11.5 Quantification of Mitral Stenosis Severity

Two major factors determine the severity of mitral stenosis:the size of the mitral orifice during diastole (mitral valve area) and the magnitude of the gradients across the valve. The mitral vale area (MVA) can be determined with 2D echo (planimetry and by Doppler techniques - the pressure half time method). The gradients are assessed with Doppler using the Bernoulli formula. Other features such as the size of the left atrium or 2D measurements of leaflet separation do not permit accurate assessment of severity. Quantification of mitral stenosis may be quite difficult. One must consider other factors which might greatly influence the estimated severity of stenosis, such as image quality, reliability of the measurements, body surface area, heart rate, cardiac output, or concomitant mitral regurgitation.

11.5.1 Planimetry of the mitral valve

Planimetry is performed from a parasternal short-axis view with the transducer positioned so that the mitral valve is imaged in the perpendicular plane, in which the mitral valve orifice is smallest. A frame is chosen during early diastolic filling, at a time when the mitral valve shows maximal opening excursion. The shape of the mitral orifice may vary greatly, depending on the affected segments of the valve and the degree of commissural fusion. Planimetry is technically challenging and requires good imaging skills. It cannot be done in all patients, such as:


a) those who cannot be imaged from a parasternal approach,

b) those with poor image quality,

c) when the cut plane cannot be optimally aligned to the smallest orifice, and

d) in calcified valves, when dropouts and artifacts do not permit the investigator to view the mitral valve orifice.

To reduce measurement error it would be advisable to perform several measurements (especially in the presence of atrial fibrillation). When feasible, planimetry is the most reliable technique to quantify the severity of MS because it is not based on hemodynamic assumptions.

The normal area of the mitral valve orifice is between 4 and 6 cm2. When the orifice area decreases below 2 cm2 one observes impedance of flow and an increase in gradients. Thus, it is not uncommon to find very mild forms of mitral stenosis in which discrete doming is the only sign of rheumatic mitral stenosis. The shape of the orifice may vary greatly, depending on the severity of the condition (more round in mild forms), whether both commissures are fused, and the distribution of rheumatic inflammation.

Several studies have shown that planimetry using 3D imaging is superior to conventional 2D planimetry. Although 3D planimetry is not yet a standard procedure, it might become one in the future.