15.4: Mitral Regurgitation I: Introduction

Mitral regurgitation is characterized by the backward circulation of blood from the left ventricle to the left atrium during systole, a phase of the cardiac cycle when the heart contracts and pumps blood out of the chambers. This abnormal flow occurs primarily due to the dysfunction of the mitral valve or its supporting structures, which include the mitral leaflets, chordae tendineae, annulus, and papillary muscles.

Etiology and Mechanisms:

Primary Mitral Regurgitation: This type arises from direct problems with the mitral valve leaflets themselves, such as:

• Mitral valve prolapse, where leaflets bulge excessively into the atrium.
• Rheumatic heart disease leads to scarring and deformation.
• Degenerative changes cause the leaflets and chordae tendineae to thicken and become fibrotic, affecting their function.

Secondary Mitral Regurgitation: This type results from conditions affecting the left ventricle or papillary muscles indirectly, such as:

• Myocardial infarction leads to papillary muscle dysfunction or rupture.
• Ischemic heart disease alters ventricular geometry and function.
• Cardiomyopathies or systemic diseases like lupus impact the ventricular structure or function.
• Infective endocarditis causes acute damage through leaflet perforation or scarring.

Pathophysiological Changes:

Regardless of the cause, the pathophysiology of MR involves the dysfunction of the mitral valve or its supporting structures (leaflets, chordae tendineae, annulus, papillary muscles). Damage or deformation in these areas leads to incomplete valve closure and blood regurgitation during Systole.

Volume Overload: MR significantly increases the volume the left atrium and ventricle must handle, as the regurgitated blood adds to the normal blood flow from the pulmonary veins.

• Adaptive Responses: The left atrium and ventricle initially adapt by enlarging (hypertrophy) and stretching (dilation) to accommodate the extra blood volume. This helps maintain cardiac output but can lead to atrial and ventricular dilation and systolic dysfunction.
• Pulmonary and Systemic Effects: The increased pressure in the left atrium due to the regurgitated blood leads to pulmonary congestion and symptoms of heart failure. Increased back pressure in the pulmonary circulation can cause pulmonary hypertension, adding strain on the right ventricle, which may culminate in right-sided heart failure.
• Chronic Consequences: Over time, chronic volume overload and increased atrial pressures result in progressive ventricular dilation and dysfunction, culminating in systolic heart failure. The heart's pumping efficiency decreases, manifesting as reduced cardiac output, fatigue, and exertional dyspnea.