True shape and area of proximal isovelocity surface area (PISA) when flow convergence is hemispherical in valvular regurgitation

The proximal isovelocity surface area (PISA) method for quantifying valvula r regurgitation uses an echocardiographic image with superimposed colour Do ppler 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 h emispherical) contours and the speed of the blood passing through it. Howev er, colour Doppler mapping measures the velocity component towards the echo probe (nu cos theta) rather than speed (nu), so that the contours of equal Doppler velocity (isodoppler velocity contours) differ from isospeed conto urs. We derive the shape of the isodoppler contour surface obtainable by co lour Doppler mapping, and show that its area is much less than that of the hemispherical isospeed contour. When regurgitant Aux is derived from an app ropriate single measure of contour dimension, an appropriate result may be obtained. However, if the true echocardiographic surface area is measured d irectly, the regurgitant flux will be substantially underestimated. Indeed, the conditions necessary for isodoppler velocity contours to be hemispheri cal are extraordinary. We should not therefore make deductions from the app arent shape for the convergence zone without considering the principles by which the image is generated. The discrepancy will assume practical signifi cance when increased resolution of colour Doppler technology makes measurem ent of apparent surface area feasible. Assuming the flow contours are indee d hemispherical, a 'correction' factor of 1.45 would be required. (C) 2000 Published by Elsevier Science Ireland Ltd. All rights reserved.