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Pathology

Acute and recurrent inflammation produces the typical pathologic features of MS due to rheumatic heart disease. These include fibrous thickening and calcification of the valve leaflets, fusion of the commissures (the borders where the leaflets meet), and thickening and shortening of the chordae tendineae.

Pathophysiology

In early diastole in the normal heart, the mitral valve opens and blood flows freely from the left atrium (LA) into the left ventricle (LV), such that there is a negligible pressure difference between the two chambers. In MS, however, there is obstruction to blood flow across the valve such that emptying of the LA is impeded and there is an abnormal pressure gradient between the LA and LV (Figs. 8-1 and 8-2). As a result, the left atrial pressure increases. Hemodynamic changes become apparent when the cross-sectional area of the valve, normally 4 to 6 cm2, is reduced to less than 2 cm2. The high left atrial pressure in MS is transmitted retrograde to the pulmonary circulation, resulting in increased pulmonary venous and capillary pressures (see Fig. 8-1). This elevation of hydrostatic pressure in the pulmonary vasculature may cause transudation of plasma into the lung interstitium and alveoli. The patient may therefore experience dyspnea and other symptoms of heart failure (as described in Chapter 9). In severe cases, significant elevation of pulmonary venous pressure leads to the opening of collateral channels between the pulmonary and bronchial veins. Subsequently, an engorged bronchial vein may rupture into a bronchus, resulting in hemoptysis (coughing up blood). The elevation of left atrial pressure in MS can result in two distinct forms of pulmonary hypertension: passive and reactive. Most patients with MS exhibit passive pulmonary hypertension, related to the backward transmission of the elevated LA pressure into the pulmonary vasculature as described in the previous paragraph. This actually represents an "obligatory" increase in pulmonary artery pressure that preserves forward flow in the setting of increased left atrial and pulmonary venous pressures. Additionally, approximately 40% of patients with MS demonstrate reactive pulmonary hypertension with medial hypertrophy and intimal fibrosis of the pulmonary arterioles. Reactive pulmonary hypertension initially serves a "beneficial" role because the increased arteriolar resistance impedes blood flow into the engorged pulmonary capillary bed and thus reduces capillary hydrostatic pressure (thereby "protecting" the pulmonary capillaries from even higher pressures). However, this benefit is at the cost of decreased blood flow through the pulmonary vasculature and elevation of the right-sided heart pressures, as the right ventricle pumps against the increased resistance. Chronic elevation of right ventricular pressure leads to hypertrophy and dilatation of that chamber and ultimately to right-sided heart failure. Chronic pressure overload of the LA in MS leads to left atrial enlargement. Left atrial dilatation stretches the atrial conduction fibers and may disrupt the integrity of the cardiac conduction system, resulting in atrial fibrillation (a rapid irregular heart rhythm; see Chapter 12). Atrial fibrillation contributes to a decline in cardiac output in MS because the increased heart rate shortens diastole. This reduces the time available for blood to flow across the obstructed mitral valve to fill the LV, and, at the same time, further augments the elevated left atrial pressure. In addition, with atrial fibrillation, there is a loss of the late diastolic atrial contraction that normally contributes to LV filling. The relative stagnation of blood flow in the dilated LA in MS, especially when combined with the development of atrial fibrillation, predisposes to intra-atrial thrombus formation. Thromboemboli to the brain and other organs may follow, leading to devastating complications such as cerebrovascular occlusion (stroke). Thus, MS patients who develop atrial fibrillation require chronic anticoagulation therapy. The consequences of MS primarily affect the left atrium and the pulmonary vasculature, as described above. Left ventricular pressures are usually normal, but impaired filling of the chamber through the stenotic valve may reduce LV stroke volume and cardiac output.

4. Clinical Manifestations and Evaluation of Mitral Stenosis

a. Presentation

The natural history of MS is variable. Survival exceeds 80% in asymptomatic or minimally symptomatic patients at 10 years. However, the 10-year survival of untreated patients after onset of symptoms is only 50-60%. Longevity is much more limited for patients with advanced symptoms and is dismal for those who develop significant pulmonary hypertension, with a mean survival of less than 3 years. The clinical presentation of MS depends largely on the degree of reduction of the valve area. The more severe the stenosis, the greater the symptoms related to elevation of left atrial and pulmonary venous pressures. The earliest manifestations are those of dyspnea and reduced exercise capacity. In mild MS, dyspnea may be absent at rest; however, it develops on exertion as LA pressure rises with the exercise-induced increase in blood flow through the heart and faster heart rate (i.e., decreased diastolic filling time). Other conditions and activities that augment heart rate and cardiac blood flow and precipitate or exacerbate symptoms of MS include fever, anemia, hyperthyroidism, pregnancy, rapid arrhythmias such as atrial fibrillation, emotional stress, and sexual intercourse. With more severe MS (i.e., a smaller valve area), dyspnea occurs even at rest. Increasing fatigue and more severe signs of pulmonary congestion, such as orthopnea and paroxysmal nocturnal dyspnea (described in Chapter 9), occur. With advanced MS and pulmonary hypertension, signs of right-sided heart failure ensue, including jugular venous distention, hepatomegaly, ascites, and peripheral edema. Compression of the recurrent laryngeal nerve by an enlarged pulmonary artery or LA may cause hoarseness (known as Ortner syndrome). Less often, the diagnosis of MS is heralded by one of its complications: atrial fibrillation, thromboembolism, infective endocarditis, or hemoptysis, as described in the earlier section on Pathophysiology.

