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HEART VALVE SURGERY
The Aortic and Pulmonary Valves are integral tissues which form a structure within the heart, therefore to replace a diseased valve with a heart valve homograft (or allograft) requires surgical implantation – open-heart surgery is required. To understand why surgical implantation is required see information below. Anatomy of the Heart Chambers and Valves of the Heart The heart contains four chambers or compartments. The heart pumps blood by contracting (squeezing blood out of its chambers) and relaxing (allowing blood to enter its chambers). The two upper chambers are the atria. They receive blood returning from your body through your veins. The two lower chambers are the ventricles. They are the chambers where blood is pumped out through your body through arteries. The two atria contract gently to push blood into the ventricles. The ventricles contract forcefully to propel blood to your lungs and the rest of your body. The chambers on the left side of the heart must work harder in order to push blood and fluids through your body, so the left side of your heart is more muscular than the right. The heart has four major valves, which allow blood to flow in only one direction. The mitral and tricuspid valves control the flow of blood from the atria to the ventricles. The aortic and pulmonary valves control the flow of blood out of the ventricles.
Figure of a Normal, Healthy, Heart Valves Human heart valves are remarkable structures. These tissue-paper thin membranes attached to the heart wall undergo a constant "beating" (flexing of the tissue) day after day, year after year. In fact, they withstand about 80 million beats a year or 5 to 6 billion beats in an average lifetime. Each beat is an amazing display of strength and flexibility. Heart valves are structures that only allow blood to flow one way. The valves are strong, thin leaflets of tissue anchored to an opening that attaches to the myocardium. There are four cardiac valves. Two of the valves are referred to as the atrioventricular (AV) valves. They control blood flow from the atria to the ventricles and prevent blood from flowing back. The AV valve on the right side of the heart is called the tricuspid valve, because it has three flaps. The AV valve on the left side is called the mitral valve, or the bicuspid valve, because it has two flaps. The other two cardiac valves, the aortic and pulmonary valves, are for outflow. They are also called the semilunar valves because their three flaps are shaped like half moons. They control blood flow out of the ventricles. The aortic valve is located at the top of the left ventricle (between the left ventricle and the ascending, or upper portion of the aorta). The pulmonic valve is located between the right ventricle and pulmonary artery. The mitral and tricuspid valves are substantially larger than the aortic and pulmonary valves. The characteristic heart sounds ("lubb, dubb") are caused by the closing of the heart valves: the mitral and tricuspid valves make the first sound, and the aortic and pulmonary valves the second.
A perfect valve minimises any obstruction and allows blood to flow freely in one direction. It closes completely and quickly, and does not allow any blood to flow back through the valve. Regurgitation is backflow through a valve as the leaflets close; leakage regurgitation is backflow through the leaflets when they are closed. In normal functioning valves, there should be no leakage or regurgitation. The orifice-annulus ratio is the ratio of the valve opening area to the valve annulus (the circular area around the valve) diameter. In an ideal valve, this ratio should be 1:1. When a heart valve opens fully and evenly, blood flows through the valve smoothly. When a valve does not open fully or evenly, blood flowing through it can become turbulent and have areas of pooling, which can form blood clots. When blood flows backwards through the valve (regurgitation), the heart has to work harder to pump the same amount of blood. Regurgitation can also occur when a heart valve does not close quickly or completely. When a natural heart valve becomes defective, it can result in
This can occur in just one valve, or in more than one valve.
A leaky (regurgitant) aortic valve allows blood to flow back from the aorta to the left ventricle when the ventricle relaxes. When the heart valves are severely damaged, the surgeon/patient may choose to replace them. The first step is to remove the diseased natural valve (excise the valve during open-heart surgery) and then implant an allograft heart valve in its place. The allograft (homograft) valves used to replace the heart’s diseased natural valves come in different sizes to fit the patient’s anatomy. There are two broad categories of heart valves that are classified according to the type of material:
There is a wide variety of tissue valves:
Heterograft (or Xenograft) – valves or tissue taken from animals (i.e., pig or cow) and preserved. A porcine valve is made from a pig’s aortic heart valve. It can be sewn onto a flexible or semi-flexible frame to make a “stented” valve, or the natural aortic root is left intact to function as the frame to make a “stentless” valve. Another type of tissue valve is a pericardial valve. Pericardial valves traditionally contain leaflets made from bovine (cow) pericardium (the sac surrounding the heart) and are sewn onto a flexible or semi-flexible frame (stent). Homograft (or Allograft) - human valves or tissue taken from cadavers and heart transplant recipient donors.
