Transesophageal Echocardiography for Surgical Repair of Mitral Regurgitation

Degenerative (Myxomatous) Disease

Degenerative disease is the most common form of mitral valve disease in developed countries and encompasses a spectrum of pathology from fibroelastic deficiency to Barlow disease. ^, Fibroelastic deficiency is characterized by thin, delicate leaflet tissue that is relatively deficient in mucopolysaccharides, collagen, and elastic fibers. There is little excess leaflet tissue, and the disease process is usually localized to a single leaflet segment. MR results from leaflet flail due to chordal rupture. The posterior leaflet is affected more commonly than the anterior leaflet, and P2 is affected more commonly than P1 or P3. Noninvolved segments are morphologically normal, and the mitral annulus is typically only mildly dilated.

Barlow disease is a generalized process characterized by an excess of thickened, redundant leaflet tissue and elongated primary chordae. Typically, there is marked annular dilation, which is maximal along the commissural axis. Annular dynamics are blunted, with diminished systolic shortening and folding compared with normal function. Histologically, there is an accumulation of glycosaminoglycans in the extracellular matrix, a process called myxomatous degeneration . MR primarily results from leaflet prolapse, which may involve multiple segments. Leaflet flail can also occur after rupture of primary chordae. Mitral clefts or deep posterior scallops are associated with Barlow disease, and they can contribute to MR.

Barlow disease may occur in isolation or in association with genetically mediated systemic diseases such as Marfan syndrome. Barlow disease is also associated with hypertrophic cardiomyopathy, bicuspid aortic valve, and dystrophic calcification. ^, Myxomatous degeneration of the tricuspid valve occurs in 40% to 50% of patients with Barlow disease. Advanced Barlow disease is commonly associated with mitral annular disjunction, in which there is a wide separation between the LV shoulder and the posterior annulus. Mitral annular disjunction is associated with ventricular arrhythmias and sudden death. ^,

The terminology used to describe degenerative disease is somewhat confusing and reflects the overlapping spectrum of pathology. The terms fibroelastic deficiency and Barlow disease are widely used in the surgical community. The term myxomatous mitral valve disease is often used synonymously with degenerative disease. The two features of degenerative disease that are important in terms of surgical repair are the distribution of disease (e.g., single segment, multiple segments, bileaflet involvement) and the amount of redundant leaflet tissue. The terms Barlow disease and fibroelastic deficiency capture these distinctions and are used throughout this chapter.

Successful repair of degenerative disease is possible in most patients. Rates of successful repair approach 100% for isolated posterior leaflet flail. Repair rates are lower in cases of complex anterior leaflet or bileaflet disease, although successful repair rates in excess of 90% have been reported from high-volume centers. ^{, ,}

Infective Endocarditis

Endocarditis of native heart valves usually occurs in the context of underlying valve disease, which for the mitral valve is typically rheumatic or degenerative. Endocarditis is also associated with diabetes, renal failure, immunosuppression, and prosthetic devices such as pacing wires. Vegetations, the characteristic lesion of endocarditis, arise from the deposition of fibrin and platelets at a site of endocardial injury, which subsequently becomes infected with microorganisms circulating in the blood. Staphylococci and streptococci are the most common causative agents, accounting for 80% of cases. MR results from leaflet perforation or chordal rupture, which often occurs at the site of attachment of vegetations. Abscesses and fistulas to surrounding structures can develop.

Valve repair is possible for most patients with mitral valve endocarditis and is associated with low mortality rates and high rates of freedom from reoperation. ^, Valve replacement is required when leaflet destruction is extensive or in the setting of abscesses or fistulas.

Rheumatic Disease

Rheumatic heart disease remains the most common cause of valvular heart disease in developing nations; it is less common in developed nations but is still responsible for about 20% of valvular heart disease cases. In Europe, rheumatic heart disease is responsible for approximately 85% of cases of mitral stenosis and 15% of cases of MR. Rheumatic disease leads to thickening, fibrosis, and calcification of the leaflets and subvalvular structures. Mitral stenosis typically results from commissural fusion, whereas MR results from leaflet and chordal retraction.

Surgical repair of rheumatic MR is challenging and is not possible in many patients, and valve replacement is commonly indicated. However, in selected patients, repair results in 80% to 90% long-term freedom from reoperation.

Radiation Necrosis

Radiation-induced valvular dysfunction typically manifests 10 to 20 years after mediastinal irradiation. Mitral valve dysfunction results from thickening, retraction, and calcification of leaflets and subvalvular apparatus. Associated abnormalities include restrictive cardiomyopathy, calcification of the great vessels, coronary stenosis, pericardial constriction, conduction abnormalities, and pulmonary fibrosis. If surgery is indicated, valve replacement is usually required.

Dystrophic Calcification

Dystrophic calcification, also known as mitral annular calcification, is common in elderly patients; its risk factors are similar to those of atherosclerosis. Calcification primarily involves the annulus but can extend into the bodies of the leaflets. Dystrophic calcification does not usually cause hemodynamically significant mitral valve dysfunction, but it does complicate surgery for other indications, notably degenerative disease.

Papillary Muscle Rupture

Papillary muscle rupture arises as a consequence of myocardial infarction. Rupture usually involves the posteromedial muscle because of its sole blood supply from the RCA. Complete rupture of a papillary muscle causes torrential bileaflet regurgitation. Urgent surgery, usually involving mitral valve replacement, is mandatory. Partial rupture, which involves a single papillary muscle head, typically causes severe (but not torrential) unileaflet regurgitation. For cases of partial rupture, surgery can often be delayed, and valve repair is usually possible.

