Bicuspid heart valve is the valve present between the left atrium and left ventricle which is also known as mitral heart valve or atrioventricle valve. It is called so because it has two leaflets or cusps and its name is derived from a Latin word bi which means two. This valve lies between the left atrium and left ventricle and controls the flow of blood from left atrium into the left ventricle.
A bicuspid valve which functions normally opens when a pressure change is formed in between the left atrium and left ventricle. As the left atrium gets filled with blood the pressure inside it increases due to which bicuspid valve opens to allow the flow of blood into left ventricle. When the lefty atrium get emptied into left ventricle the pressure is changed again and this pressure change results in the closure of the bicuspid heart valve. Flow of blood occurs due to atrial contraction and at the end of this contraction the bicuspid valve is closed.
The area of a normal bicuspid valve is 4-6 square cm. It has two cusps named as the anteromedial leaflet and the posterolateral leaflet and these leaflets control the opening and closing of the bicuspid valve. The opening of the valve is surrounded by a fibrous ring known as the mitral valve annulus. Anterior leaflet comprises most of the part of the ring while posterior valve has larger surface area and the chordae tendineae are attached to the posterior surface of the valve to prevent the prolapsing of the leaflets.
These inextensible chordae tendineae are attached from one side with the leaflets and from other side to the papillary muscles. Papillary muscles are finger-like projections from the wall of the left ventricle. During diastole when the left ventricle contracts the intraventricular pressure force the leaflets to close and the tendons avoid the opening of the leaflets in wrong direction thus preventing the back flow of blood into left atrium. The chords are of different thickness and the thinnest and the thickest chords are present at the free margin of the leaflet and away from free margin respectively.
During the left ventricular diastole the ventricular myocardial relaxes and results in the drop of pressure in left ventricle and the bicuspid valve opens to allow the flow of blood from left atrium into left ventricle. The active relaxation of the ventricular myocardium causes a pressure gradient which results in the rapid flow of blood from left atrium through bicuspid valve. About 70-80% of the blood is passed into left ventricle by this early filling phase of left ventricle. On Doppler echocardiography this early filling is shown as E wave. After this early filling there is a phase of slow filling.
Left atrial contraction or left atrial systole which occurs during left ventricular diastole causes added blood to flow across the mitral valve immediately before left ventricular systole. This late flow across the open mitral valve is seen on doppler echocardiography of the mitral valve as the A wave. The late filling of the LV contributes about 20% to the volume in the left ventricle prior to ventricular systole, and is known as the atrial kick.
The shape and size of the mitral annulus changes throughout the cardiac cycle. The annulus becomes small during atrial systole as this reduction in size is necessary for the proper coapting of the leaflets of the bicuspid valve.