Mitral Regurgitation

Mitral regurgitation (MR) is the abnormal retrograde flow of blood from the left ventricle into the left atrium during systole due to incomplete mitral valve closure. It represents the second most common valvular lesion after aortic stenosis and affects approximately 2-3% of the general population, with prevalence increasing with age. MR can be classified as primary (organic), resulting from structural valve pathology, or secondary (functional), caused by left ventricular dilatation or annular dilatation. The condition is clinically significant because severe MR can progress insidiously to left ventricular dysfunction and heart failure, making early identification and appropriate timing of surgical intervention essential. Understanding the hemodynamic consequences, varied etiologies, and prognostic implications of MR is critical for board examinations and clinical practice.

The pathophysiology of mitral regurgitation involves multiple mechanisms that disrupt normal valve coaptation and alter left heart hemodynamics:

During systole, incomplete mitral valve closure permits blood to reflux into the left atrium, which normally has low compliance and limited distensibility. This sudden increase in atrial volume and pressure is transmitted retrograde to the pulmonary vasculature, causing acute elevation in pulmonary venous and capillary wedge pressures. This mechanism explains the acute pulmonary edema seen in acute severe MR (e.g., papillary muscle rupture post-MI). The volume of regurgitant flow directly correlates with the orifice area of the regurgitant jet and the pressure gradient between the left ventricle and atrium during systole—factors described by the Bernoulli equation.

In chronic MR, the left atrium undergoes eccentric hypertrophy and progressive dilatation to accommodate the regurgitant volume at lower pressure (reservoir function increases). Simultaneously, the left ventricle experiences volume overload from both normal forward stroke volume and the regurgitant volume. This leads to eccentric left ventricular hypertrophy with elongation of myocytes in series, increasing chamber size while initially maintaining normal wall thickness. The Frank-Starling mechanism operates at increased preload, allowing the ventricle to eject both forward and backward flow while maintaining a normal ejection fraction for prolonged periods. However, prolonged volume overload eventually exhausts compensatory mechanisms, leading to afterload mismatch, progressive contractile dysfunction, and transition to dilated cardiomyopathy with reduced ejection fraction—a critical threshold for surgical intervention.

Primary MR results from mechanical disruption of normal valve anatomy. In rheumatic mitral disease, inflammatory damage causes leaflet thickening, commissural fusion, and chordal shortening. In myxomatous degeneration (mitral valve prolapse), collagen and elastin structural abnormalities lead to leaflet redundancy and elongated chordae tendineae, causing systolic bulging into the atrium and incomplete coaptation. Endocarditis causes vegetations and valve perforation, directly preventing closure. Ischemic MR (papillary muscle dysfunction or rupture) disrupts the complex papillary muscle-chordal-leaflet apparatus that normally maintains leaflet geometry and coaptation height.

Secondary MR develops when the mitral apparatus is structurally intact but geometric distortion prevents proper leaflet closure. Left ventricular dilatation displaces papillary muscles laterally and apically, increasing papillary muscle-to-annulus distance (papillary muscle displacement). Simultaneously, the mitral annulus dilates, increasing annular circumference. These geometric changes increase the coaptation deficit—the distance between leaflet edges that fail to meet during systole. Importantly, the systolic tenting area (the area bounded by the annulus and the two leaflets at their point of maximal systolic displacement) quantifies this mechanism on echocardiography. Left atrial remodeling also contributes: atrial dilatation pulls on the annulus circumferentially, further increasing annular diameter. Unlike primary MR, functional MR is a consequence of ventricular dysfunction and represents a pathophysiologic marker of disease severity.

Chronic MR activates compensatory neurohormonal systems (sympathetic nervous system, renin-angiotensin-aldosterone system) that increase contractility and promote sodium retention to maintain cardiac output. However, these mechanisms eventually become maladaptive, promoting further remodeling through increased afterload, increased wall stress, and stimulation of myocardial apoptosis. Progressive loss of contractile function, increased afterload intolerance (paradoxically worsening forward output as afterload increases), and diastolic dysfunction culminate in overt heart failure.

