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Heart, lungs, arteries and veins constitute the cardiovascular system of the human body. Heart, has the most critical function in human body which is pumping of blood through-out the body. Heart diseases are of many types. One of them is , disease of heart valves. Heart valve diseases are the major threat to human life. The technology has increased rapidly and provided us with solutions to treat these heart valve diseases. But still the number of deaths due to these valve diseases is high. Heart valve diseases do not show obvious symptoms, making it very difficult to diagnose, for example a person was having valve disease at the age of 12yrs but it was diagnosed at the age of 31yrs. Heart valve diseases can be due to rheumatic fever, due to infection in valve , calcification or it can be congenital. (1)(Cardiology explained)
Structure and function
Heart valve allows the blood to flow through heart in different stages or levels. Pumping of blood is a continuous process of opening and closing of the heart valves. Four cardiac chambers are present in heart, the upper two being atria and lower two ventricles. These are again sub-divided into left atrium, right atrium, left ventricle and right ventricle. The de-oxygenated blood flows into the heart through superior vena cava and inferior vena cava and enters right atrium. Then from right atrium it flows through right ventricle through tricuspid valve, then into pulmonary arteries to lungs through pulmonary valve.The deoxygenated blood is oxygenated in lungs and it flows back into pulmonary veins into left atrium and then through mitral valve it enters left ventricle which pumps the blood to aorta through aortic valve which flows to all parts of the body. Heart valves ensure that they do not allow the pumped blood to flow back.(cardiology, tortora).
Mitral valve and tricuspid valves have similar structures with one difference tricuspid valves have three semi lunar shaped leaflets or cusps whereas mitral valve has only two. These leaflets are connected to chordae tendineae and papillary muscles. The difference in blood pressure in atria and ventricle leads to the opening and closing of the leaflets. Any infection or damage to the leaflets or chordae tendineae leads to backward flow or leakage of blood into different chamber which results in serious heart valve problems. The other two valves are aortic valve and pulmonary valve. These valves have three cusps which are crescent moon shaped attached to a valve ring. The valves open when the pressure in ventricles is more than the pressure in arteries and flows into pulmonary trunk and aorta. Blood flows back to the heart due to relaxation of ventricles thus filling the cusps and closing the valves. (tortora, cardiology).
AORTIC VALVE DISEASES:
The proper function of aortic valve happens when the leaflets are opening properly during systole and closing completely during diastole. Aortic valve disease occurs when leaflets or cusps of the valve are damaged due to calcification, fibrosis and bacterial infection. The main aortic valve diseases are aortic stenosis and aortic regurgitation.
Aortic stenosis (AS) is the most common valve disease in Europe and is mostly observed in middle or old age patients.(4). Calcification occurs due settlement of calcium which narrows the aortic valve. Thus, the total amount of oxygenated blood does not flow through it creating a backwards flow of blood resulting in increased pressure in left ventricle. Severe calcification requires immediate surgery, otherwise calcification might close the orifice of the valve resulting in complete valve failure (Julian). Chan states that calcification might start from lipid accumulation resulting in calcium deposit. Though it is a severe problem but still there is no explanation of occurrence of calcification. Damage to the cusps leads to fibrosis or thickening of the leaflet and causes rheumatic aortic stenosis. Bicuspid aortic valve(two cusps instead of three), subvalvar and supravalvar aortic stenosis are congenital. They arise due to genetic defects such as fibrous tissue in the outflow tract. These can lead to left ventricle hypertrophy.
When there is large amount of back flow of blood in the left ventricle, increasing the pressure and resulting in dilatation of the left ventricle due to sudden increase in flow of blood and improper functioning of the aortic cusps is aortic regurgitation. Aortic regurgitation arises due to rheumatic fever, infective endocarditis, syphilitic aortitis and results in damage of cusps or dilatation of aorta. While Julian states that rheumatic fever leads to the cusps thickening, shortening and may be commissural fusion chan claims that it leads to valve neovascularization, inflammation, commissural fusion and thickening. Lymphocytes cause infiltration of the valve, if neovascularization and inflammation is present and it leads to the damage in valve tissues. Damage in valve tissues results in dysfunction of the valve. Infection in endocarditis affects the structure of the cusps, cusps deteriorate and there is leak in blood flow as the cusps do not close or open properly.
MITRAL VALVE DISEASES:
Selzer states that early view of rheumatic disease was that it occurs in left side cardiac valves, which is not true in present conditions. Mitral valve diseases are the only valve diseases occurring mainly due to rheumatic fever. Mitral valve cusps are connected to papillary muscles and chordae tendineae any damage to these will cause mitral valve disease. Mitral stenosis and mitral regurgitation are the two main mitral valve diseases.
Mitral stenosis starts with acute carditis and rheumatic carditis which damages heart muscles and cusps of mitral valve, but this does not create narrowing of mitral valve. Mitral stenosis cannot be detected easily in its early stages, selzer concludes that in most of the patients mitral stenosis fully develops in a decade. The pathology of mitral stenosis is deterioration of the cusps in commissures, due to this the cusps does not close properly. As stated by Julian shortening of the chordae tendineae also results in improper closing of the cusps. The blood pressure in mitral valve is normally low but there is increase in the pressure due to reduction in the orifice size resulting in severe haemodynamic consequences. Comparing Julian and sezler, Julian states that normal size of valve orifice is approximately 5cm² and it becomes chronic when valve size reduces to 1cm² or less while sezler states that normal size of valve orifice is approximately 3cm² and as it reduces to 0.9cm², it is severe stenosis and the patient has to undergo surgery. Pressure develops in the left atrium in chronic mitral stenosis and the cardiac blood flow increases rapidly and affects pulmonary veins and capillaries. Atrial fibrillation, pulmonary embolism and respiratory problems are developed due to mitral stenosis.