Secondary Regurgitation

Secondary MR can arise from atrial fibrillation, decreased LV systolic function, and increased LV systolic function. All three are ventricular cardiomyopathies.

MR resulting from atrial fibrillation is known as atrial functional MR. Patients with chronic atrial fibrillation develop enlarged atria. Atrial enlargement or loss of atrial contraction is not sufficient to cause MR. Systolic annular contraction is linked to ventricular contraction. Atrial functional MR results from subtle impairment of ventricular dynamics adjacent to the mitral annulus. Echocardiographically, atrial functional MR appears as isolated annular enlargement. Compensatory leaflet area adaptation eventually becomes insufficient. Systolic LV dimensions and function are preserved, and there is no leaflet tethering. Atrial functional MR is classified as Carpentier type I dysfunction (i.e., normal leaflet motion).

The mechanism of secondary MR associated with decreased LV function is LV dilation. LV dilation causes lateral and apical displacement of the papillary muscles, which results in leaflet tethering in systole. ^, This leads to Carpentier type IIIb dysfunction (i.e., leaflet motion restricted in systole).

The relationship between LV dilation and MR is complex. MR is more common after inferior and posterior infarction than after anterior infarction, despite greater ventricular dilation with the latter. ^, This apparent anomaly may be explained by regional differences in myocardial function affecting the orientation of the papillary muscles to each other and to the valve leaflets. In particular, an increased interpapillary distance and reduced systolic shortening of the interpapillary distance have been implicated as mechanisms of secondary MR that are independent of the severity of LV dilation. Annular dilation is a consistent finding. ^, Adaptive remodeling of the leaflet area and chordae tendineae eventually becomes insufficient, leading to MR.

Two patterns of leaflet tethering are recognized: asymmetric and symmetric. Asymmetric tethering is the most common form and typically results from localized remodeling of the basal inferior or inferolateral LV wall after inferior or posterior infarction, with resultant lateral displacement of the posteromedial papillary muscle. Posterior leaflet tethering is more marked than anterior leaflet tethering, and P3 is more severely affected than P1 or P2. ^, Distortion of the anterior leaflet also occurs, mainly caused by increased tension due to secondary chordae originating from the posteromedial papillary muscle. Annular dilation is asymmetric and is most marked along the anteroposterior axis. Ventricular dimensions and LV ejection fraction (LVEF) may be relatively normal. ^,

In contrast, symmetric tethering results from global LV dilation, most commonly caused by dilated cardiomyopathy or global remodeling after a large (usually anterior) myocardial infarction. ^, Apical and lateral displacement of both papillary muscles occurs with symmetric annular dilation. The LV typically demonstrates marked spherical enlargement, and LVEF is usually severely reduced. Symmetric leaflet tethering can also occur in patients with degenerative disease when spherical LV enlargement arises as a consequence of chronic severe MR.

There are seemingly conflicting data on the role of surgical repair for treating secondary MR. In a small randomized trial of patients with severe ischemic regurgitation, repair with a simple reductive ring annuloplasty was associated with increased recurrence of moderate or severe regurgitation compared with valve replacement (32% vs. 2.3%, P < 0.001). Image guidance was not used in this study. Nonetheless, the investigators were able to demonstrate that for durable repairs, reverse remodeling was superior to replacement.

Challenging data were offered from the Surgical Treatment for Ischemic Heart Failure trial, which compared coronary revascularization, coronary revascularization plus ventricular reconstruction, and medical therapy in patients with a low LVEF. Among the 1460 patients randomized to coronary revascularization, 234 also underwent mitral valve surgery, which involved mitral valve repair in 94% of cases. Mitral valve surgery performed at the time of coronary revascularization was associated with significantly improved short- and long-term survival. Part of the difficulty with these data is that reductive annuloplasty, the only widely accepted repair technique at the time, is associated with early failure when leaflet tethering is severe. ^{, ,}

The Cardiovascular Assessment of MitraClip Percutaneous Therapy for Heart Failure Patients with Functional Mitral Regurgitation (COAPT) trial used detailed TEE imaging before enrollment to limit patient selection to those with valve anatomy suitable for durable percutaneous edge-to-edge repair. Using image guidance, they achieved a 95% rate of freedom from recurrent MR at 12 months and improved survival at 24 months.

These three trials demonstrate that durable repair in suitable patients improves ventricular remodeling and prolongs survival in patients with severe secondary MR. The trials also demonstrate that detailed image guidance is mandatory. Echocardiography-guided patient selection and more complex repair techniques can overcome the high failure rate for repair of secondary regurgitation.

Secondary MR can arise from dynamic LV outflow tract (LVOT) obstruction in patients with hypertrophic cardiomyopathy or isolated asymmetric septal hypertrophy. The main mechanism of regurgitation is septal hypertrophy causing systolic anterior motion (SAM), in which the anterior mitral leaflet is entrained into the LVOT (see Chapter 13 ). However, some patients with hypertrophic cardiomyopathy develop LVOT obstruction and severe MR in the absence of septal hypertrophy. Mechanisms of MR in this circumstance include an elongated anterior mitral leaflet, abnormal chordal attachments, and a bifid papillary muscle. Mitral valve repair with or without myectomy is typically indicated.

Distinct from hypertrophic cardiomyopathy, asymmetric septal hypertrophy refers to isolated hypertrophy of the basal anterior septum. The condition is associated with increased age, hypertension, diabetes, aortic stenosis, and dystrophic calcification. Asymmetric septal hypertrophy typically requires no surgical intervention but predisposes to postoperative SAM in patients undergoing repair for degenerative disease.