MVP is the most common cause of isolated primary MR in developed countries, occurring in 2-3% of the population. The condition results from genetic abnormalities affecting fibrillin, collagen, and other structural proteins in the valve matrix, leading to leaflet redundancy, elongation of chordae tendineae, and systolic bulging. MVP is associated with connective tissue disorders including Marfan syndrome (FBN1 gene mutations), Ehlers-Danlos syndrome, and osteogenesis imperfecta. Risk factors include female sex (female predominance 2-3:1), hyperthyroidism, and familial clustering. While many patients with MVP remain asymptomatic with no hemodynamic consequences, approximately 2% progress to severe MR requiring intervention, particularly when chordal rupture occurs.

Historically the leading cause globally and still prevalent in developing nations, acute rheumatic fever (ARF) causes valvulitis with leaflet inflammation, edema, and subsequent fibrosis and calcification. Rheumatic MR typically involves both stenosis and regurgitation ("mixed" lesion) due to commissural fusion combined with leaflet restriction and annular dilatation. Risk factors include group A streptococcal pharyngitis (inadequate or untreated), recurrent ARF episodes, and socioeconomic factors affecting access to antibiotics.

Bacterial or fungal infection causes vegetations on leaflets, leading to leaflet perforation, destruction, and regurgitation. Viridans group streptococci (most common), Staphylococcus aureus (particularly in IV drug users and healthcare-associated infections), and enterococci are frequent pathogens. Acute endocarditis presents with acute severe MR and fulminant heart failure; chronic endocarditis causes progressive scarring and calcification.

Results from myocardial infarction affecting papillary muscles (posteromedial papillary muscle territory supplied by a single right coronary artery is most vulnerable). Acute papillary muscle rupture causes sudden-onset severe MR and cardiogenic shock, while chronic ischemic MR develops from papillary muscle dysfunction and fibrosis or from global left ventricular dilatation post-MI.

Blunt chest trauma can rupture chordae tendineae or papillary muscles, presenting with acute MR. Motor vehicle accidents and steering wheel injuries are common mechanisms.

Radiation to the mediastinum (Hodgkin lymphoma therapy) causes valve fibrosis and thickening. Antiphospholipid syndrome causes valve thickening and regurgitation via thromboinflammatory mechanisms. Systemic lupus erythematosus causes lupus-associated valve disease (Libman-Sacks endocarditis). Carcinoid syndrome affects mitral valve (less commonly than tricuspid) through serotonin-mediated fibrosis.

Any condition causing left ventricular enlargement and reduced ejection fraction—including dilated cardiomyopathy (ischemic or non-ischemic etiology), prior myocardial infarction, hypertensive heart disease, and aortic valve disease—can cause secondary MR through the geometric mechanisms described in pathophysiology. The degree of functional MR correlates with ventricular size and dysfunction severity.

Atrial fibrillation causes left atrial dilatation, which mechanically dilates the mitral annulus and increases the coaptation deficit. This creates a bidirectional relationship where baseline MR predisposes to atrial fibrillation, which then worsens MR.

Chronic elevation of pulmonary vascular resistance causes right ventricular dilatation and rightward septal bowing, which mechanically affects left ventricular geometry and can contribute to secondary MR (though mechanisms are complex).

Associated with left ventricular diastolic dysfunction and increased afterload, contributing to secondary MR, particularly in patients with hypertension.

The clinical presentation of MR varies dramatically based on acuity of onset, severity, and left ventricular compensatory capacity:

Presents with sudden-onset dyspnea, orthopnea, and acute pulmonary edema as the non-compliant left atrium experiences abrupt volume and pressure overload. Patients appear acutely ill with respiratory distress. This occurs in papillary muscle rupture post-MI, spontaneous chordae tendineae rupture, acute endocarditis, or acute trauma. Hemodynamically, the V wave (ventricular systolic wave) becomes prominent in the pulmonary capillary wedge pressure tracing, reflecting rapid transmission of regurgitant flow into the atrium.

