Assessment of the mitral valve
The mitral valve is the first structure of the heart identified through echocardiogram. The echocardiographic evaluation of the mitral valve is composed of multiple 2D views, M-mode tracing, and Doppler flow evaluation. If these three techniques are contraindicated, a transesophageal echocardiogram can be performed. Together, these procedures are able to give accurate information regarding the mitral valve’s functions and abnormalities, if present.
The orientation of the mitral valve’s anterior leaflet is to the anterior chest wall, making the anterior leaflet ideal target for sound reflection. The anterior leaflet also has a large margin-to-base ratio, making it highly mobile. The anatomic windows which can be used to record the mitral valve through ultrasound are the apical, precordium, and subxiphoid.
M-mode echocardiogram is done from the precordium and guided from 2D short and long axis views. Normally, the anterior leaflet of the mitral valve shows a motion pattern which is the reflection of the ventricular filling’s phasic nature and produces an M-shaped pattern. The posterior image, on the other hand, shows a W-shaped pattern with smaller excursion.
The E-point, which is the initial opening diastolic movement of the mitral valve, occurs because of the rapid left ventricular filling. During the F-point, the valve assumes an almost closed position during the mid-diastole. This reflects the deceleration of the inflow as the pressure gradient between the ventricle and the atrium is reduced. This early diastolic closure, which is usually more than 60 mm/s, called the E-F slope.
There is continuous blood flow from the pulmonary veins to the left ventricle during the mid-diastolic phase known as the conduit phase, During this phase, the atrium acts as a passive channel instead of a reservoir. The mitral valve opens a second time, termed as A point, after atrial contraction, completing the letter M’s second peak. The deceleration of atrial inflow and the isometric LV contraction results in final closure.
The 2D echocardiogram appearance of a normal mitral valve is dependent on the imaging plane used.
- Parasternal short axis: ovoid orifice appearance or “fish mouth”
- Parasternal long axis and apical: clapping hands
But generally, the mitral valve should appear as a two-leaflet structure, mobile enough to respond to diastolic filling but still able to form a stable coaptation plane during systole.
The mitral valve leaflets are translucent and thin and the chordae of each leaflet is connected to both papillary muscles. Each mitral valve leaflet is represented by two linear echoes on M-mode. On 2D imaging, the valve can be seen as thin and homogeneous and less than 4 mm thick. This thickness, however, is also dependent upon transducer frequency.
A lower frequency of 2.5 mHz is preferred by many sonographers as it permits adequate images even in difficult patients and is ideal for Doppler flow. Although with this frequency, even normal valves may appear thicker so if an abnormality is suspected, a higher frequency of 3.5 mHz is used to determine if it is indeed thickened. In general, compared to an aortic root, a normal mitral valve is thinner when imaged under the same conditions.
Using the 2D echocardiographic exam, the chordate, papillary muscles, and the mitral annulus can also be appreciated. The papillary muscles are best seen in the parasternal short axis anterolaterally and psoteromedially. On 2D imaging, examination of the mitral annulus is limited to diameter measurement and determination of the presence of calcification.
Using Doppler echocardiogram on the mitral valve would reveal that the velocity patter of the blood going into the left ventricle at diastole resembles the M-shaped pattern seen in the M-mode of the same structure. Blood flow is rapid at the early phases of filling, very low during the conduit phase of the mid-diastole, and accelerates at the atrial contraction.