Mitral valve replacement is a procedure whereby the diseased mitral valve of a patient's heart is replaced by either a mechanical or tissue (bioprosthetic) valve. The mitral valve may need to be replaced because:
The valve is leaky (mitral valve regurgitation) The valve is narrowed and doesn't open properly (mitral valve stenosis) Causes of mitral valve disease include infection, calcification and inherited collagen disease. Current mitral valve replacement approaches include open heart surgery and minimally invasive cardiac surgery (MICS).
Normal mitral valve anatomy and physiology
The mitral valve is a bileaflet valve sited between the left atrium and left ventricle, responsible for preventing blood flowing from the ventricle to the atrium when the heart contracts. It is elliptical, and its area varies from 5.0 to 11.4 cm2. The valve leaflets are separated by two commissures, and each leaflet of the valve (anterior leaflet, the large one, and posterior leaflet, the small one) has three sections (p1, p2, p3). Histologically, each leaflet is composed of the solid fibrosa, the spongiosa at the atrial surface and another fibroelastic layer covering the leaflets. Two papillary muscles originating from the base of the left ventricle hold the mitral leaflets in place through chordae tendinae, which insert the edge of the leaflets, preventing them from leaking during left ventricle systole.
Vortex formation During normal mitral valve function fluid jets from the left atrium through the mitral valve into the left ventricle. The vortex created from this jetting travels towards the apex of the left ventricle because of the asymmetric shape of the mitral valve leaflets. This vortex rotates clockwise until the isovolumetric contraction of the left ventricle opens the aortic valve and redirects the fluid flow from the apex of the left ventricle to the systemic circulation and the rest of the body. The asymmetry of the mitral valve is very important in the diastolic flow patterns of transmitral flow. Additionally the entire systems; the mitral annulus, papillary muscles and the chordae tendinea all play a vital role in forming a sophisticated vortex that optimizes the fluid flow in the left heart. Simulations have been performed showing how all of these aspects of the mitral valve contribute to the normal vortex formation in the left heart.
Mitral stenosis and regurgitation The most common cause of mitral stenosis is rheumatic fever, seen mostly in the developing world. Other causes are mitral degenerative disease, severe calcification (elderly), congenital deformities, malignant carcinoid syndrome, neoplasm, left atrial appendage thrombus, endocarditic vegetations, certain inherited metabolic diseases, or complications of previous procedures at the aortic valve. Mitral stenosis causes left atrial pressure to increase, which, if left untreated, can lead to ventricular dilation, hypertrophy, atrial fibrillation, and thrombus creation. Symptoms include shortness of breath (dyspnea) on exertion, when lying flat (orthopnea) or during the night (paroxysmal nocturnal dyspnea), and fatigue. If mitral leaflets don't coapt (close) effectively, blood flows backwards (regurgitation) from the left ventricle towards the left atrium during systole. The most common causes are myxomatous degeneration (Barlow disease), ischemic heart disease, dilated cardiomyopathy, rheumatic valve disease, mitral annular calcification, infective endocarditis, congenital anomalies, endocardial fibrosis, myocarditis, and collagen-vascular disorders. The most used system to classify mitral valve regurgitation is Carpentier's classification, which separates mitral regurgitation into three types, depending on the leaflet motion in relation to the mitral annular plane:
Type I: the leaflets are moving normally Type II: leaflet motion is excessive Type III: leaflet movement is restricted.
Artificial valve types There are two main types of artificial mitral valve: mechanical valves and tissue (bioprosthetic) valves. They come in various sizes (commonly starting from an external diameter of 19 mm and increasing by 2 mm per model).
Mechanical valves Mechanical valves are made from metal and/or pyrolitic carbon, and can last 20–30 years. The risk of blood clots forming is higher with mechanical valves than with bioprosthetic valves. As a result, patients with mechanical valves must take blood-thinning medication (anticoagulants) for the rest of their lives, making them more prone to bleeding. There are three types of mechanical valves:
Caged ball valve (not in use any more) Tilting disc Bileaflet disc Bileaflet valves are the most common type of mechanical valve, offering desirable haemodynamics. The two leaflets of a bileaflet disc valve open during diastole and close during systole.
Bioprosthetic valves Bioprosthetic valves are made from animal tissues. Most people with bioprosthetic valves don't need to take anticoagulants long term. However, bioprosthetic valves may only last 10–15 years. They tend to deteriorate more quickly in younger patients. Valve failure prevalence at 10 years is 30%, increasing to 35–65% at 15 years. New tissue preservation technologies are being studied to try to increase the durability of bioprosthetic valves.
Valve selection The choice of valve depends upon the patient's age, medical condition, preferences, and lifestyle. Typically, patients younger than 65 years old will receive a mechanical valve unless they are unable to take long-term anticoagulation, and patients older than 70 years will receive a bioprosthetic valve.
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