What is the structure and function of the mitral valve apparatus?

Anatomy and Function of the Mitral Valve Apparatus

The mitral valve apparatus is a complex, coordinated structure consisting of the saddle-shaped annulus, multiscalloped leaflets connected by commissures, chordae tendineae, papillary muscles, and left ventricular wall attachments, all working together to ensure unidirectional blood flow from the left atrium to the left ventricle. 1

Structural Components

Mitral Annulus

• Hyperbolic paraboloid (saddle-shaped) structure that serves as the attachment point for the mitral valve leaflets 1
• Oval in shape, which cannot be adequately appreciated with 2D echocardiography alone 1
• Forms a dynamic structure that changes shape throughout the cardiac cycle 1

Mitral Leaflets

• Two asymmetric leaflets: anterior and posterior 1
• The anterior leaflet:
  • Has the larger radial surface despite being attached to only about one-third of the annular circumference 1
  • Sail-shaped and in continuity with the left and noncoronary cusps of the aortic valve (aortic-mitral curtain) 1
• The posterior leaflet:
  • Has a larger circumferential attachment (two-thirds of the annulus) 1
  • Quadrangular in shape 1
• Both leaflets are segmented into three individual scallops 1:
  • Anterior leaflet: A1, A2, A3 (from left to right)
  • Posterior leaflet: P1, P2, P3 (from left to right)
• A1 and P1 correspond to the anterolateral portion near the anterolateral commissure and left atrial appendage 1
• A3 and P3 are located near the posteromedial commissure and tricuspid annulus 1
• The coaptation line has an upward concavity and can be checked for leaks when closed 1

Subvalvular Apparatus

• Chordae tendineae:

  • Classified based on their insertion points on the leaflets 1:
    • Primary chordae: insert on the tips of the leaflets
    • Secondary chordae: insert on the body of the leaflets
    • Tertiary chordae: insert on the base of the leaflets
  • The posteromedial papillary muscle chordae are typically fewer in number but longer and stiffer than those from the anterolateral papillary muscle 2

• Papillary muscles:

  • Two main papillary muscles: anterolateral and posteromedial 1
  • Attach to the left ventricular wall 1
  • Work in coordination with the chordae to prevent valve prolapse during systole 1

Functional Aspects

• The mitral valve apparatus requires intricate coordination between all anatomic components, each with unique functional geometry 1
• Normal valve function depends on the complex interaction between the leaflets, subvalvular apparatus, annulus, and left ventricle 1
• During systole, the valve closes to prevent regurgitation of blood from the left ventricle back into the left atrium 1
• During diastole, the valve opens to allow blood flow from the left atrium into the left ventricle 1
• The saddle shape of the annulus helps distribute mechanical stress across the valve components 1

Imaging of the Mitral Valve

• Three-dimensional echocardiography is ideal for examining the mitral apparatus components 1
• Visualization perspectives:
  • The posterior leaflet is best visualized from the parasternal window 1
  • The anterior leaflet can be well seen from both apical and parasternal windows 1
  • The ventricular perspective from the apical window is recommended for visualizing the anterior leaflet 1
  • Prolapsing segments are best appreciated from the surgical view of the valve 1

Clinical Significance

• Dysfunction of any component can lead to mitral valve disease 1:
  • Primary (organic) mitral regurgitation: intrinsic valvular disease affecting the leaflets or chordae
  • Secondary (functional) mitral regurgitation: results from left ventricular dysfunction despite structurally normal valve components
• Understanding the complex anatomy is essential for:
  • Precise localization of pathology 1
  • Planning surgical or transcatheter interventions 1
  • Predicting outcomes of repair techniques 3

Mechanisms of Mitral Regurgitation

• Carpentier's classification of leaflet motion helps understand mechanisms of mitral regurgitation 1:
  • Type I: normal leaflet motion
  • Type II: excessive motion (prolapse)
  • Type III: restrictive motion (A: systolic/diastolic, B: systolic)
• Left ventricular remodeling and tethering of the mitral valve play major roles in secondary mitral regurgitation 1

The mitral valve apparatus's complex anatomy and function highlight the importance of comprehensive evaluation in patients with suspected valve disease, as proper understanding is crucial for appropriate management decisions and optimal outcomes.