An overview of the anatomy of the valve and the way the valve functions is an important starting point, as it is quite complex in its function. It is an integral part of left sided function.
The mitral valve sits between the left atrium and left ventricle (and hence is one of the atrioventricular valves). It has two leaflets that make it look a bit like a bishop’s hat - a mitre, and hence the name.
he mitral valve sits within a fibrous ring that divides the left atrium and ventricle. Within this annulus sits the valve itself. It consists of 2 assymetrical valve leaflets:
Anterior - about ⅔ of the valve area, but ⅓ of the annular circumference
Posterior - about ⅓ of the valve area, but ⅔ of the annular circumference
The aortic valve is closely apposed to the mitral valve, with continuity of the fibrous rings, and surgery on one can have an impact on the other. The circumflex artery also closely follows the annulus of the valve, and may also be at rsik during valve surgery.
The posterior leaflet is divided into 3 ‘scallops’ which are labelled P1, P2, and P3. The anterior leaflet isn’t particularly scalloped, but the adjacent areas are labelled in a similar way according to the corresponding posterior leaflet area (A1, A2, A3). The leaflets overlap slightly (by about 5mm) thus acting to seal against backwards flow. The joining of the two valve leaflets are termed the anterior commissure and posterior commissure.
Each cusp of the valve receives insertions from chordae tendineae, both at the margins (primary cord) and away from them (secondary cord). The chordae tendineae are continuous with the papillary muscles of the left ventricular muscle wall. There are 2 main papillary muscles: Anterolateral - Supplied by both the LAD and circumflex arteries. Posteromedial - Supplied only by the RCA (in right dominant circulation) as so more at rsik of ischaemia, such as in an inferior MI.
The mitral valve’s total area is normally 4-6 cm^2
The main purpose of the valve is to provide one directional flow of blood. This is therefore to allow smooth flow of blood into the left ventricle in diastole, and prevent backward flow of ventricular blood into the left atrium in systole.
During systole, contraction of the papillary muscles (simultaneously with the rest of the left ventricular muscle) pulls taut on the chordae tendineae and as such the valve leaflets. This tension holds the cusps in the correct location, allowing them to resist displacement by the pressure gradient between the left ventricle and atrium (looking a bit like a ship’s sail in the wind). As such, blood instead flows out the left ventricular outflow tract and into the aorta.
Each cusp receives insertions from more than one papillary muscle, with the papillary muscles being on the anterolateral and posteromedial walls. As part of the myocardium, it is useful to be aware of their blood supply. The posteromedial papillary muscles are usually supplied by the RCA (in the normal right sided dominant circulation) and thus may be affected by an inferior MI. The anterolateral muscles have mixed supply from the LAD and circumflex arteries.