Like with the aortic valve, in this chapter we will be primarily interested in the identification of catastrophic, gross valve failure or dysfunction that is severe enough to impact patient hemodynamics. This is the primary goal of the focused cardiac ultrasound (FoCUS) assessment and heavily relies on 2D ultrasound technology. A comprehensive evaluation of the valves involves color flow doppler (CFD), pulsed wave doppler (PWD) and continuous wave doppler (CWD) which are out of the scope of FoCUS but that we will briefly touch base. It is through these other techniques that we can have a better and more precise assessment of the different degrees of valve dysfunction.
Mitral Valve Anatomy and Function Recap
The mitral valve is a bicuspid structure that allows directional flow of blood. Both the posterior and the anterior mitral valve leaflets are divided into eight segments with a thickness ranging between 1 to 5mm. On diastole, the mitral valve opens on the early filling stage when the left atrial pressure is greater than that of the left ventricle allowing it to fill with blood. Most of the blood flows during this initial phase of left ventricular relaxation. Atrial contraction contributes up to 25% of the cardiac output before the onset of systole. On systole, the pressure generated by the LV moves the scallops back to their closed position. The chordae tendinea prevent the valve from prolapsing into the atrium. The mitral annulus is the fibrous ring that supports the mitral valve leaflets and changes its shape throughout the cardiac cycle.
Mitral valve. On the left, its bileaflet structure with their corresponding scallops as seen from the left atrium. On the right we appreciate a 3D cine of the MV viewed from the left ventricle. Notice the movement of anterior mitral valve on early and late diastole as the anterior mitral valve appears to flicker. Also notice the shape of the MV annulus on systole and diastole. Images courtesy of Innotata and Kjetil Lenes.
Mitral Valve Stenosis
The most common cause of mitral valve stenosis is rheumatic heart disease. The typical immobility of the valve tips creates a typical hokey stick configuration.
On ultrasound we typically observe a hyperechoic and heavily calcified valve with significant reduction of movement. The diagrams display the movement of the valve on systole without and with stenosis.
Normal valve function on diastole
Mitral valve stenosis
We can appreciate the restriction of movement of the anterior leaflet of the mitral valve in a hockey stick configuration on diastole when seen on the parasternal long axis and apical 4 chamber views. Compare the normal findings with that of a patient with MS.
Mitral valve stenosis. Clips 1 and 3, normal MV movement. Clips 2 and 4, a heavily calcified, thickened mitral valve with restriction of movement.
Diagnosis of the severity of mitral valve stenosis requires Doppler measurements and is beyond the scope for focused cardiac ultrasound (FoCUS). On the apical 5 chamber view, continuous wave doppler on the mitral valve will measure all velocities across that scan line and we are interrogating diastole. The following are examples of level 1 recommendations for measuring the severity of MS.
As we can imagine we would typically expect large fast flows into the LV. We can measure pressure half time which is the time the max pressure takes to half (this is a load dependent measure). A high number means that the pressure never equilibrates between the LA and LV. A low number implies that the equilibration of pressures happens fast. We estimate MV area by the formula MV area= 220/PHT. In the case below the pressure half time appears traced by the white dots seen on the screen with a MV area of 220/228 or <1cm square area or severe MS. Also by tracing the CWD envelope we can also calculate the mean pressure gradient and a value >10mmHg is considered severe.