Book Text

Mitral Stenosis

Mitral stenosis (MS) is almost always rheumatic in origin. Thus, though still a common indication for mitral valve replacement, its overall incidence in developed countries continues to decline with improved socioeconomic conditions. MS exhibits a 2:1 female preponderance. Infrequent causes include congenital defects (seen in infancy and childhood), malignant carcinoid (although right heart lesions are its usual cardiac manifestation), rheumatoid arthritis, and prolapse of left atrial myxomas into the mitral valve opening.

Although rheumatic fever causes pancarditis, the long-term sequelae are confined to the endocardium (including the valves). Pure MS occurs in 25% of patients who develop long-term rheumatic sequelae. The remaining patients have either combined lesions in one valve (e.g., combined stenosis and regurgitation occur in 40% of patients) or combined valvular disease (aortic plus mitral disease). Rheumatic-induced chronic aortic valve disease never occurs in isolation; concurrent mitral valve disease is also present. Although the interval between rheumatic fever and the development of MS is at least 2 years, patients are usually asymptomatic for 2 decades and symptoms generally develop in the 3rd and 4th decades of life. Progression is much more rapid in underdeveloped countries. It is unclear whether the chronic valvular sequelae of rheumatic fever reflect ongoing smoldering rheumatic fever or the consequences of turbulent blood flow patterns induced by the initial rheumatic process in a manner analogous to the development of aortic stenosis in a patient with a bicuspid aortic valve. In patients with chronic

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Figure 50-22 Hemodynamics of aortic regurgitation. A, Normal conditions. B, The hemodynamic changes that occur in severe acute aortic regurgitation. Although total stroke volume is increased, forward stroke volume is reduced. Left ventricular end-diastolic pressure rises dramatically. C, Hemodynamic changes occurring in chronic compensated aortic regurgitation are shown. Eccentric hypertrophy produces increased end-diastolic volume, which permits an increase in total as well as forward stroke volume. The volume overload is accommodated, and left ventricular filling pressure is normalized. Ventricular emptying and end-systolic volume remain normal. D, In chronic decompensated aortic regurgitation, impaired left ventricular emptying produces an increase in end-systolic volume and a fall in ejection fraction, total stroke volume, and forward stroke volume. Further cardiac dilatation occurs along with re-elevation of left ventricular filling pressure. E, Immediately after valve replacement, preload estimated by end-diastolic volume decreases, as does filling pressure. End-systolic volume is also decreased, but to a lesser extent. The result is an initial fall in ejection fraction. Despite these changes, elimination of regurgitation leads to an increase in forward stroke volume. AoP, aortic pressure; EDV, end-diastolic volume; EF, ejection fraction; ESV, end-systolic volume; LVEDP, left ventricular end-diastolic pressure; RF, regurgitant fraction. (From Carabello BA: Aortic regurgitation: Hemodynamic determinants of prognosis. In Cohn LH, DiSesa VJ [eds]: Aortic Regurgitation: Medical and Surgical Management. New York, Marcel Dekker, 1986.)

rheumatic mitral valve disease, the pathology (fibrosis/adhesions) affects the commissures predominantly in 30% of cases, the cusps in 15%, the chords in 15%, and a combination in the remainder. This differential distribution of pathology does have functional implications. For example, when rheumatic fever affects mainly the chords, the dominant lesion is likely to be mitral regurgitation.^[143] Moreover, the severity of calcification in MS correlates with the transvalvular pressure gradient.^[144] Severe MS, if untreated, results in progressive elevations in left atrial pressure, pulmonary vascular pressures, left atrial size, elevation of the left main stem bronchus, atrial fibrillation, left atrial thrombus formation, and obliterative (and thus irreversible) pulmonary vascular changes and resultant right heart failure.

The essential characteristics of a “normal” mitral valve are

Area greater than 4.0 cm²
Diastolic mitral valve flow of 150 to 200 mL/sec/diastole
Diastolic transvalvular pressure gradient of less than 2 mm Hg
LVEDP of approximately 5 mm Hg.

Intuitively, we would predict that for any given mitral valve area, the higher the flow the higher the gradient. This relationship is not linear. Very high flows do not result in adverse hemodynamic consequences (i.e., elevated left atrial and pulmonary vascular pressures) in the presence of a normal mitral valve area ( Fig. 50-23 ). More important than the lack of linearity is divergence of the relationship between diastolic flow and the transvalvular pressure gradient as the mitral valve area progressively decreases (see Fig. 50-23 ). At any given flow, the pressure gradient increases as mitral valve area decreases. As a corollary, at any given mitral valve area, the gradient also increases as flow increases. Thus, patients with normal flow and moderate MS are subject to acute elevations in left atrial and pulmonary vascular pressures and thus symptoms when flow increases precipitously (e.g., pregnancy, hypervolemia, hyperthyroidism) and/or diastolic time decreases (e.g., new onset of atrial fibrillation with rapid ventricular response, which differentially shortens diastolic time).

