Proximal Isovelocity Surface Area (PISA) method for estimation of mitral regurgitation
Proximal Isovelocity Surface Area (PISA) method is based on the continuity equation. When a flow passes through a narrow orifice, as it approaches the narrowest region, there is a flow convergence and flow acceleration. PISA is the surface area of the hemisphere at the aliasing region of the flow convergence. PISA increases as the flow increases and also with lower aliasing velocity. To reduce errors in measurement, smaller aliasing velocity has to be set, to get higher PISA measurement with lower chance for errors.
Regurgitant flow rate can be calculated as:
2 Pi r2 x Valiasing
Radius is measured from the orifice to point of colour change. If the flow convergence is not a true hemisphere, the angle subtended by the flow convergence at the orifice has to be measured and divided by 180 to get a correction factor. Good correlation between angiographic estimates of regurgitant flow and PISA based estimates have been reported.
Limitations of PISA method
Dynamic changes in orifice
Non optimal flow convergence
Vena contracta is the narrowest part of the regurgitant jet. Roughly, vena contracta diameter between 3 – 6 mm can be taken as moderate mitral regurgitation; below 3 mm can be considered mild and more than 6 mm as severe.
Colour M-mode will identify whether the mitral regurgitation is pansystolic or not.