CARDIAC VALVES

• Ensure unidirectional blood flow through the heart and great vessels.
• Comprise fibroelastic tissues; specifically, a collagenous core covered by endocardium.
• Damage to the valves and/or their supporting structures can be acute or chronic: cumulative damage is caused by over 30 million contractions per year that deform the valves.
• Stenosis occurs when the valve orifice is obstructed.
• Insufficiency, which can lead to regurgitation, occurs when the orifice remains open due to an incomplete seal (we address the effects of stenosis and regurgitation, elsewhere).

Valvular Anatomy

We’ll use standard names for the valve (cusps), but be aware that nomenclature varies based on author’s preference for fetal or adult position and other considerations.

Semilunar Valves:

Comprise three cusps, aka, leaflets, that prevent backflow to the ventricles.
Pulmonary valve comprises an anterior, right, and left leaflet; they ensure unidirectional flow of deoxygenated blood from the right ventricle to the lungs.
Aortic valve comprises right coronary, left coronary, and posterior non-coronary leaflets; leaflet names reflect their relationships to the ostia of the coronary arteries. The aortic semilunar valve ensures unidirectional flow of oxygenated blood from the left ventricle to systemic circulation.

Atrioventricular (AV) valves:

Bicuspid valve, aka, mitral valve, is on the left.
– It comprises two primary cusps (hence, “bi” & “cuspid”), anterior and posterior, which ensure unidirectional flow of oxygenated blood from the left atrium to left ventricle. Each primary cusp can be further subdivided into three regions (Anterior 1-3 and Posterior 1-3).
Tricuspid valve is on the right.
– It comprises anterior, posterior, and septal leaflets, which ensure unidirectional flow of deoxygenated blood from the right atrium to right ventricle.

Structural details of the Valves and Supporting Structures

• Semilunar valves
• Features of the external heart

Semilunar valves

• Trap blood within the sinuses of the aorta and pulmonary trunk.
  During diastole, the semilunar valve leaflets fall open to trap blood in the sinuses, thus preventing backflow into the ventricles.
  The coronary arteries run into the right and left aortic sinuses.
• Annulus is the ring-like network of fibrous tissue that attaches the leaflets to the vessel wall. It’s not really a perfect circle of continuous fibrous tissue, but instead comprises elements that are dynamically responsive to heart contractions.
• Nodule (aka, nodule of Arantius), is a thickened spot in the middle of the free edge of the leaflet.
• Lunule is the free edge of the leaflet.
• Commissures are where the leaflets attach to the wall.
• Sinotubular junction passes through the commissures and signifies the transition from the sinus to the vessel.

Atrioventricular Valves
Are attached to the papillary muscles via chordae tendineae(tendinous cords), which comprise a network of collagenous and elastic fibers.

• Papillary muscles are special extensions of the myocardium that anchor the valve leaflets.
• Annular ring is a fibrous structure that anchors the leaflets to the heart.

Damaged chordae tendineae and/or papillary muscles causes functional regurgitation; though not a primary valve defect, dysfunction of the supporting structures impedes valve functioning.

Acquired aortic and mitral valve dysfunctions

• Aortic valve degeneration, calcification, and subsequent stenosis is one of the most common valvular dysfunctions.
  – Is the result of long-term buildup of hydroxyapatite on the valvular cusps; the calcified masses project into the sinuses, preventing valve opening, and, therefore, blood flow.
  – Hydroxyapatite is a calcium salt found in bone; the presence of osteoblast-like cells on the cardiac valves indicates that valvular degeneration and calcification involves a process similar to that of bone formation.
  – Normal “wear and tear” of the valves can lead to calcification over time, but chronic injury, as from hyperlipidemia, hypertension, and other factors related to atherosclerosisincreases the risk, as does the presence of a bicuspid aortic valve, which is subjected to more mechanical stress.
  – Bicuspid aortic valve can be congenital, as in approximately 1% of the population, or can be a complication of rheumatic valve disease.

• Aortic valve insufficiency is commonly caused by aortic root dilation (aka, aneurysm)
  – Dilation is associated with Marfan syndrome, which is a connective tissue disorder that can affect the blood vessels and aortic valves, which alter the blood flow direction, and, therefore, wall sheer stress.
  – Also, hypertension appears to be associated with aortic dilation, though the exact relationship may depend on additional factors.
  – Dilation itself may be asymptomatic, and is often undiagnosed until imaging; however, it should be monitored to prevent dissection and rupture. In some cases, root remodeling and valve replacement is necessary.

• Mitral valve calcification can lead to stenosis and/or regurgitation
  – The calcified masses can block the electrical conduction system, leading to arrhythmias, and raises the risk of endocarditis (discussed elsewhere).
  – Calcified masses tend to be in the annular ring (as opposed to the cuspal aortic calcification); the leaflets themselves become rubbery and thick due to myxomatus deposits in the spongiosa layer.
  – Furthermore, mitral valve calcification increases the risk of thrombus formation, and, therefore, stroke.
  – Calcification as result of chronic, recurrent injury is a complication of mitral valve prolapse, in which leaflets balloon into the atrium during ventricular systole. Prolapse prevents complete sealing of the mitral valve, which allows blood regurgitation and associated heart complications (discussed in detail, elsewhere).

• Mitral stenosis is largely attributed to rheumatic heart diseasefollowing one or more episodes of rheumatic fever.
  – Inflammation and scaring from rheumatic heart disease produces vegetations along the free edge of the valve leaflet; the chordae tendineae thicken and fuse together.
  – Valvular stenosis is visible with a characteristic “button hole” or “fish mouth” appearance.

• Endocarditis refers to inflammation of the endocardial lining of the heart; it tends to affect the valves, particularly on the left side of the heart.
  – In our histologic sample, we can see the inflamed valvular endocardium (and notice the leukocytes, which are indicative of inflammation) and the vegetation.
  – Vegetations can break off from the valve, travel in the circulation, and cause stroke; thus, endocarditis can have fatal consequences (we discuss the causes and consequences of endocarditis, elsewhere).

Valvular Replacement & Complications

Valvular disease often warrants valvular replacement; however, serious complications are common: Approximately 60% of valve recipients develop prosthetic-related complications within 10 years.
Complications depend on valve type:

• Mechanical valves produce more turbulent flow, and, therefore, are more susceptible to thromboembolism formation. Thus, patients are prescribed long-term anticoagulants (vitamin K antagonists such as Warfarin and aspirin).
• Bioprosthetic valves, which are derived from bovine, porcine, or even the patient’s own valvular tissues, are more susceptible to deterioration over time.
  Both prosthetic valve types increase the risk of infectious endocarditis and leakage; thus, antibiotics are prescribed for any oral procedures expected to breach the gingivae.