Left atrioventricular valve [Bicuspid valve; Mitral valve]

The left atrioventricular valve (valva atrioventricularis sinistra), also called in humans the bicuspid valve or mitral valve, regulates blood flow from the left atrium to the left ventricle and prevents regurgitation during ventricular systole.

The valve is located between the left atrium and left ventricle at the left atrioventricular orifice. It is anchored to the fibrous cardiac skeleton, specifically on the anulus fibrosus (mitral annulus in humans), which provides structural support and electrical insulation between atrial and ventricular myocardium.

The left atrioventricular (mitral valve) apparatus consists of:

1- Two valve cusps (leaflets): a septal cusp and a parietal cusp

In domestic animals, the left atrioventricular valve [Bicsupid valve; Mitral valve] is described typically with two major cusps (but the division of the left atrioventricular valve into cusps may be indistinct like in dogs):

a) Septal cusp (cuspis septalis) (also called the anterior or aortic cusp). It is larger and more robust. It is positioned adjacent to the interventricular septum and is in fibrous continuity with the aortic valve via the aortic-mitral curtain.

b) Parietal cusp (cuspis parietalis) (lso called the posterior cusp by analogy with human anatomy). It is attached to the free (lateral) wall of the left ventricle and is usually broader but shorter in radial height compared to the septal cusp.

The cusps are thin, pliable, and semilunar in shape. Their atrial surfaces are smooth, while the ventricular surfaces give attachment to the chordae tendineae.

2 - The chordae tendineae, that are collagenous fibrous cords that connect the free edges and ventricular surfaces of the cusps to the papillary muscles. They prevent cusp prolapse into the left atrium during ventricular systole.

Chordae tendineae from both papillary muscles attach to each mitral leaflet. The chordae typically divide into several branches before inserting either at the free edge or onto the ventricular surface of the leaflet. Chordae that support the leaflet’s free edge are known as commissural chordae tendineae. Chordae that insert into the leaflet’s ventricular surface (some- times called “mitral valve leaflet chordae”) provide major sup- port for the leaflet during systole; they usually are thicker than the commissural chordae. Other classification schemes also have been used for chordae tendineae, including designation of primary chords (attaching to the leaflet edge) and secondary chords (attaching to the ventricular surface of the valve).

3 - Two main papillary muscles in the left ventricle: subauricular (paraconal) papillary muscle and subatrial (subsinuosal) papillary muscle. Each papillary muscle gives rise to chordae that attach to both cusps. This cross-distribution ensures balanced tension and coordinated closure. The papillary muscles contract during ventricular systole, tightening the chordae tendineae and stabilizing the valve leaflets.

Degenerative mitral valve disease is the most important cause of heart disease in the dog.