Mitral Regurgitation

The incidence of mitral regurgitation (MR) in the population reflects its underlying causes. MR can occur either acutely or chronically and is a relatively common indication for cardiac surgical intervention. The latter usually involves either mitral valve replacement or mitral valve repair. The underlying cause of MR has a significant influence on the decision to replace or repair the valve; for example, MR associated with idiopathic calcification is unlikely to be amenable to repair, whereas that associated with prolapse frequently is.

MR is best assessed not only in the context of the mitral valve per se but also in the context of the "mitral apparatus" as a whole. The mitral apparatus includes (1) the mitral annulus, (2) the valve leaflets, (3) the chordae tendineae, and (4) the papillary muscles. Abnormalities and dysfunction of any of these structures can cause MR. MR has many causes ( Table 50-5 ), but acute MR is most often caused by infective endocarditis, ruptured chordae tendineae, and coronary artery disease with acute global left ventricular dysfunction. Chronic MR is most frequently caused by rheumatic disease, degenerative disease (mitral valve prolapse syndrome, Marfan's syndrome), or dilatation of the mitral valve annulus and left ventricular cavity. The mitral valve prolapse syndrome with degenerative disease of the mitral valve involving myxomatous infiltration and fibroelastic deficiency has many names, including Barlow's syndrome, billowing of the mitral leaflet, mitral valve prolapse, floppy mitral valve, and flail leaflet. This syndrome was first described in the 1960s by Reid.^{[147] [148]} Their clinical observation of an apical mid-to-late systolic click occurs frequently and has been described in up to 5% of the population. However, the clinical consequences of degenerative disease of the mitral valve represent a spectrum, with many patients being asymptomatic ( Fig. 50-24 ). Application of more rigorous echocardiographic criteria indicates that clinically relevant problems occur in up to 2.4% of the population. It is more frequent in females (2:1 female-to-male ratio) and does not, as once suggested, have an autosomal dominant inheritance.^[149] Though usually confined to the mitral valve, myxomatous degeneration can also involve the aortic, pulmonary, and tricuspid valves, especially in patients with Marfan's syndrome.

The degree of MR is directly related to orifice size and the pressure gradient between the left ventricle and left atrium. Importantly, both these variables are dynamic and can be manipulated and thus modulate the magnitude of regurgitation. Increasing preload (because it increases left ventricular size), increasing afterload, and decreasing contractility (leading to increased left ventricular size) all tend to accentuate MR, whereas the opposite changes in these parameters tend to attenuate regurgitation.

TABLE 50-5 -- Causes of mitral regurgitation

Acute *

Mitral Annulus Disorders

Infective endocarditis (abscess formation)

Trauma (valvular heart surgery)

Paravalvular leak

Mitral Leaflet Disorders

Infective endocarditis (perforation or interfering with valve closure by vegetation)

Trauma (tear during percutaneous mitral balloon valvotomy or penetrating chest injury)

Tumors (atrial myxoma)

Myxomatous degeneration

Systemic lupus erythematosus (Libman-Sacks lesion)

Rupture of Chordae Tendineae

Idiopathic

Myxomatous degeneration (mitral valve prolapse, Marfan's syndrome)

Infective endocarditis

Acute rheumatic fever

Trauma (percutaneous balloon valvotomy, blunt chest trauma)

Papillary Muscle Disorders

Coronary artery disease (causing dysfunction and rarely rupture)

Acute global left ventricular dysfunction

Infiltrative diseases (amyloidosis, sarcoidosis)

Trauma

Primary Mitral Valve Prosthetic Disorders

Porcine cusp perforation (or degeneration)

Mechanical failure (strut fracture)

Immobilized disk or ball of the mechanical prosthesis

Chronic †

Inflammatory

Rheumatic heart disease

Systemic lupus erythematosus

Scleroderma

Degenerative

Myxomatous degeneration of mitral valve leaflets

Marfan's syndrome

Ehlers-Danlos syndrome

Infective

Infective endocarditis affecting normal, abnormal, or prosthetic mitral valve

Structural

Ruptured papillary muscle or chordae tendineae (spontaneous or secondary to myocardial ischemia)

Rupture or dysfunction of papillary muscle (ischemia or myocardial infarction)

Dilatation of mitral valve annulus and left ventricular cavity

Hypertrophic cardiomyopathy

Paravalvular prosthetic leak

Congenital

Mitral valve clefts or fenestrations

Parachute mitral valve abnormality

*Modified from Jutzy KR, Al-Zaibag M: Acute mitral and aortic valve regurgitation. In Al-Zaibag M, Duran CMG (eds): Valvular Heart Disease. New York, Marcel Dekker, 1994, pp 345–382.
†Modified from Haffajee CI: Chronic mitral regurgitation. In Dalen JE, Alpert JS (eds): Valvular Heart Disease, 2nd ed. Boston, Little, Brown, 1987, p 112.

1963

Figure 50-24 Left, The dynamic spectrum, time in years, and progression of mitral valve prolapse (MVP) are shown. A subtle gradation (crosshatched area) exists between normal mitral valves and valves with mild MVP but without mitral regurgitation (no MR). Progression from the level MVP-no MR to another level may or may not occur. Most patients with MVP syndrome occupy the area above the dotted line, whereas those with progressive mitral valve dysfunction occupy the area below the dotted line. Right, The large circle represents the total number of patients with MVP. Patients with MVP may be symptomatic or asymptomatic. Symptoms may be directly related to mitral valve dysfunction (gray circle) or to autonomic dysfunction (red circle). Certain patients with symptoms directly related to mitral valve dysfunction may present with and continue to have symptoms secondary to autonomic dysfunction. (Redrawn from Boudoulas H, Wooley CF: Mitral Valve Prolapse and the Mitral Valve Prolapse Syndrome. Mt Kisco, NY, Futura Publishing, 1988.)

Assessment of ventricular function is an important and challenging component of the perioperative evaluation of these patients. Ejection phase indices, such as ejection fraction and fractional fiber shortening, are poor indices of contractility because they are inversely related to after-load and, in patients with MR, the left ventricle ejects into the left atrium, as well as into the aorta. Indeed, normal ejection phase indices (e.g., ejection fraction greater than 60%) in patients with MR may reflect impaired myocardial function. Certainly, ejection fractions of 40% to 60% and those less than 40% are associated with moderately impaired and severely impaired ventricular function, respectively, when observed in the presence of MR. As a corollary, it has been clearly demonstrated that survival after mitral valve replacement for MR is decreased in patients with ejection fractions less than 60%. ^[151] Because left ventricular function is an important determinant of outcome in patients with MR, non-ejection indices of function, such as left ventricular end-systolic pressure-volume relationships, have been used to assess function in the setting of MR.^{[152] [153]} Patients with normal end-systolic volumes (less than 40 mL/m² ) exhibit good preservation of function postoperatively, whereas those with enlargement (end-systolic volume greater than 80 mL/m² ) have increased perioperative mortality and poor postoperative function. An end-systolic volume of 55 mL/m² has been used to segregate patients with high and low perioperative morbidity and mortality. ^[154] The importance of afterload in assessing function is further illustrated by the fact that for any given end-systolic volume, patients with MR have more severe dysfunction than do those with aortic regurgitation.