Assessment of mitral valve area by echocardiography
Mitral valve cross section for planimetry Assessment of mitral valve area by echocardiography can be done by following methods:
Two dimensional planimetry: The valve area is traced out using an electronic caliper and the machine calculates the valve area. Smallest full circle is taken as the mitral valve area as larger full circles may be proximal measurements of the belly of the mitral leaflets and hence not representative of the actual severity of the obstruction. If it is not full circle, it may be a distal measurement or oblique measurement.
Three dimensional planimetry: This advanced technique is possible only on 3-D echocardiography with a 3-D scan head which gives ‘voxels’ or volume elements instead of ‘pixels’ or picture elements given by 2-D echocardiographs.
Pressure half time (PHT): This involves Doppler echocardiography. A pressure half time of 220 ms corresponds to a valve area of 1 sq cm. Pressure half time is measured from the descend of the E wave in the transmitral Doppler, usually obtained in apical four chamber view. PHT is the time taken for the peak transmitral gradient to fall to half its value. It will also correspond to the time taken for the transmitral velocity to fall by square root of two as the pressure gradient is proportional to the square of the velocity. MVA (mitral valve area) = 220/PHT
Continuity equation: Continuity equation uses mitral and aortic flow as well as cross sectional area of the left ventricular outflow tract, to derive the mitral valve area. The area is measured by planimetry and flow measured by Doppler assessment of velocity time integral (VTI).
Proximal isovelocity surface area (PISA) method utilizes the flow convergence region proximal to the high velocity trans mitral jet in diastole. If the flow convergence is not a true hemisphere, the angle subtended by the flow convergence at the orifice must be measured and divided by 180 to get a correction factor. The diastolic flow rate is calculated as follows :
Mitral flow (ml/s) = 2Pπr2 x angle α/180 x Valias (cm/s)
r: maximal radius of the flow convergence region in early diastole measured on the centreline of the flow convergence region
Valias: aliasing velocity
α /180: correction factor accounting for the angle α between the mitral leaflets
Mitral valve area is then determined by dividing maximal diastolic flow rate by peak continuous wave Doppler velocity of mitral inflow Vmax.
PISA method is based on the same principle as the continuity equation. It has been shown to be accurate and reproducible. Proximal flow convergence can be easily visualized .