Heart Valves PDF

Professor Claudio Sette Sbobbers Benedetta Teresa Iovine, Capaldi Liliana Checker Esra Hoxha Date 11/10/2023 THE VALVES...

Professor Claudio Sette Sbobbers Benedetta Teresa Iovine, Capaldi Liliana Checker Esra Hoxha Date 11/10/2023 THE VALVES OF THE HEART The fibrous skeleton is the only solid tissue that is present in the heart and it mainly consists of the membranous part of the interventricular septum and of of four fibrous rings, each surrounding one of the valves. In this image, we can see the atrioventricular valves, which connect the atria with the ventricles. The valve on the right is called tricuspid, the one on the left is called mitral. There are also two additional, smaller, valves, which are anterior to the atrioventricular valves: the anterior is the pulmonary valve and the one in the middle is the aortic valve. In addition to anuli, which are the circular structures that support the attachment of the leaftlets of the valves, you can also see some thickening of the fibrous skeleton: these are called the trigones of the fibrous skeleton of the heart and are distinguished into left and right fibrous trigones. The trigones correspond to the point of connection between the anuli of the two atrioventricular valves. At the atrioventricular septum, there’s a thicker portion of connective tissue that links the valves, reinforces the whole structure and prevents the electrical impulse from passing from atria to ventricles. This insolation, however, is not absolute: at a certain point, we need the transition of the impulse, but it will occur in a very accurate way at a specific point. This point of electrical connection relies on a small hole in the right fibrous trigone: the atrioventricular bundle. THE ATRIOVENTRICUAL VALVE Each atrioventricular valve has an anulus, which is an incomplete circumferential ring, where we have the attachment of leaflets which can open or close, allowing blood passage. In the case of the right atrioventricular valve, we have three leaflets and the valve is called tricuspid. The leaflets are quite triagonal and the base is represented by the point of attachment to the anulus. In the image, each leaflet is called cuspid, although the name may not be totally correct because of its geometry, since, especially anterior and posterior leaflets are elongated structures. The anterior one has the longest distance between the base and the end, but it has the shortest attachment to the anulus. The posterior one (which should be properly called the inferior one) is, instead, shorter but wider and protects 2/3 of the anulus. The third leaflet is called the septal cusp which is the most fixed and it stabilizes the structure. The leaflets are attached to muscular protrusion in the ventricular part of the heart.  In the right ventricle, we have the presence of the septo marginal trabecula, that originates at the level of the interventricular septum and project towards the apex of the right ventricle, until it reaches the base of the anterior papillary muscle (look at the second image). This muscle has its base that originates from the trabeculae of the ventricle of the heart and it ends into the cavity of the ventricle where it attaches to tendinous cords. Leaflet: endocardial fold which contains cords of fibrous tissue that is in continuity with the fibrous tissue of the anulus. Each leaflet has a fibrous core which is not even throughout its length: it has a very thick and strong base that fuses with the anulus, an intermediate portion, the mid zone, that is much thinner and, at the end, we have the attachment of tendinous cords (which are made by fibrous connective tissue) at the rough zone. In the ventricular cavity, we have several papillary muscles, but only three of them are directly attached, through tendinous cords, to the leaflets of the valve and these are: the antero-superior, the septal and the posterior (which should, more correctly, be called the inferior one). We need this precise organization in order to ensure that valves only open from atria to ventricles. In the normal position of the heart, the atria and the ventricles are not one exactly above the other, indeed the heart is slightly rotated anticlockwise and this make the atrioventricular septum inclined with an angle of approximatively 45° from the vertical axis and corresponds to the line that connects the acute angle of the heart with the obtuse one. So the right atrioventricular valve, faces its aperture anteriorly, inferiorly and towards the left. In the left ventricle, the mitral valve is protected only by two leaflets, and, therefore, its name is bicuspid. Here the name cuspid is even less appropriate since the two leaflets are very flat. We also have only two papillary muscles, the anterior and posterior muscles, which protrude into the chamber cavity and are attached to leaflets through tendinous cords. The circumference of the mitral valve is about 7.2-9 cm.  