b. Examination

On examination, there are several typical findings of MS. Palpation of the chest may reveal a right ventricular "tap" in patients with increased right ventricular pressure. Auscultation discloses a loud S1 (the first heart sound, which is associated with mitral valve closure) in the early stages of the disease. The increased S1 results from the high pressure gradient between the atrium and ventricle, which keeps the mobile portions of the mitral valve leaflets widely separated throughout diastole; at the onset of systole, ventricular contraction abruptly slams the leaflets together from a relatively wide position, causing the closure sound to be more prominent (see Chapter 2). In late stages of the disease, the intensity of S1 may normalize or become reduced as the valve leaflets thicken, calcify, and become less mobile. A main feature of auscultation in MS is a high-pitched "opening snap" (OS) that follows S2. The OS is thought to result from the sudden tensing of the chordae tendineae and stenotic leaflets on opening of the abnormal valve. The interval between S2 and the OS relates inversely to the severity of MS. That is, the more severe the MS, the higher the LA pressure and the earlier the valve is forced open in diastole.

The OS is followed by a low-frequency decrescendo murmur (termed diastolic rumble) caused by turbulent flow across the stenotic valve during diastole (see Fig. 8-2). The duration, but not the intensity, of the diastolic murmur relates to the severity of MS. The more severe the stenosis, the longer it takes for the LA to empty and for the gradient between the LA and LV to dissipate. Near the end of diastole, contraction of the LA in patients in sinus rhythm causes the pressure gradient between the LA and LV to rise again (see Fig. 8-2); therefore, the murmur briefly becomes louder at that time (termed presystolic accentuation). This final accentuation of the murmur does not occur if atrial fibrillation has developed because there is no effective atrial contraction in that situation. Murmurs caused by other valve lesions are often found concurrently in patients with MS. For example, mitral regurgitation (discussed later in this chapter) frequently coexists with MS. Additionally, right-sided heart failure caused by severe MS may induce tricuspid regurgitation as a result of right ventricular enlargement. A diastolic decrescendo murmur along the left sternal border may be present owing to coexistent aortic regurgitation (because of rheumatic involvement of the aortic leaflets) or pulmonic regurgitation (because of MS-induced pulmonary hypertension).

The electrocardiogram in MS routinely shows left atrial enlargement and, if pulmonary hypertension has developed, right ventricular hypertrophy. Atrial fibrillation may be present. The chest radiograph reveals left atrial enlargement, pulmonary vascular redistribution, interstitial edema, and Kerley B lines resulting from edema within the pulmonary septae (see Chapter 3). With the development of pulmonary hypertension, right ventricular enlargement and prominence of the pulmonary arteries appear. Echocardiography is of major diagnostic value in MS. Structural findings include thickened mitral leaflets with abnormal fusion of their commissures and restricted separation during diastole. The degree of left atrial enlargement can be quantified, and if present, intra-atrial thrombus may be visualized. The mitral valve area can be measured directly on cross-sectional views or calculated from Doppler velocity measurements (a technique known as the "diastolic pressure half-time"). Patients can be stratified into stages of disease severity based partly on the mitral valve area. A normal mitral valve orifice measures between 4 and 6 cm2. Current guidelines define clinically important "severe" MS as a valve area ≤1.5 cm2, a state that is typically accompanied by LA enlargement and elevated pulmonary artery systolic pressure. A valve area ≤1.0 cm2 is termed "very severe" MS. If the findings determined by echocardiography seem milder than the patient's history and examination suggest, an exercise test with accompanying Doppler assessment, or cardiac catheterization may be warranted to further define hemodynamic measurements.

5. Treatment of Mitral Stenosis

Salt intake restriction and diuretic therapy may improve symptoms due to vascular congestion. Heart rate slowing agents, such as β-blockers or nondihydropyridine calcium channel blockers (e.g., diltiazem or verapamil, see Chapter 17), increase diastolic LV filling time and therefore ease symptoms that occur during exercise. These drugs, or digoxin, are similarly useful to slow the ventricular rate in patients with accompanying rapid atrial fibrillation. Anticoagulant therapy to prevent thromboembolism is recommended for MS patients with atrial fibrillation, or an identified atrial thrombus, or prior embolic events. Percutaneous or surgical valve interventions are the only treatments that alter the natural history of MS and are indicated in patients with severe, symptomatic MS. Percutaneous balloon mitral valvuloplasty is the treatment of choice in appropriately selected patients (those without advanced anatomic deformity of the valve, mitral regurgitation, or left atrial thrombus). During this procedure, a balloon catheter is advanced from the femoral vein into the right atrium, across the atrial septum (by intentionally puncturing the interatrial septum), and through the narrowed mitral valve orifice. The balloon is then rapidly inflated, thereby "cracking" open the fused commissures. The short- and long-term results of this procedure are typically excellent and compare favorably with those of surgical treatment in anatomically appropriate patients. In young adults with the most suitable anatomy for the procedure, the event-free survival rate approaches 80% to 90% over 3 to 7 years of follow-up. Approximately 5% of patients undergoing balloon mitral valvuloplasty are left with a residual atrial septal defect due to the transseptal puncture. Less frequent complications include cerebral emboli at the time of valvuloplasty, cardiac perforation by the catheter, or the unintentional creation of substantial mitral regurgitation. Open mitral valve commissurotomy (an operation in which the stenotic commissures are separated under direct visualization) may be undertaken in patients for whom percutaneous balloon valvuloplasty is not feasible or successful. It is effective in relieving obstruction, and restenosis occurs in fewer than 20% of patients over 10 to 20 years of follow-up. Perioperative mortality rates are low (2%). Mitral valve replacement is considered in patients who are not appropriate candidates for balloon valvuloplasty or open commissurotomy.