Cardiovascular disorders

II./1.2.: The structure of the cuspidal valves

II./1.2.1.: General characteristics of the cuspidal valves

The cuspidal valves are located at the atrioventricular (venous) orifices. Histologically these are triangular endocardium duplicates, containing collagenous connective tissue between the two endocardial layers. One of the endocardial membranes covers the atrial, the other membrane lines the ventricular side of the valve. The normal valves contain no blood vessels, the vasculature of them is limited to a few small vessels near their origin. The heart valves are thin, membranous structures.

Thickenings of any origin or the calcification of the heart valves make the appropriate orifice narrower (stenosis), causing difficulty in the propelling of the blood through that orifice. The same occurs as a result of developmental disorders (eg. only two instead of three aortic semilunar valve leaflets develop – this malformation is called 'bicuspid' aortic valve) as well.

If -by any reason- the closing function of a valve exhibits problems (insufficiency) the blood -due to the imperfect closure of the orifice- flows back, regurgitates (eg. aortic regurgitation, insufficiency).

The cuspidal valves originate from the fibrous skeleton (the annulus fibrosus) of the heart; the valves are formed by 2 or 3 downwardly projected cusps (flap, cuspis) and smaller accessory cusps (Fig. 2A). The cusps are connected to papillary muscles (cylindrical myocardial structures covered outside by endocardium) by thin tendinous cords made by collagen fibers, covered also by endocardium (heart strings, chordae tendineae). The chordae tendineae -in a smaller part- attach to the ventricular wall as well. From the tip of one papillary muscle 6-12 real, thin chordae tendineae arise, which can make branching before reaching the valves. Usually chordae tendineae from one papillary muscle run to 2 adjacent cusps.

According to the attachment of chordae tendineae  there are several types of them: inserting on the edge of the cusp (marginal), ending on the ventricular surface of the cusp (parietal), reaching the origin of the cusp on the annulus fibrosus (basal) or attaching the tip of the cusp (apical)

Figure 1: The structure of the cuspidal:

(1) Truncus pulmonalis; (2) Aorta ascendens; (3) Valvula semilunaris dextra; (4) Valvula semilunaris posterior; (5) Valvula semilunaris sinistra; (6) Arteria coronaria dextra; (7) Arteria coronaria sinistra; (8) Cuspis anterior; (9) Ventriculus sinister.

Figure 2: Semilunar valves:

(1) Septum interatriale; (2) Valvula foraminis ovalis; (3) Vene pulmonales; (4) Atrium sinistrum; (5) Cuspis posterior (valve atrioventricularis sinister) ; (6) Chordae tendineae; (7) Musculus papillaris posterior.

The function of the cuspidal valves: at the end of ventricular diastole (relaxation of ventricular myocardium) the valves begin to close behind the blood flow which moves from the atria into the ventricles; this happens initially due to the flexibility of the valves. Their total closure occurs due to the increase of ventricular pressure during the ventricular systole (contraction).

During ventricular contraction, papillary muscles are also shortened so can keep the system of their chordae tendineae (bundles of collagen fibres!) always tight, preventing the valves from flapping back into the atria (the flaps spread out, approaching each other, resulting the closure of the valve). So, during the ventricular systole, when the longitudinal diameter of the chambers gets shorter, the contraction of papillary muscles compensates the shortening of the chamber, thereby inhibiting the valves flapping back (prolapsing) towards the atria.

The cusps of the atrioventricular valves develop from the endocardium cushions of the atrioventricular canal; the papillary muscles are separated from the developing ventricular myocardium.

II./1.2.2.: Tricuspid valve, right atrioventricular valve

The tricuspid valve is located at the inlet of the right ventricle, at its venous orifice. This valve has 3 cusps: anterior, posterior and septal cusps. The septal cusp originates from the membranous part of the interventricular septum. Therefore, below the origin of the septal cusp the interventricular septum is found, while above its origin the so called atrioventricular septum is situated (between the right atrium and the left ventricle). The anterior papillary muscle is found at the anterior wall of the right ventricle, originating from the septomarginal trabecula.

There are often more posterior papillary muscles seen at the posterior wall of the right ventricle but usually no –or very rudimentary- papillary muscles start from the interventricular septum (septal papillary muscles), so here the chordae tendineae originate usually directly from the interventricular septum. The reason of it is that contraction of the ventricle occurs mainly along its longitudinal diameter (the distance between the apex of the heart and the annulus fibrosus); the narrowing of the chamber in its transverse diameter is less prominent.

II./1.2.3.: Bicuspid valve, mitral valve or left atrioventricular valve

The mitral valve is located in the orifice between the left atrium and the left ventricle– in a very close vicinity to the aortic orifice (the mitral valve is named after the bilobed miter of the bishops). Two larger cusps can be distinguished: the anterior and posterior cusps. The larger anterior cusp (other names: aortic cusp, septal cusp, cuspis maior or anteromedial cusp) is situated in front, is slightly placed to the right, in the direction of the aortic orifice. The anterior cusp originates from the anterior 2/5 part of the mitral annulus. The anterior cusp separates the blood inflow and outflow in the left ventricle.

The smaller posterior cusp (its other names: ventricular cusp, mural cusp, cuspis minor or posterolateral cusp) is placed towards the left side; it arises from the posterior 3/5 part of the mitral fibrous ring. The posterior cusp has two major fissures, dividing it into 3 units (P1, P2 and P3 "scallops"). Between the anterior and posterior cusps two commissures (anterolateral and posteromedial) are located in the left atrioventricular orifice (Figure 3). At valve closure, the so-called coaptation surfaces (the narrow surfaces of both cusps facing each other) meet and attach together.

Figure 3.: The fine structure of the mitral valve: the segments (A1, A2, A3 and P1, P2, P3) and the commissures are shown on the cusps. The "aortic-mitral curtain" is the continuation of the connective tissue between the anterior cusp and the left and posterior aortic valve leaflets ("fibrous continuity" between the right and left fibrous trigones). (Modified: Carpentier A, Adams DH, Filsoufi F. Carpentier's Reconstructive Valve Surgery . From Valve Analysis to Valve Reconstruction. Elsevier Saunders, 2010) (1) Valva aortae; (2) Valva bicuspidalis (mitralis); (3) Aortic mitral curtain; (4) Valvula semilunaris dexter; (5) Valvula semilunaris sinister; (6) Valvula semilunaris posterior; (7) Cuspis anterior; (8) Cuspis posterior,

The mitral valve has 2 strong papillary muscles - these muscles originate from the left ventricle according to the gaps between the anterior and posterior cusps: the anterior papillary muscle is on the left side (anterolateral), the posterior papillary muscle arises from the right side of the posterior wall of the ventricle (posteromedial). These papillary muscles vary in shape, are often split or doubled. Chordae tendineae start from both papillary muscles to both main cusps.