Here we are looking at atrioventricular valves and the right ventricle, where we can notice the anterior papillary muscle and tendinous cords. The atrial part of the leaflets is always very flat since we have no tendinous cords attachment, whereas the ventricular surface is thicker in order to sustain the increasing pressure found in the ventricles. In the cycle of the heart, each chamber has two states: relaxation (diastole) and contraction (systole) and position of the valves changes during each phase. During the diastolic phase we will have the filling of the atria, with blood returning from pulmonary veins and venae cavae. When the pressure becomes higher than the one in the ventricles, then the weight of the blood pushed and opens the leaflets of atrioventricular valve and it flows into the ventricle. During this phase, we have the closure of the pulmonary valves and the aortic valves.  During the systolic phase, we have the contraction of the ventricles, the blood pressure of ventricles pushes blood superiorly; this allows the closure of the atrioventricular valve and the aperture of the pulmonary and aortic valve. The shape of the leaflets is different according to the different functions of the valves: if the valve must close towards the ventricles, its concavity faces the atria; if it must close towards the atria, its concavity faces towards the ventricles. In this way, when pressure increases, it pushes the valve towards where it has to close. In the case of aortic and pulmonary valves, we don’t have papillary muscles, however, the base of their leaflets is sustained by muscles that maintain the staidness of the valve. when, during the diastolic phase, you have the weight of the blood in the atria, that opens the leaflets because the papillary muscles are attached to the ventricles which is in diastole. When you have the contraction of ventricles, papillary muscles contract and pull the valves. We said that the atrioventricular septum contains the fibrous skeleton of the heart and the valves. This is not fully connected because the pulmonary valve doesn’t lay on the same plane, whereas is slightly superior and anterior and therefore does not take direcly part to the formation of the fibrous skeleton and is connected by the tendon of the pulmonary valve or tendon of the conus. The other two valves are connected through the trigones.  This image allows to see the geometry of the fibrous skeleton of the heart. The medial aspect of the two anuli originate from the fibrous trigones and they extend, laterally, on opposite directions. The most distal part of the anulus is discontinuous and we have loose connective tissue, needed to allow dilation and aperture of the valve. When the valve is closed, the anulus becomes smaller and the alveolar part contracts. A different situation is at the level of aortic and pulmonary valves because we have no annuli and the point of insertion is onto the wall of ventricles. The apex of leaflets define a triangular region: sinuses of aorta and pulmonary trunk, which play an important function in ensuring rapid closure of the valve. The pulmonary valve has leaflets with semilunar insertion in the pulmonary trunk. AORTIC ROOT There are several differences. The attachment of leaflets is reinforced by muscles in the case of the aortic valve. Secondly, there are orifices from which the two coronary arteries that vascularize the heart originate. So, we define the leaflets and the sinuses as left coronary sinus and left coronary leaflet and right coronary sinus and right coronary leaflet. In some individuals, there can be also a third coronary sinus. Bulges of the sinuses of the aortic trunk are more pronounced than those of the pulmonary trunk and they are generally referred to as Valsalva’s sinuses. If we draw a virtual line that connects all the apexes of the leaflets which attach to each other, we will obtain a sinutubular junction that represents the highest point reached during the open conformation. The point in which all leaflets contact the other defines inferiorly the triangle shown before. The sinutubular junction is important because it defines approximately the upper end of the bulge region. Incongruency between the leaflet which is flat and the aortic valve which is concave allows the presence of some blood between the two even when the valve is opened. In this way, it will be easier and faster to close the valve during the diastolic phase. In the normal situation (when the aortic valve is perfectly functioning), there is less than 4% of the blood that flows back into the ventricle at each diastolic phase. If you remove the leaflets, the amount of blood that flows back may reach the 20%. Of course, this decreases the efficiency of the circulation and therefore imposes more work on the heart that must pump more frequently to assure a continuous flow of blood through the systemic circulation. The position of the orifices can be at the level of the free end of the leaflet or slightly superior to that. Nevertheless, when the leaflet opens, it reduces the flow of the blood into the orifices and therefore less blood enters the coronaries during systolic phase. This is functionally acceptable  because given the systolic phase, the contraction affects also the lumen of the arteries so less circulation will be allowed. Whereas circulation in the arteries is more efficient during the diastolic phase of the heart when the musculature relaxes. Contrary to what we have seen at the level of the AV valve in which you have a flatter anulus that forms the attachment of the leaflet, here you don’t have a real ring. Rather you have this complex organization of triangle and semilunar attachment