The proximal isovelocity surface area (PISA) method for quantifying valvular regurgitation
uses an echocardiographic image with superimposed colour Doppler mapping to visualise
the contours of velocity in the blood travelling towards the regurgitant orifice.
The flux of blood through the regurgitant orifice is obtained as the product of the
area of one of these (presumed hemispherical) contours and the speed of the blood
passing through it. However, colour Doppler mapping measures the velocity component
towards the echo probe (v cos theta;) rather than speed (v), so that the contours
of equal Doppler velocity (isodoppler velocity contours) differ from isospeed contours.
We derive the shape of the isodoppler contour surface obtainable by colour Doppler
mapping, and show that its area is much less than that of the hemispherical isospeed
contour. When regurgitant flux is derived from an appropriate single measure of contour
dimension, an appropriate result may be obtained. However, if the true echocardiographic
surface area is measured directly, the regurgitant flux will be substantially underestimated.
Indeed, the conditions necessary for isodoppler velocity contours to be hemispherical
are extraordinary. We should not therefore make deductions from the apparent shape
for the convergence zone without considering the principles by which the image is
generated. The discrepancy will assume practical significance when increased resolution
of colour Doppler technology makes measurement of apparent surface area feasible.
Assuming the flow contours are indeed hemispherical, a 'correction' factor of 1.45
would be required.