Porth's Essentials of Pathophysiology, 4e

463

Disorders of Cardiac Function

C h a p t e r 1 9

calcification associated with age-related degenerative changes or the normal “wear and tear” of either pre- viously normal aortic valves or congenitally bicuspid valves with which approximately 1% of the popula- tion are born. 9 The incidence of acquired aortic valve stenosis is increasing with the rising average age of the population. 6,40 The progression of calcific aortic stenosis is usu- ally slow and varies widely among individuals. It usu- ally becomes clinically evident in the sixth and seventh decades in persons with bicuspid aortic valves, and not until the eighth and ninth decades in those with pre- viously normal valves. Valve changes range from mild thickening without obstruction to severe calcification with impaired leaflet motion and obstructed left ven- tricular outflow. 41 Processes in the development of cal- cific aortic valve disease have been shown to be similar to those in CAD. Both conditions are more common in men, older persons, and persons with hypercholester- emia, and both derive in part from an active inflamma- tory process. 40 Early lesions of aortic sclerosis show focal subendo- thelial plaquelike lesions, similar to the initial phases of an atherosclerotic lesion. Aortic sclerosis is distinguished from aortic stenosis by the degree of valve impairment. In aortic sclerosis the valve leaflets are abnormally thick- ened, but the obstruction to outflow is minimal, whereas in aortic stenosis the functional area of the valve has decreased enough to cause measurable obstruction to outflow. Calcification of the aortic valve progresses from the base of the cusps to the leaflets. This reduces leaflet motion and effective valve area, but without com- missural fusion. As calcification progresses, the leaflets become more rigid, there is worsening of obstruction to left ventricular outflow, and fusion of the commissures leads to aortic stenosis. Because aortic stenosis usually develops gradually, the left ventricle has time to adapt. With increased systolic pressure from obstruction, the left ventricular wall becomes thicker, or hypertrophies, but a normal chamber volume is maintained. With this increase in wall thickness, called concentric remodeling, the left ventricular ejection fraction is maintained within nor- mal limits. Hemodynamic variables remain stable even though the valve opening area is reduced to half of its normal size. However, an additional reduction in the valve opening area from one half to one fourth of its normal size produces severe obstruction to flow and a progressive increase in the pressure overload on the left ventricle. At this point, the increased work of the heart begins to exceed the coronary blood flow reserve, caus- ing both systolic and diastolic dysfunction and signs of heart failure. 38,40,41 Aortic stenosis is usually first diagnosed with aus- cultation of a loud systolic ejection murmur or a single or paradoxically split-second heart sound. Eventually, the classic symptoms of advanced aortic stenosis such as angina, syncope, and heart failure develop, although more subtle signs of a decrease in exercise tolerance or exertional dyspnea should be monitored closely. Angina occurs in approximately two thirds of

apparatus as the leaflets prolapse. Two-dimensional and Doppler echocardiography is used to diagnose mitral valve prolapse. The treatment of mitral valve prolapse focuses on the relief of symptoms and the prevention of compli- cations. 38 Persons with palpitations and mild tachyar- rhythmias or increased adrenergic symptoms, and those with chest discomfort, anxiety, and fatigue often respond well to therapy with beta blockers. In many cases, the cessation of stimulants such as caffeine, alco- hol, and cigarettes may be sufficient to control symp- toms. Although infective endocarditis is an uncommon complication in persons with a murmur, antibiotic pro- phylaxis may be recommended before extensive den- tal or surgical procedures associated with bacteremia. Transient ischemic attacks occur more frequently in per- sons with mitral valve prolapse. Therefore, daily aspirin therapy is recommended in persons with documented events who have a sinus rhythm and no atrial thrombi. Persons with severe valve dysfunction may require valve surgery. Aortic Valve Disorders The aortic valve, which is located between the left ven- tricle and aorta, has three cuplike cusps and sometimes is referred to as the aortic semilunar valve because its leaflets are crescent or moon shaped. The aortic valve has no chordae tendineae. Although their structures are similar, the edges of the aortic valve leaflets are thicker than those of the mitral valve, particularly at the middle of the leaflet (see Chapter 17, Fig. 17-10). The mecha- nism of action of the aortic valve is also different from that of the mitral valve. During ventricular systole when the ventricles contract and blood is being ejected into the aorta, the cusps of the valve are flattened against the aor- tic wall and blood rushes past them; when the ventricles relax and the blood (no longer propelled forward by the pressure of ventricular contraction) begins to flow back- ward toward the heart, it fills the cusps and this closes the valve. An important feature of the aortic valve is the loca- tion of the orifices for the two main coronary arteries, which are located behind the valve and at right angles to the direction of blood flow. It is the lateral pressure in the aorta that propels blood into the coronary arteries. During the ejection phase of the cardiac cycle, the lateral pressure is diminished by conversion of potential energy to kinetic energy as blood moves forward into the aorta. This process is grossly exaggerated in aortic valve steno- sis because of the high flow velocities. Aortic Valve Stenosis. Aortic valve stenosis is charac- terized by narrowing of the valve orifice with increased resistance to ejection of blood from the left ventricle into the aorta (see Fig. 19-15). The most common causes of aortic valve stenosis are congenital valve malformations and acquired calcification of a normal aortic valve. Congenital malformations may result in unicuspid, bicuspid, or misshapen valve leaflets. Acquired aortic stenosis is usually the consequence of