points which support the valve. This is a real image of the aortic valve in its closed position. Under the label, you see the nodules that represent the thickened part of the free end of the leaflets where the closure becomes reinforced. We are looking at the valve from the aortic point of view, in fact you can see concavity of the leaflets. CARDIAC CYCLE Contraction of the heart is a circular event. The heart contracts 70 times per minute. Conventionally, the description starts when ventricles are relaxed. a) During the ventricular diastole (also called diastole of the heart), all four chambers are relaxed and the blood flows from the vessels to the atria. The more blood fills the atria, the more pressure is exerted on the wall of these chambers pushing blood through the AV valve. So, the AV valves begin to open during the diastolic phase of the heart. When this occurs, you have the beginning of the impulse that as you can see begins from atria. Indeed, the pacemaker of the heart is in the right atrium, on the lateral side of the superior vena cava in the sulcus terminalis. b) The impulse starts there and then rapidly propagates to the two atria, that contract almost simultaneously. The atrial systole (contraction of the atria) increases further the pressure within the chamber and opens even more the AV valve. At this point, all the blood is transferred to the ventricles. c) Once ventricles are filled, propagation of the stimulus is passed from atria to the ventricles (atrial diastole). Ventricles start to contract from the apex, so the blood is pushed towards the two major arteries. The fact that the AV valves close forces the blood to exit from the ventricles. AV valves have a larger orifice than the aorta, therefore if there was not a valve protecting it, most of the blood would flow up and forth from atria to ventricles and vice versa. d) Once you have the ventricular systole, the cycle basically ends. The blood is pushed into the aorta and pulmonary trunk. Because these arteries have elastic walls, they tend to swell during systole of the ventricle and then they return to their original position. When the arteries go back to their initial dimension, they push the blood inside the concavity of the valves which are filled. Valves are now closed, and the blood cannot flow back inside the ventricle. e) Finally, during the ventricular diastole, ventricles relax. Pressure within them is very low and the pressure of the blood inside the vessels is high. CONDUCTING SYSTEM OF THE HEART Myocardium is composed of myocardial cells. These are single, striated cells capable of contracting autonomously but in a very slow manner. However, there are specialized cells within the heart, called nodal cells, which have higher contractility than myocardial cells. Nodal cells are concentrated in 3 major structures. The first one is the sinoatrial node; it is the real pacemaker because its cells are the fastest. It is where the contraction starts. From here, the propagation extends to the whole atrial compartment where it encounters a second node (atrioventricular node) in the interatrial septum. From the AV node, it is transferred to the ventricular compartment through the AV bundle (or bundle of His). It travels to the two sides of the interventricular septum until it reaches the apex. In the apex, the impulse propagates to the surface of the heart and then the cycle starts again.  This cycle is very efficient because it allows superficial contraction to begin from the apex to the top. It is like when you squeeze the toothpaste tube from the bottom, all the toothpaste will come out. In the same way, if ventricles start contracting from the apex, we can push out all the blood into the arteries. Therefore, the conducting system of the heart ensures the maximal efficiency for the ejection of the blood from the heart. To do this, you need to separate the contraction of the atria from the ventricles. This means that gap junctions present between myocardial cells need to be interrupted. This function is carried out by the fibrous skeleton located strategically at the border between atria and ventricles. PATHWAY OF THE IMPULSE 1. SA node: real pacemaker of the heart. 2. Interatrial septum. 3. Interventricular septum: the first portion is membranous. Once the impulse reaches the muscular part, the bundle divides into the right and left branch of the AV bundle. 4. The apex of the ventricles: nodal cells are surrounded by intermediate cells that contract slower. This ensures that it reaches the apex before propagating to myocardial cells. The fibers here become more superficial, in contact with myocardium. RIGHT SIDE OF CONDUCTING SYSTEM This is a picture of the right side of the conducting system. Connection between SA node and AV node is not direct (there are not fibers that connect them directly). However, the propagation of the stimulus in the atrial compartment will eventually reach the AV node. These create a delay in between the contraction of the atria and that of the ventricle and allow complete emptying of the atrium before the ventricle starts contracting. Once the AV node is reached, impulse is transferred through the right trigone to the membranous part of the interventricular septum until it reaches the thick muscular part of the septum. Here, it splits