The mitral valve controls blood flow from the left atrium to the left ventricle. Named for its resemblance to a bishop’s mitre, it is a complex structure that depends on the coordinated function of multiple components.
Location and Relations
The mitral valve is located at the left atrioventricular junction, posterior and to the left of the aortic valve. It is in fibrous continuity with the aortic valve via the intervalvular fibrosa (aortomitral curtain).
The mitral valve is related to:
- Superiorly: Left atrium and left atrial appendage
- Inferiorly: Left ventricle
- Anteriorly: Aortic valve and left ventricular outflow tract
- Posteriorly: Left ventricular free wall
- Laterally: Left circumflex coronary artery (in the left atrioventricular groove)
Components of the Mitral Valve
Fibrous Annulus
The mitral annulus is a D-shaped fibrous ring that anchors the leaflets. It is not a complete circle; the anterior portion is interrupted by the aortic-mitral continuity.
Characteristics:
- Saddle-shaped in three dimensions (peaks at the anterior and posterior commissures)
- Dynamic throughout the cardiac cycle
- Contracts during systole (reducing orifice area by 20-30%)
- Expands during diastole
- The posterior annulus is more susceptible to dilation
Leaflets
The mitral valve has two leaflets:
Anterior Leaflet (Aortic Leaflet):
- Triangular, attaching to approximately one-third of the annular circumference
- Longer than the posterior leaflet
- Occupies a more anterior position adjacent to the aortic valve
- Separates the left ventricular inflow and outflow tracts
- Has a smooth ventricular surface that contributes to the outflow tract
- The basal portion is in fibrous continuity with the non-coronary and left coronary cusps of the aortic valve
Posterior Leaflet (Murral Leaflet):
- Quadrangular, attaching to approximately two-thirds of the annular circumference
- Smaller surface area than the anterior leaflet
- Divided into three scallops: P1 (anterolateral), P2 (middle), P3 (posteromedial)
- The middle scallop (P2) is the largest and most commonly involved in prolapse
Commissures
The two commissures are the points where the anterior and posterior leaflets meet:
- Anterolateral commissure: Adjacent to the left atrial appendage
- Posteromedial commissure: Adjacent to the interatrial septum
Chordae Tendineae
The chordae are classified by their attachment pattern:
Primary (Marginal) Chordae: Insert at the free edge of the leaflets. Prevent edge eversion during systole.
Secondary (Basal) Chordae: Insert on the ventricular surface of the leaflets. Provide structural support and contribute to ventricular function.
Tertiary Chordae: Connect the ventricular wall directly to the posterior leaflet.
Commissural Chordae: Attach at the commissures.
Papillary Muscles
Two groups of papillary muscles support the mitral apparatus:
Anterolateral Papillary Muscle:
- Located on the anterior lateral wall
- Receives dual blood supply (LAD + circumflex)
- Provides chordae to the anterior and posterior leaflets (A1, P1 scallops)
Posteromedial Papillary Muscle:
- Located on the posterior medial wall
- Single blood supply (PDA, right dominant circulation)
- More vulnerable to ischemia/infarction
- Provides chordae to anterior and posterior leaflets (A3, P3 scallops)
Mitral Valve Apparatus
The mitral valve functions as an integrated unit comprising:
- Left atrial wall (contractile function)
- Mitral annulus
- Leaflets
- Chordae tendineae
- Papillary muscles
- Left ventricular wall
Dysfunction of any component can lead to mitral regurgitation.
Function
The mitral valve opens during ventricular diastole, allowing blood to flow from left atrium to left ventricle. The anterior leaflet moves toward the interventricular septum, creating a triangular orifice.
During ventricular systole, the leaflets coapt along a curvilinear line. The area of coaptation is approximately 5-7 mm in depth. The papillary muscles contract simultaneously with the ventricular wall to maintain leaflet competence.
Normal Dimensions
- Annular diameter: 3-3.5 cm
- Valve area: 4-6 cm²
- Mitral stenosis significant when: < 1.5 cm²
- Coaptation length: 5-7 mm
- Tenting area: < 1.5 cm² (3D echocardiography)
Clinical Significance
Mitral Valve Prolapse (MVP)
Systolic displacement of one or both leaflets into the left atrium, most commonly involving the posterior leaflet middle scallop (P2). Usually benign but can cause regurgitation, arrhythmias, or endocarditis. Physical exam reveals a mid-systolic click and late systolic murmur.
Mitral Regurgitation (MR)
Incomplete coaptation of the leaflets during systole. Causes include:
- Primary: Myxomatous degeneration, flail leaflet, endocarditis, rheumatic disease
- Functional: Annular dilation from left ventricular enlargement, papillary muscle displacement
- Acute: Papillary muscle rupture (post-MI), chordal rupture
Mitral Stenosis (MS)
Obstruction to left atrial emptying, almost always due to rheumatic heart disease. The valve leaflets become thickened, calcified, and fused at the commissures. Presents with dyspnea, atrial fibrillation, and signs of pulmonary hypertension.