Mitral regurgitation is back flow of blood in mitral valve. It is consequence of many diseases affecting the valve such as rheumatic endocarditis, calcification, dilatation of mitral valve, mitral valve prolapse, papillary muscle malfunction. The shortening of chordae tendineae and damage in cusp results in change of the valve orifice structure(Julian). Mitral valve prolapse(MVP) occurs when the mitral cusp is thickened due to damage of the different layers of the cusp resulting in balloon like shape of the cusp. It can be congenital or a developed disease. Schoen states that the present explanation for improper working of the cusps is possibly due to genetic disorder which weakens the valvular tissue.
In 1960's prosthetic heart valves were introduced for the replacement of damaged heart valves. Prosthetic heart valves have certainly decreased the rate of deaths due to failure of heart valves but the life of prosthetic valve depends on many factors like type of valve, severity of disease in the patient and age of the patient. They can be divided mainly into two kind of mechanical valves and biologic valves, these can be subdivided into many types. Ball -cage valves, tilting disk valves and bileaflet valves are the types of mechanical valves. Biologic valves can be sub-divided into homograft valves, autograft valve and bio-prosthetic valves. The question of selection of better prosthetic valve still remains unanswered.
Mechanical valve and biologic valves
Ball cage valve (Starr-Edwards) was the first valve to be introduced in 1960. It has a ring shaped base which was mounted by an alloy cage with a bio-compatible ball in it. It had reduced thrombosis but had other drawbacks such as bulky design and hemolysis(p1), to overcome this tilting disc valves were introduced. Tilting disc valves have a disc attached to the base ring, it opens at a specific angle and then closes back as flat disc. Many changes were made to the original tilting disc valve i.e. Bjork-shiley valve, to improve opening angle and to reduce thrombosis. But it showed defects, such as fracture in the metal strut. An improved version was introduced, Medtronic valve with titanium casing and carbon coated disc which showed less thrombosis and produced mechanically good results. Some of these valves are still in use but the first bileaflet valve, St. Jude medical valve is used more than any other valve. It is made of pyrolytic carbon and tungsten, with two semi-circular cusps attached to a hinge system. It shows less thrombosis and better functioning than other valves.Much more advances were made in bileaflet valves and the current valve is Medtronic advantage valve, it has some design changes and is suitable for supra-annular implantation.
Biologic valves can be mainly divided into bio-prosthetic valves, homografts and autografts. Bio-prosthetic valve consists of porcine and bovine valve material attached to prosthetic base. Porcine valves are preferred more than bovine valves. Stented porcine valves have porcine valve material sewed on stent for example, hancock bio-prosthesis valves which does not show any structural damage and provides better hemodynamics. Mosaic bio-prosthetic valves abide anti-calcification treatment. Homograft valves are the valves that are stored from unsuccessful heart transplants, pulmonary and aortic valves are commonly used as homografts. Autografts are replacement of pulmonary valve of the patient from its position and placing it in the damaged valve position of the same patient and implanting bio-prosthetic valve in pulmonary valve position.
The selection of valve for replacement is always a topic of debate. With the technology advancement and research, solutions for valve replacement has been found but still has its own drawbacks and leaves the patient in dilemma to decide the type of valve to be implanted.
Mechanical valves have life-long structural durability and freedom from reoperation but leads to life-long problem of anti-coagulation whereas bio-prosthetic valve does not show any thrombosis so anti-coagulation is not required. But bio-prosthetic valve deteriorates and is not durable, patients have to replace the bio-prosthetic valve in every 10-15 years. Age of patient plays an important role in selection of valve, younger patients can have bio-prosthetic replacement as re-operation might not be complicated unless they have other problems related to heart as on the other hand older patients can have mechanical replacement as due to their age it is not suggested to have re-operation. The above criteria can be considered but it is not possible completely, it is not true for all the patients. Some younger patients might have multiple valve problems and re-operation might not be the correct option for them.
Hemolysis might arise as a problem in mechanical valves which is not a problem in bio-prosthetic valve. Mechanical valves show bleeding complications and has life-long risk of endocarditis. The Bjork-shiley mechanical valves have defect of fracture in the metal struts, so the durability depends on the type of mechanical valve used. Ball cage valve have bulky design thus are not recommended for surgery involving small cavities. Homografts are natural form of valves but the success rate is very less. Patient's body might reject the valve, patient has high risk of getting some viral infection from the donor. As these valves are preserved, preservation and improper sterilization methods can also be a factor leading to failure of the valve. The advantage it has is hemodynamics of the valve is good.
Ross procedure is performed for implantation of autografts. As the pulmonary valve has similarities in structure with aortic valve, Ross procedure is performed on defective aortic valve. This procedure is risky but has showed good results on young patients.
After having tremendous options for valve replacement, till now no such valve has been developed which provides less or no complications. The factors to be considered are too many and each patient is not the same, so the success rate is different in different patients. It might depend on the lifestyle and care taken after surgery. The suggestion is that, as age plays a vital role in selection of valve so considering age a specific valve should be introduced for certain age group. The best way would be to find a better solution for thrombosis in mechanical valves.