into the right branch of the AV bundle. This is thinner and more cylindrical respect to the left side. It travels to the base of the septomarginal trabecula and then the papillary muscles. This is the first point in which Purkinje fibers reach the subendocardial layer and release the impulse through myocardial cells. In this way, thanks to cell-to-cell junctions’ impulse is propagated to all cells. LEFT SIDE OF CONDUCTING SYSTEM The bundle here is flatter and larger and it splits toward the apex into several branches, called Purkinje fibers that project immediately to the papillary muscles and then to the rest of the myocardium. So, Purkinje fibers initially ride the superficial aspect of the left side of the interventricular septum and then they reach the apex. In the right part, they attach to interventricular septum all the way to the cardiac notch and then they transfer anteriorly and posteriorly to the papillary muscles. ECG: Contraction of the heart, being an electrical impulse, can be monitored electrically. A typical device is the electrocardiograph. The 2 waves correspond to the stimulation of the SA node at the beginning of the heart contraction. Then, the strongest electrical change occurs at the level of the ventricular contraction, and this is recorded as QRS curve. VASCULARIZATION OF THE HEART Vascularization is performed by the coronary arteries. The first is called the right coronary artery that originates from the ascending aorta and the other one is the left coronary artery. They are very different in course and in behavior. The right coronary artery (RCA) is long and ride almost entirely along the atrioventricular groove. It originates from the right Valsalva’s sinus that comes from anterior right side of the aorta. Its origin is hidden because the aorta is covered anteriorly by the pulmonary trunk and by the protrusion of the atrium, called the right auricle. So, infundibulum, right auricle and pulmonary trunk cover the origin of aorta. The aorta travels in a diagonal manner pointing inferiorly and reaches anteriorly the atrioventricular groove. It remains in the AV groove until it reaches the acute angle of the heart, then it passes beyond the acute angle and transfers onto the diaphragmatic surface. The left coronary artery (LCA) is short and originates from the left Valsalva’s sinus of the aorta. Before reaching the AV groove on the left side, it splits in the terminal arteries called the descending coronary artery or anterior interventricular artery and the circumflex artery. The circumflex artery behaves similarly to the RCA: it remains in the interventricular groove; it reaches the obtuse angle of the heart and eventually goes to the diaphragmatic surface. The descending coronary artery descends into the anterior interventricular groove until the cardiac notch, where it passes beyond reaching the diaphragmatic surface. ARTERIAL CIRCULATION ON THE STERNOCOSTAL SURFACE: From the RCA, there are 4 or 5 deriving ventricular arteries which originate at right angle from the RCA pointing toward the apex of the heart. Right coronary angle is oriented at 45 degrees with respect to the vertical axis of the body. They descend along anterior sternocostal surface of the heart covering much of the right ventricle. They do not take a proper name, instead they are just called ventricular arteries. The only one described with a proper name is one of the first which is called the right conus artery. The latter reaches the infundibulum of conus arteriosus of the right ventricle and forms an anastomosis with a homologous artery that originates from the descending coronary artery of the LCA. So, here you have an anastomotic vascularization of the infundibulum. There is another important artery that in most individuals originates from RCA, called sinoatrial artery. These projects go superiorly towards the atria, passes underneath the auricle, reaches the superior vena cava and here it splits into two arteries that loop around the termination of the superior vena cava (SVC). The right branch enters the sulcus terminalis and produces the nodal artery which enters the atrium and reaches the SA node. In fact, nodal cells are organized around the termination of the nodal artery, so they are highly oxygenated. Therefore, at the core of SA node there are nodal cells, then there are cells with intermediate contractive capability and finally, you have myocardial cells (the slowest). In 65 to 70% of the people, the sinoatrial node artery originates from the RCA. However, in the remaining part of the individuals, it can originate from the LCA. In this case, it has a longer course because it must descend on the left side of the descending aorta until it reaches the SVC and then it behaves in the same way of the other. From the LCA, its short course does not release any collateral arteries so, the first branches are the terminal ones (anterior interventricular arteries). The anterior interventricular artery produces a few diagonal arteries. The ones on the right side of the groove have a short course and vascularize a small portion of the sternocostal surface of the right ventricle close to the interventricular groove. Those to the left side have a longer course and reach to different extent the sternocostal portion of the left ventricle. One of them, which