angle of the leaflet to the valve base, the diameter of the valve base, and the radial length of the leaflet change significantly. The diameter of the sinotubular junction, the circumferential length of the free edge of the leaflet, and the height of the sinuses change to a lesser extent.

Thus, the radial length of the leaflet was maximum in the diastolic phase of isovolumic decrease in intraventricular pressure and minimum in the systolic phase of reduced ejection. The radial systolic-diastolic stretch of the leaflet was, on average, 63.2±1.3%. The leaflet was longer in diastole with a high diastolic gradient and shorter in the phase of reduced blood flow, when the systolic gradient was close to zero. The circumferential systolic-diastolic stretch of the leaflet and sinotubular junction was, respectively, 32.0±2.0% and 14.1±1.4%. The radial angle of inclination of the leaflet to the valve base changed, on average, from 22 in diastole to 93° in systole.

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

1. the preparatory period occurred during the phase of isovolumic increase in intraventricular pressure; the valves straightened, shortened somewhat 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 onset of blood expulsion, a wave of inversion was formed at the base of the valves, which quickly spread in the radial direction to the bodies of the valves and further to their free edges;
3. the peak of the valve opening occurred during the first phase of maximum expulsion; during this period, the free edges of the valves were maximally bent towards the sinuses, the shape of the valve opening approached a circle, and in profile the valve resembled the shape of a truncated inverted cone;
4. the period of relatively stable opening of the valves occurred during the second phase of maximum expulsion, the free edges of the valves straightened along the flow axis, the valve took the form of a cylinder, and the valves gradually closed; by the end of this period, the shape of the valve opening became triangular;
5. the period of rapid valve closure coincided with the phase of reduced ejection. At the base of the cusps, a reversion wave was formed, stretching the contracted cusps in the radial direction, which led to their closure first along the ventricular edge of the coaptation zone, and then to complete closure of the cusps.

Maximum deformations of the aortic root elements occurred during periods of rapid opening and closing of the valve. With rapid changes in the shape of the cusps that form the aortic valve, high stresses can occur in them, which can lead to degenerative changes in the tissue.

The mechanism of opening and closing of the valve with the formation of, respectively, an inversion and reversion wave, as well as an increase in the radial angle of inclination of the valve to the base of the valve in the phase of isovolumic increase in pressure inside the ventricle can be attributed to the damping mechanisms of the aortic root, reducing the deformation and stress of the valve valves.