11.5.3 Pressure Half Time
Quantification using the pressure half-time method is based on the assumption that the rate at which the gradient drops during diastole corresponds to the severity of mitral stenosis. The larger the mitral valve orifice area is, the quicker the left ventricle will fill and the more rapidly the gradients will drop. The speed with which the gradient drops can be measured from the slope of the diastolic inflow signal (deceleration slope). The steeper the curve, the larger is the mitral valve orifice area (and the less severe is mitral stenosis). To standardize the calculation one uses the slope between maximal inflow velocity (maximal pressure gradient) to the time when half of the pressure gradient has dropped (pressure half-time). Modern echo scanners automatically derive this time point. From the practical point of view, all one needs to do is align the measurement calipers to the curve, starting from the maximal velocity at early diastolic filling. In atrial fibrillation one should use average values of several measurements. In addition, avoid PHT and measurements from cycles with a short diastole.
In severe mitral stenosis you will not see the E wave of the Doppler signal return to the base line
Mitral valve area is estimated by dividing 220 (empirical value) by the pressure half-time.
MVA = 220 / PHT
Although the PHT technique is easy to use, there are numerous conditions in which the calculated mitral valve area is unreliable and this method should be used with caution. Especially in the following settings:
- Diastolic dysfunction -can lead to over estimation of MS severity
- Aortic regurgitation - can lead to under estimation of MS severity
- After valvuloplasty -- PHT is unreliable
- Heavily calcified valves - - unreliable
Difficulties may arise when the shape of the Doppler spectrum is not a distinct continuous slope but a concave tracing. It this case one should approximate the slope to the best fit and ignore the initial portion of the spectrum.
The pressure half-time method is based on hemodynamic assumptions and works less well in the elderly and multimorbid patient population with mitral stenosis.11.5.4 Other less commonly used methods for quantification
Several other methods of quantification have been suggested. These include calculations based on the continuity equation and the PISA method.
The main limitation of the continuity method is that it cannot be employed in the presence of mitral regurgitation and/or aortic regurgitation. Thus, it is rarely used.
MVA = (D2LVOT/4) x (VTIAortic / VTIMitral)
The proximal isovelocity surface area (PISA) method uses the aliasing velocity of mitral inflow to calculate the mitral flow volume, which is divided by the peak velocity of diastolic mitral inflow to derive the mitral valve orifice area. Although this method is quite appealing, it is limited by the fact that the PISA is usually not truly hemispheric, as assumed in the formula.
MVA = ∏(r 2) x (Valiasing) ⁄ Peak Vmitral x ∝ ⁄ 180