The Gorlin formula^[130] is used to calculate mitral valve area and dictates, as inferred earlier, that flow and the

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Figure 50-23 The gradient across mitral valves of various sizes is plotted according to mitral valve flow. Note that when mitral valve flow is in the normal range (150 to 200 mL/sec of diastole), the gradient across a mitral valve area (MVA) of 2 cm² is 4 to 6 mm Hg. With an MVA of 1 cm² , the gradient with normal mitral valve flow is 18 to 28 mm Hg. (Redrawn from Dalen JE, Alpert JS [eds]: Valvular Heart Disease, 2nd ed. Boston, Little, Brown, 1987.)

pressure gradient be measured simultaneously. The formula for calculating mitral valve area is

where DM signifies diastolic mean. In any specific patient, mitral valve area is relatively fixed, whereas flow and the gradient are changing variables. Rearranging the above formula reveals that flow is proportional to the square of the gradient. Thus, if one doubles flow, one will quadruple the transvalvular pressure gradient. Although the Gorlin formula and the assumption that mitral valve area is fixed have been used for years to assess patients with mitral valve disease, the concept that flow per se may influence mitral valve area (increased flow leading to orifice distention) has legitimacy and has led to the suggestion that mitral valve resistance (the quotient of flow and pressure gradient) be used to quantify stenosis.^[145] Nevertheless, the calculation of mitral valve area remains in universal use. MS is considered mild when the mitral valve area is between 1 and 2 cm² and critical if less than 1.0 cm² . When MS is critical, a transvalvular pressure gradient of 20 mm Hg is required to maintain normal cardiac output. An LVEDP of 5 mm Hg implies that left atrial pressure is 25 mm Hg, a level that causes pulmonary congestion and the onset of dyspnea symptoms. As MS progresses, left ventricular filling becomes increasingly dependent on adequate diastolic filling time and on atrial kick, which may be responsible for up to 30% of left ventricular filling. The onset of atrial fibrillation eliminates the latter and compromises the former to a degree that is proportional to the ventricular response. Hence, there is a precipitous onset of symptoms with atrial fibrillation (initially often paroxysmal) and relief of symptoms with rate control by the use of cardiac glycosides, β-blockers, or both. Atrial fibrillation in the setting of MS is not simply a function of left atrial enlargement; it also reflects the effects of the rheumatic process (inflammation, fibrosis) on intra-atrial electrophysiologic characteristics.

Left ventricular end-diastolic pressure and volume are usually normal in patients with pure MS, although left ventricular end-diastolic volume is decreased in a minority of patients. Indices of overall systolic function indicate that systolic function is compromised in up to 25% of patients. Moreover, regional wall motion abnormalities are common. These changes may reflect the underlying rheumatic process or concomitant coronary artery disease and may explain why a subpopulation of patients with MS do poorly after mitral valve replacement.

Patients with MS may have cardiac symptoms or noncardiac symptoms secondary to thromboembolic phenomena. The dominant cardiac symptoms reflect the underlying pathophysiology, specifically, the ability to maintain normal cardiac output and the status of the pulmonary vasculature. Patients with moderate stenosis (mitral valve area of 1 to 1.5 cm² ) may not necessarily have significant secondary pulmonary vasculature changes, but experience profound dyspnea secondary to pulmonary congestion if the valvular gradient increases acutely (e.g., new-onset atrial fibrillation, increased cardiac output with exercise). Conversely, patients with severe stenosis (mitral valve area less than 1 cm² ) are likely to have significant secondary pulmonary vascular changes and an inability to generate an adequate cardiac output and thus complain primarily of fatigue and weakness. Patients with MS are always at risk of infective endocarditis and may have hemoptysis or compression of local structures secondary to left atrial expansion (e.g., recurrent laryngeal nerve and esophageal compression leading to dysphagia).

Patients with asymptomatic MS are managed medically, whereas those with mild symptoms should have their management customized to their specific medical and domestic circumstances. Patients with symptomatic critical stenosis and those with thromboembolic phenomena are managed surgically. Surgical options include closed balloon commissurotomy, closed and open surgical commissurotomy, and mitral valve replacement. All commissurotomy procedures are associated with mitral regurgitation and thromboembolism, although this intervention may be appropriate in selected patients, such as those with little calcification, minimal mitral regurgitation, or subvalvular disease involvement.^[146]

Successful intraoperative management of patients with MS requires not only knowledge of what constitutes a normal/abnormal mitral valve area but also an understanding of (1) the implications of mitral annular and mitral valve calcification; (2) the possibility of concurrent non-mitral valve disease; (3) the fundamental interaction between flow, diastolic time, and the transvalvular mitral valve gradient, plus the implications that will ensue proximally (e.g., left atrial pressure, pulmonary artery pressure, pulmonary congestion), and distally (cardiac output); (4) the influence of rheumatic disease on left ventricular function; (5) the implications of visualizing the left atrial appendage and confirming the presence or absence of thrombus; (6) the reversibility of pulmonary vascular

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changes (increased pulmonary resistance in this setting has three components: passive congestion, vasoconstriction, and obliterative irreversible vascular changes); (7) right ventricular function; and (8) parenchymal pulmonary changes (decreased compliance secondary to vascular congestion) with its implications for intraoperative and postoperative ventilator management.