Criteria and Selection of Heart Valves The surgeon will choose between mechanical and tissue valves based on an individual assessment of the benefits and risks of each valve and patient’s lifestyle, age and medical condition. The choice of implanting a tissue or mechanical valve is often based on age. However, there is no clear agreement on the exact age cut-off where a tissue valve may be preferable to a mechanical valve. The patient’s condition is extremely important in determining which type of valve to use. Because allograft tissue valves do not require long-term anti-clotting medication, very sick patients who might need surgery for coronary artery disease or kidney problems can be candidates for tissue valves. Women of childbearing age, young children and some elderly people also benefit from tissue valves. Heart Valve AbnormalitiesStenosisStenosis is the narrowing of a valve opening that result in lower blood flow through the valve. The primary cause of stenosis is calcification as a result of rheumatic fever, infection, congenital abnormalities, bacterial endocarditis or venereal disease. Except for stenosis caused by congenital abnormalities, the development of a stenotic valve begins with scarring of the valve cusps. Over time, two processes might occur: Scar tissue may accumulate to the point where the leaflets become thickened and begin to lose their flexibility, or the rougher scar tissue may become a site for calcification. The accumulated calcified masses reduces the flexibility of the leaflets. As the leaflets lose their flexibility, the valve becomes more stenotic. The area through which blood flows is gradually reduced, so less and less blood can pass through the valve with each contraction. As it becomes harder to push blood through the valve, the heart chamber before that valve feels greater pressures. Over time, this increasing pressure gradually stretches the muscles of the chamber wall, and they become thicker. An enlarged heart can lead to an increased likelihood of blood clots as the blood backs up in the enlarged chamber and pools. The more blood pools in one spot, the greater the possibility for it to form a clot. This complication arises more frequently when the heart is not in sinus rhythm (normal, coordinated contraction of heart muscles). Regurgitation (Insufficiency/Incompetence) When a valve cannot close completely, blood may flow backward through the valve, which is called regurgitation. The major causes of regurgitation are rheumatic fever, bacterial endocarditis, coronary artery disease, and venereal disease. There are three ways infections can cause regurgitation. First, infections can cause lesions (scars or rough spots) on the flaps. This scar tissue can build up to the point where the cusps are so rigid (and stenotic) that they cannot close properly. This results in backflow through the valve. A valve can be regurgitant and stenotic. Second, infections may cause tissue to break down, tear or form holes in the valves or around the perimeter of the valve (perivalvular leaks). Third, coronary artery disease can block the artery and cause papillary muscle dysfunction (inadequate or impaired function) because nutrients and oxygen do not reach the muscle tissue. Papillary muscles are structures that contract to make the mitral and tricuspid valves open. Infection and myocardial infarction can also cause other problems with the supporting valve structures and valve closure. Over time, regurgitation can become more severe. Generally, even though the initial damage to a valve may happen early in life, the early effects are mild and the defect progresses slowly. Although some regurgitation occurs with all valves, when the regurgitation rate attains a significant level, clinical problems may become noticeable and you may experience symptoms. Surgical InformationThe aortic or pulmonary valve allograft must permanently replace, totally or partially, the diseased aortic or pulmonary valve, and will remain with the patient on discharge. It is always the intention of everyone involved that the allograft heart valve will not be removed from the patient either surgically or otherwise at a later date. The reality is, at present there is no “perfect” heart valve prosthesis that can last forever in a patient. Durability depends on the type of valve replaced, the prosthesis used, the skills of the surgeon, the condition of the patient, the age and sex of the patient, patient’s care and compliance and other factors. Speak to your cardiologist or cardiothoracic surgeon for more information. With so many types of heart valve surgeries (repairs or replacements), results vary among patients. In the case of aortic or pulmonary valve replacement, patients can expect to return to their preoperative condition or better after they have had sufficient time to heal. After full recovery from a heart valve operation, activity should be almost unrestricted. A doctor can determine the patient’s risks, benefits, and post-surgical treatment plan. Longevity of an allograft tissue valve depends on many patient variables and medical conditions. This makes it impossible to predict how long a valve or repair device will last in any one patient. However, symptoms indicating that a tissue valve may need replacement typically occur over time and allow the doctor to schedule elective surgery, if the need arises. A well-implanted allograft valve normally lasts up to 20 years and in rare cases more. The patient should have regular check-ups with a cardiologist to monitor the performance of the implanted allograft. Re-operation to repair replacement valves is quite variable and depends on the reason for the repair. If a valve is calcified, it probably would be replaced with a new valve. Source: St. Vincent's Hospital Guides to Heart Surgery - Pictures courtesy of ADAM |
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