Porth's Essentials of Pathophysiology, 4e

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Circulatory Function

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Paroxysmal atrial tachycardia and atrial fibrillation develop in 30% to 40% of persons with symptomatic mitral stenosis. 38 Together, fibrillation and distention of the atria predispose the person to mural thrombus for- mation. The risk of arterial embolization, particularly stroke, is significantly increased in persons with atrial fibrillation. Mitral Valve Regurgitation. Mitral valve regurgita- tion is characterized by incomplete closure of the mitral valve, with the left ventricular stroke volume being divided between the forward stroke volume that moves blood into the aorta and the regurgitant stroke volume that moves it back into the left atrium during systole (see Fig. 19-15). Mitral valve regurgitation can result from many processes that result in a rigid and thickened valve that does not open or close completely, such as IE or RHD. It can also result from rupture of the chordae tendineae or papillary muscles due to an MI, papillary muscle dysfunction, or stretching of the valve struc- tures due to dilation of the left ventricle or valve orifice. Mitral valve prolapse is a common cause of mitral valve regurgitation in lean, thin women. The hemodynamic changes associated with chronic mitral valve regurgitation occur gradually, allowing the left atrium to undergo compensatory responses. An increase in left ventricular end-diastolic volume permits an increase in total stroke volume, with restoration of forward flow into the aorta. Augmented preload and reduced or normal afterload (provided by unloading the left ventricle into the left atrium) facilitate left ven- tricular ejection. At the same time, a gradual increase in left atrial size allows for accommodation of the regurgi- tant volume at a lower filling pressure. A characteristic feature of mitral valve regurgitation is an enlarged or hypertrophied left ventricle, a hyperdynamic left ven- tricular impulse, and a pansystolic (throughout systole) heart murmur. Mitral regurgitation, like mitral stenosis, predisposes to atrial fibrillation. The increased volume work associated with mitral regurgitation is relatively well tolerated, and many per- sons with the disorder remain asymptomatic for many years, developing symptoms between 6 and 10 years after diagnosis. The severity of regurgitation reflects the degree of left ventricular enlargement and may correlate with murmur intensity 39 and typically correlates with the patients’ symptoms. 38 As the disorder progresses, left ventricular function becomes impaired, the forward (aortic) stroke volume decreases, and the left atrial pres- sure increases, with the subsequent development of pulmonary congestion. It is usually recommended that valve surgery be performed before the onset of these symptoms. Mitral Valve Prolapse. Sometimes referred to as floppy mitral valve syndrome, mitral valve prolapse occurs in 1% to 2.5% of the general population. 37 The disorder is seen more frequently in thin women, and there may be a familial basis. Familial mitral valve prolapse is transmit- ted as an autosomal dominant trait, and several chro- mosomal loci have been identified. Although the exact

cause of the disorder usually is unknown, it has been associated with Marfan syndrome, osteogenesis imper- fecta, and other connective tissue disorders, and with cardiac, hematologic, neuroendocrine, metabolic, and psychological disorders. Pathologic findings in persons with mitral valve pro- lapse include a myxedematous (mucinous) degenera- tion of mitral valve leaflets that causes them to become enlarged and floppy so that they prolapse or balloon back into the left atrium during systole 37 (Fig. 19-16). Secondary fibrotic changes reflect the stresses and injury that these ballooning movements impose on the valve. Certain forms of mitral valve prolapse may arise from disorders of the myocardium that place undue stress on the mitral valve because of abnormal movement of the ventricular wall or papillary muscle. Mitral valve pro- lapse may or may not cause mitral regurgitation. Most people with mitral valve prolapse are asymp- tomatic and the disorder is discovered during a routine physical examination. A minority of people have symp- toms such as chest pain, dyspnea, fatigue, anxiety, pal- pitations, and light-headedness. Unlike angina, the chest pain often is prolonged, ill defined, and not associated with exercise or exertion. The pain has been attributed to ischemia resulting from traction of the prolapsing valve leaflets. Palpitations, arrhythmias, and anxiety may result from abnormal autonomic nervous system function that commonly accompanies the disorder. Rare cases of sudden death have been reported for persons with mitral valve prolapse, mainly those with a family history of similar occurrences. The disorder is characterized by a spectrum of aus- cultatory findings, ranging from a silent form to one or more midsystolic clicks followed by a late systolic or pansystolic murmur heard best at the apex. The clicks are caused by the sudden tensing of the mitral valve

FIGURE 19-16. Mitral valve prolapse. A view of the mitral valve from the left atrium shows redundant and deformed leaflets that billow into the left atrial cavity. (From Saffitz JE. The heart. In: Rubin E, Strayer DS, eds. Rubin’s Pathology: Clinicopathologic Foundations of Medicine. 6th ed. Philadelphia, PA: Wolters Kluwer Health | Lippincott Williams & Wilkins; 2012:518.)