The pressure half-time is the time required for the peak transvalvular pressure gradient to be reduced by one-half and is quantitatively related to the degree of mitral stenosis. The pressure gradient is measured by calculating the velocity of flow across the mitral valve and converting the velocity to pressure, using the modified Bernoulli equation: P = 4V2, where P = pressure and V = velocity of flow. The figure below shows a typical velocity envelope across the mitral valve. The maximum velocity is 296 cm/s. A normal pressure half-time is about 70 ms. In severe mitral stenosis, the pressure half-time is greater than 200 ms. The mitral valve area can be calculated using the formula MVA = 220/pressure half-time. For example in a patient with severe mitral stenosis, the pressure half-time might be 250 ms and the MVA = 220/250, or 0.7 cm2.
Continuous-wave Doppler measurement of the velocity across the mitral valve.
How do regurgitant lesions affect the calculation of valve areas?
The presence of severe aortic regurgitation will cause a slight underestimation of the severity of the mitral stenosis (overestimation of the valve area) determined by Doppler pressure half-time, because the gradient between the left atrium and left ventricle decreases faster due to the aortic regurgitation (hence a shorter pressure half-time). On the other hand, the valve area as determined by cardiac catheterization will tend to overestimate the severity of the mitral stenosis (underestimate the valve area), because the cardiac output is in the numerator of the Gorlin equation. The cardiac output used in the formula is the forward cardiac output measured by the Fick equation and therefore does not take into account the regurgitant volume, underestimating the flow across the valve. Mitral regurgitation does not affect the pressure half-time significantly, but it does affect the Gorlin area, as with aortic regurgitation.
What treatment options are available?
• Surgical replacement of mitral valve
• Surgical repair (open commissurotomy), where the fused leaflets are separated
• Balloon mitral valvuloplasty (commisurotomy done with a percutaneous balloon technique)
What is a normal mitral valve area? When is the mitral valve orifice small enough to warrant surgical or other intervention?
A normal mitral valve orifice is 4-6 cm2, and intervention is usually considered warranted when the orifice is 1.0 cm2. However, some patients with valve areas of 1.0-1.5 cm2 may have pulmonary hypertension or exertional dyspnea, warranting intervention.
What echocardiographic parameters are used to predict the success of mitral valvuloplasty?
The mitral valve is more amenable to valvuloplasty if the valve is pliable and the valve and subvalvular apparatus are not severely calcified or thickened. An echocardiographic grading system has been developed by Weyman et al. based on valve mobility, thickening or calcification, and subvalvular thickening. Each of these four variables is assigned a score from 1-4 (4 being most severe), with a maximum score being 16. In general, a score of 8 or less suggests that the valve would respond well to valvuloplasty. The presence of more than mild mitral regurgitation precludes valvuloplasty, because in most cases this procedure will increase the degree of mitral regurgitation.