Porth's Essentials of Pathophysiology, 4e

461

Disorders of Cardiac Function

C h a p t e r 1 9

Pulmonary veins Aortic valve

Left atrium

Mitral valve

Systole

Left ventricle

Aortic valve stenosis

Mitral valve regurgitation

FIGURE 19-15. Alterations in hemodynamic function that accompany aortic valve stenosis, mitral valve regurgitation, mitral valve stenosis, and aortic valve regurgitation.The thin arrows indicate direction of normal flow, and thick arrows the direction of abnormal flow. Expand description.

Diastole

Mitral valve stenosis

Aortic valve regurgitation

The effects of valvular heart disease depend on the valve involved, the degree of involvement, the rapidity of onset, and the rate and adequacy of compensatory mechanisms. For example, sudden destruction of an aortic valve cusp by infection can cause massive regurgi- tation and rapid heart failure, whereas rheumatic mitral stenosis usually develops over years without obvious symptoms. Abnormal turbulent flow through diseased valves typically produces abnormal heart sounds called murmurs . Mitral Valve Disorders The mitral or left atrioventricular (AV) valve controls the directional flow of blood between the left atrium and the left ventricle. The edges or cusps of both AV valves, which are thinner than those of the semilunar (i.e., pul- monic and aortic) valves, are anchored to the papillary muscles by the chordae tendineae. During much of sys- tole, the mitral valve is subjected to the high pressure generated by the left ventricle as it pumps blood into the systemic circulation. During this period of increased pressure, the chordae tendineae prevent the eversion of the valve leaflets into the left atrium. Mitral Valve Stenosis. Mitral valve stenosis represents the incomplete opening of the mitral valve during dias- tole, with left atrial distension and impaired filling of the left ventricle (see Fig. 19-15). Mitral valve stenosis is most commonly the result of rheumatic fever. 37,38 Less frequently, the defect is congenital and manifests dur- ing infancy or early childhood or calcification in elderly

patients. Mitral valve stenosis is a continuous, progres- sive, lifelong disorder consisting of a slow, stable course in the early years and progressive acceleration in later years. Mitral valve stenosis is characterized by fibrous replacement of valvular tissue, along with stiffness and fusion of the valve apparatus (see Fig. 19-14). Typically, the mitral cusps fuse at the edges and the chordinae ten- dinae thicken and shorten pulling the valvular structures into the ventricles. As the resistance to flow through the valve increases, the left atrium dilates and left atrial pressure increases. The increased left atrial pressure eventually is transmitted to the pulmonary venous sys- tem, causing pulmonary congestion. A characteristic auscultatory finding in mitral stenosis is an opening snap following the second heart sound, which is caused by the stiff mitral valve. As the stenosis worsens, there is a localized low-pitched diastolic murmur that increases in duration with the severity of the stenosis. The clinical presentation of mitral valve stenosis depends on the severity of the obstruction or the degree of reduction in the valve area—the more severe the stenosis, the greater the symptoms. Manifestations are related to the elevation in left atrial pressure and pulmonary congestion such as dyspnea with exer- tion, decreased cardiac output owing to impaired left ventricular filling, and left atrial enlargement with the development of atrial arrhythmias and mural thrombi. More severe stenosis is associated with symptoms of pulmonary congestion, including paroxysmal noctur- nal dyspnea (PND) and orthopnea. Palpitations, chest pain, weakness, and fatigue are common complaints.