is often referred to as the diagonal branch, is very long and reaches the real apex of the heart. In addition, the anterior interventricular artery produces a perforating artery that penetrates in the interventricular septum and vascularizes the anterior two thirds of the septum. The circumflex artery travels in the interventricular groove and releases 2 or 3 arteries which irrorate in a parallel manner with respect to the diagonal arteries. Then, along the obtuse and acute angle you have generally two large arteries, called the right and left marginal arteries. ARTERIAL CIRCULATION ON THE DIAPHRAGMATIC SURFACE: From RCA, it loops around the acute angle, proceeds in the AV groove and continues until the crux cordis. In approximately 90% of individuals, these arteries descend in the posterior interventricular groove. In these cases, we say that the right dominance of the heart is present. It releases an atrioventricular node artery which enter the interatrial septum and serves the AV node. It also releases another branch that continues in the AV groove in tight contact with the coronary sinus and contributes to the vascularization of part of the diaphragmatic surface of the left side. The major branch is the inferior interventricular artery that behaves similarly to anterior interventricular artery. It also releases some arteries that penetrate the interventricular septum.  In most cases, there is also a sino-interventricular artery or there can be 2 or 3 interventricular arteries that run parallel. When you have more interventricular arteries, the marginal arteries will be limited. From LCA, this contributes to the vascularization of the left ventricle. Circumflex artery also produces some atrial arteries which ascend on the posterior side of the left atrium. VENOUS CIRCULATION ON THE HEART SURFACE The venous return of the blood occurs through 3 major cardiac veins and then smaller ones. The major cardiac veins are: Great cardiac vein, the longest. It originates at the level of the cardiac notch and ascends parallel to the descending artery into the interventricular groove. When it reaches the point of separation of LCA, it bends laterally to the left in the AV groove. It follows the circumflex and loops around the obtuse angle. Once it reaches the diaphragmatic surface, it continues its course in the atrioventricular groove until it reaches the oblique vein or Marshall vein in the left atrium. The point where Marshall and great cardiac vein meet form the coronary sinus. Coronary sinus follows the course of the great cardiac vein until it reaches the crux cordis where it receives another vein called middle cardiac vein. Middle cardiac vein originates at cardiac notch of the inferior interventricular groove. It ascends until it reaches the coronary sinus and merges with it. Coronary sinus passes beyond the crux cordis and reaches and enters the right atrium. Small cardiac veins not always present. It originates at the sternocostal surface in the AV groove and runs into it until it reaches the diaphragmatic surface. It merges with coronary sinus. Then, there is the right marginal vein that is the most inconstant one ascends in the acute angle until it merges with the small cardiac vein. In other cases, it can pass above the small cardiac vein and the right coronary artery, reaches the right atrium and penetrates it. The left marginal vein ascends onto the obtuse angle of the heart until it reaches the great cardiac vein and drains into it. Furthermore, there is the posterior vein of the left ventricle which irrorate much of its diaphragmatic surface and drains into the coronary sinus. In addition to cardiac veins, there are also the so-called ventricular veins. These are particularly present on the sternocostal surface of the right ventricle from which they originate and then they ascend to the AV groove. They bypass the right coronary artery and go beyond the AV groove until they reach the right atrium where they open into it. There are generally 3 to 4 ventricular veins in the right atrium, however their number, shape and position are variable. Finally, there are Thebesian veins or minimal veins of the atria. Present both on the right and left atrium, they have a very short course and are very thin which open directly in the wall of the corresponding atria.  Basically, what you can see in this picture from top to bottom is a change in pressure in chambers and in the major vessels. Then, you see the opening and closure of the valves and finally you can monitor the changes occurring at the level of the pressure and contractility of the heart with ECG. The resting pressure within the aorta is 80 mmHg. When there is the contraction of the left ventricle and the opening of the aortic valve, the blood is pushed into aorta and pressure decreases. Pressure of the left ventricle during diastole is near 0, but then it suddenly increases. When its pressure is lower than aortic pressure, the valve remains closed. When it exceeds, the aortic valve opens, and the two pressures becomes equal. So, the pressure in the aorta reaches its maximum few milliseconds after the systolic contraction and then rapidly decreases. Changes in aortic pressure within the aorta spans between 0 and 20 mmHg. Changes in ventricle pressure are much more dramatic (from 0 to 120 mmHg).