In a healthy heart, the forward flow of blood through the heart chambers is controlled by four valves: mitral, tricuspid, aortic, and pulmonic. The mitral and tricuspid (atrioventricular) valves are situated between the atria and ventricles. The aortic and pulmonic (semilunar) valves are situated between the ventricles and their respective great vessels (aorta and pulmonary artery). The atrioventricular valves control the flow of relatively low velocity blood between the atria and ventricles and thus are "floppier" and seated around a much larger sized orifice than are the semilunar valves, which are smaller and stiffer and designed to control high velocity blood ejected out of the ventricles. The opening and closing of the valves through the cardiac cycle is passive and results from differences in hydraulic pressure between the chambers on either side of the valve. The atrioventricular valves open in diastole, allowing blood to pass from blood-filled right and left atria into the empty right and left ventricles, which are at rest. When the blood-filled ventricles contract, pressure within the ventricles rapidly increases, slamming shut the atrioventricular valves (preventing "backward" flow of blood into the atria) and forcing open the semilunar valves, resulting in forward ejection of blood into the great vessels.
Two disease processes can affect the heart valves: stenosis and regurgitation. In valvular stenosis, a valve may become stiffened and immobile. Such a stiffened valve does not open properly and actually impedes the forward flow of blood. One example of this pathology is rheumatic mitral valve stenosis. In this disease, the mitral valve becomes damaged by an immunologic reaction resultant from a childhood episode of rheumatic fever. The mitral valve becomes stiff, calcified, and unable to open properly. Thus, blood cannot easily pass from left atrium to left ventricle. This leads to enlargement of the left atrial chamber and backup of blood into the lungs. As the disease progresses, the lungs become increasingly engorged with blood and lung scarring occurs. This causes symptoms of shortness of breath, weakness, and dizziness.
Valvular stenosis may also be congenital, such as congenital tricuspid stenosis, or congenital bicuspid aortic valve leading to aortic stenosis in the adult.
Mild valvular stenosis is treated medically. Severe valvular stenosis must be treated surgically, with valve repair or replacement. In some cases where the valve has not become severely calcified, valvuloplasty may be performed percutaneously with a balloon catheter inserted through the groin. The balloon is placed across the valve and inflated, stretching the valve open.
A valve may also become regurgitant (leaky). When a normal, healthy valve closes, the leaflets do not come together in a perfect, seamless fashion and a trivial amount of "leakage" does occur. However, a valve that becomes diseased can leak, or regurgitate, severely. One example of this occurs in bacterial endocarditis, when a heart valve becomes infected by bacteria. Bacterial organisms grow on and destroy valve tissue such that the valve leaflets do not close properly, resulting in leakage of blood through the closed valve. Another example is severe mitral valve prolapse with regurgitation.
Valvular regurgitation may be treated medically when it is "mild" or "moderate." Severe regurgitation must be treated with surgical valve replacement.