Many patients remain asymptomatic for prolonged periods, particularly with moderate MR, because eccentric left ventricular hypertrophy and atrial enlargement permit accommodation of regurgitant volume at relatively low pressures. Asymptomatic MR is often discovered incidentally on echocardiography during evaluation for other conditions.

As MR becomes more severe or as left ventricular function declines, patients develop exertional dyspnea (difficulty breathing with exertion due to impaired cardiac output and elevated pulmonary pressures with effort), fatigue, and reduced exercise capacity. The left atrium's inability to further increase compliance and volume accommodation manifests as elevated pulmonary venous pressures during increased demand.

Progressive worsening leads to orthopnea (shortness of breath when lying flat), paroxysmal nocturnal dyspnea, lower extremity edema, and hepatomegaly. Right ventricular dilatation from chronic pulmonary hypertension may occur, causing secondary tricuspid regurgitation and elevated jugular venous pressure.

Atrial fibrillation commonly develops in chronic severe MR due to chronic atrial stretch and inflammation. Patients report palpitations or may be asymptomatic with incidental discovery of atrial fibrillation on electrocardiography. Atrial fibrillation dramatically worsens MR through annular dilatation.

Uncommon in pure MR, syncope suggests concurrent aortic stenosis, severe pulmonary hypertension, or arrhythmic complications.

The volume-overloaded left ventricle produces a hyperdynamic (hyperkinectic, "bounding") point of maximum impulse (PMI) that is displaced laterally and inferiorly beyond the fifth intercostal space at the midclavicular line—reflecting left ventricular enlargement and increased stroke volume.

The classic finding is a holosystolic (high-pitched, blowing) murmur heard best at the apex with the patient in the left lateral decubitus position, radiating to the left axilla and sometimes the back. The murmur begins with S1 and extends throughout systole to or past S2 (extends through A2). This holosystolic quality reflects regurgitant flow persisting throughout systole whenever left ventricular pressure exceeds atrial pressure. The murmur may obscure S2 or create an apparent single S2. Severity correlation: Interestingly, the loudness of the murmur does not reliably correlate with MR severity—a soft murmur can represent severe MR if the left atrium is very compliant, while a loud murmur may accompany mild MR if atrial compliance is reduced.

A prominent left ventricular S3 (ventricular gallop) occurs in moderate-to-severe MR due to rapid early diastolic filling from the dilated left atrium. S3 indicates significant volume overload and correlates with more severe MR.

An irregular pulse and irregularly irregular rhythm on auscultation indicate concurrent atrial fibrillation, common in chronic severe MR.

In acute severe MR or acute decompensation, hypotension and cardiogenic shock may develop with reduced pulse pressure and cool extremities.

Presents with sudden hemodynamic collapse, acute severe MR, pulmonary edema, and cardiogenic shock within days of acute MI (typically 3-7 days as necrotic muscle weakens).

Presents with acute severe MR, septic shock, and systemic emboli; often accompanied by fever and new cardiac murmur on serial examinations.

May present with benign palpitations, chest pain (atypical), and anxiety; murmur is late systolic (after a click) rather than holosystolic, indicating less hemodynamic consequence.

Detailed history should assess symptom onset and progression (acute vs gradual), exertional limitations, orthopnea and paroxysmal nocturnal dyspnea, prior cardiac disease (prior MI, valve disease, hypertension), and risk factors for endocarditis (IV drug use, prosthetic valves, recent dental procedures). History of rheumatic fever, connective tissue disease, or family history of valve disease should be elicited. Symptom severity guides urgency of intervention.

ECG findings reflect chronic adaptation but are non-specific:

CXR findings depend on severity and acuity:

Echocardiography is the definitive diagnostic test and provides anatomic and hemodynamic assessment:

Two-dimensional findings:

Not indicated: routine infective endocarditis antibiotic prophylaxis for native-valve MR or mitral valve prolapse (AHA prophylaxis guidance). MR alone does not mandate warfarin; anticoagulation for atrial fibrillation follows CHA₂DS₂-VASc, and DOACs are acceptable since "valvular AF" requiring warfarin means mechanical valve or rheumatic mitral stenosis.