Mitral and tricuspid valves

The fibrous skeleton of the heart

The aortic, mitral and tricuspid valves form one continuous structure, with each valve in close approximation to the other and connected by the fibrous skeleton of the heart. They are arranged as a triangle at the base of the heart; the pulmonary valve is separate and placed anteriorly ( Fig. 51.1 ). The fibrous skeleton of the heart surrounds the aortic valves and partially surrounds the mitral and tricuspid valves ( Fig. 51.2 ). The anulus of each valve is therefore not homogenous, but is made of a combination of fibrous tissue for some of its diameter and muscular tissue for the remainder, which results in asymmetrical dilation of each valve under pressure and/or volume loading.

Ultimately, the effect of valvular and ventricular disease processes is to lead to pathological changes in the structures of the tricuspid and mitral valves. Pressure and/or volume overload of the chambers of the heart will lead to chamber enlargement and then to anular dilation. The normal shape of the valve anulus becomes distorted and enlarged, and these changes, which are initially reversible, become progressive and irreversible. Valve disease is common in the UK, where it occurs in 2–5% of the population; the prevalence increases with age. There are a variety of causes but the most common mitral condition is known as degenerative disease, which causes mitral valve prolapse. It is thought that 10% of patients with degenerative mitral valve disease will go on to develop regurgitation (leaking) through the valve that is sufficiently severe to warrant surgical intervention.

There is strong evidence that surgical mitral valve repair in degenerative disease has better outcomes than mitral valve replacement. These include early postoperative survival, freedom from stroke after surgery, long-term survival and freedom from reoperation in all patient cat­egories. Mitral valve repair can be technically challenging and requires surgical experience and judgement. The principles of mitral valve repair are restoration of anular geometry and normal leaflet motion, and creation of a large surface area of coaptation. An understanding of the surgical anatomy of the mitral valve is key to performing a successful repair.

The mitral valve is located centrally within the thorax, posterior to the greater mass of the heart and great vessels. The entire cardiac output flows through it. These factors make surgical access challenging. Adequate access requires cardiopulmonary bypass, exclusion of the heart from the circulation, retraction of adjacent structures and incision through the walls of the cardiac chambers. Surgical access should provide adequate exposure of all of the anatomical components of the valve (leaflets, anulus and subvalvular apparatus) both in the relaxed, empty and motionless heart and under dynamic pressure testing of the valve post repair. Visualization must occur without distortion of the valve and without damage to adjacent structures in order for the function of the valve to be able to be assessed properly.

The standard approach to the valve is through the right atrium and through the interatrial septum, requiring knowledge of the anatomy of these structures. The anatomy of the valve, the interatrial septum and the fibrous skeleton of the heart, together with the various surgical approaches to the mitral valve.

Surgical anatomy of the mitral valve

The dynamic functional unit of the mitral valve is created by the leaflets and commissures, anulus, chordae tendineae and papillary muscles. The function of the mitral valve during the heart cycle is complex and dynamic, and involves the atrium, left ventricle and aortic valve. Optimal function and coaptation of the mitral valve depend on the correct functioning and precise geometrical relationship of the atrioventricular apparatus. In diastole, the anterior and posterior leaflets open and there is high-velocity blood flow through the mitral valve orifice. Just before the contraction of the atrium, the mitral valve leaflets are moving back to their semi-open position and are then pushed widely open by the flow of blood during the atrial systole. At the beginning of left ventricular systole, the papillary muscles contract and position the leaflets of the mitral valve opposite to each other to create the position for competent closure. The pressure of blood in the left ventricle will close the mitral valve by pushing both leaflets towards each other. The large surface area of the coapting leaflets contributes to the competency of the valve during systole, regardless of the physiological variation of the volume and the pressure in the left ventricle. Both leaflets play a role in organizing the flow of blood, first by directing the flow towards the apex of the heart during diastole and then by creating the tunnel of the left ventricular outflow tract for the blood to be pushed towards the aorta during systole.

Leaflets and commissures

The mitral valve is described as having two leaflets, anterior and posterior, which are each attached to the anulus of the valve. On the ventricular aspect of the leaflets a variable number of fibrous chordae tendineae connect the leaflets to the wall of the left ventricle, principally to the papillary muscles. Although the surface area of each leaflet is the same, the anterior leaflet is attached to one-third of the anulus while the posterior leaflet is attached to two-thirds of the anulus ( Fig. 51.3 ). As a result, the anterior leaflet is narrower and longer, while the posterior leaflet is wider and shorter. The narrow anular attachment of the anterior leaflet means that it can swing on a single hinge mechanism off the anulus. In contrast, the wider attachment of the posterior leaflet means that multiple scallops separated by partial clefts or folds in the valve are required to enable it to open fully on its hinge without presenting an obstruction to the forward flow of blood through the valve during diastole.

Each leaflet is divided into two separate zones that can be clearly defined: an atrial zone, which is thin and smooth, and a coaptation zone, which is rough and thicker. The coaptation zones of each leaflet contact each other directly during systole and are responsible for valve competency. The posterior leaflet is divided by indentations resulting in scallops, usually three in number and termed P1, P2 and P3, respectively. The corresponding adjacent areas on the anterior leaflet are called A1, A2 and A3; they are rarely seen as individual scallops in a healthy valve. Anterolateral and posteromedial commissures separate the anterior and posterior leaflets of the mitral valve. The commissures do not reach the hinges of the valves directly and therefore a com­petent continuity between both leaflets is formed.