CHAPTER 48: DEGENERATIVE MITRAL VALVE DISEASE

INTRODUCTION

The normal mitral valve permits one-way blood flow from the left atrium to the left ventricle in an efficient, nearly frictionless fashion.¹ Although even a normal competent valve may allow a trivial amount of reversed flow, more than a trace of mitral regurgitation is considered pathologic.² Mild-to-moderate mitral regurgitation is tolerated indefinitely as long as it does not worsen. However, severe mitral regurgitation causes left ventricular remodeling reduced forward cardiac output, neurohumoral activation, left ventricular damage, heart failure, and ultimately death.³ The natural history of mitral regurgitation depends intimately on its etiology, the severity of left ventricular volume overload as well as its contractile performance, and the appearance of overlapping clinical conditions secondary to reversal flow, such as atrial fibrillation and pulmonary hypertension.⁴ In this setting, myxomatous degeneration of the mitral valve, a very common pathologic substrate of mitral valve billowing (normal valve coaptation) and prolapse (deficient valve coaptation), is the most prevalent cause of isolated severe mitral regurgitation requiring surgical intervention in the United States.⁵ The following is a review of the normal mitral valve anatomy as well as a summary of causes, consequences, and treatment of degenerative mitral valve regurgitation.

MITRAL VALVE ANATOMY

The mitral valve is located in the left atrioventricular groove, and allows unidirectional flow of oxygenated blood from the left atrium into the relaxed left ventricle during diastole. The mitral valve apparatus is a very complex three-dimensional assembly of separate anatomical components including the annulus, the leaflets and commissures, the chordae, the papillary muscles, and the ventricle.⁶ During systole, a coordinated interaction of these anatomical components closes the valve against ventricular pressure. Therefore, its anatomy should be scrutinized systematically to identify the lesions (the abnormalities in valve structure) that lead to the valve’s dysfunction (the alteration in closure that results in mitral regurgitation).⁷

Mitral Annulus

The mitral annulus is a fibromuscular ring located in the left atrioventricular groove, which serves as an attachment and hinge point for the mitral valve leaflets. The mitral annulus is subjectively divided into anterior and posterior segments based on the attachments of the anterior and posterior mitral leaflets, but can also be segmented by location into septal and lateral components. The anterior portion of the mitral annulus is in continuity with the fibrous skeleton of the heart, defined by the right and left fibrous trigones and the aortic mitral curtain. This portion of the mitral annulus is thus fibrous in nature, and is much less prone to dilation in comparison to the posterior portion of the annulus (Fig. 48–1). Because the fibrous skeleton is discontinuous along the posterior portion of the mitral annulus, this portion dilates or increases its circumference in the setting of chronic mitral valve regurgitation with associated atrial and ventricular dilatation.⁸ The resultant increase in mitral annular dimension tends to make the annulus more circular ...