Proximal Isovelocity Surface Area (PISA) method for estimation of mitral regurgitation

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 [1]. 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:

2πr² x V_aliasing

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.

PISA method has been used for intraoperative assessment of severity of mitral regurgitation. It provides a quantitative assessment of severity of MR and is useful in clinical decision making in the operating room [1]. Effective regurgitant orifice, regurgitant volume and regurgitant fraction can also be calculated using the PISA method.

Effective regurgitant orifice area will be (2πr² x Nyquist limit)/V_max. V_max is the maximum velocity of the mitral regurgitation get measured by continuous wave (CW) Doppler. Regurgitant volume will be effective regurgitant orifice area multiplied by the velocity time integral (VTI) of the MR jet. Regurgitant fraction is calculated as the ratio of regurgitant volume to total stroke volume [1].

Limitations of PISA method

1. Dynamic changes in orifice
2. Oval or irregular regurgitant orifice
3. Non optimal flow convergence
4. Not useful when there are multiple regurgitant orifices

Hemielliptic proximal isovelocity surface area method has been suggested for more accurate quantification of mitral regurgitation by PISA method [2]. Another method to overcome the variation in shape of PISA from a true hemisphere is three dimensional echocardiography. In a pediatric population, it was found that PISA is rarely hemispherical, but more often prolate or oblate hemispheroid [3].

References

1. Lambert AS. Proximal isovelocity surface area should be routinely measured in evaluating mitral regurgitation: a core review. Anesth Analg. 2007 Oct;105(4):940-3.
2. Fujii H, Kibira S, Izumi C, Saito T, Ryabikov A, Miura M. Hemielliptic proximal isovelocity surface area method modified for clinical application: more accurate quantification of mitral regurgitation in Doppler echocardiography. Jpn Circ J. 2001 Sep;65(9):820-6.
3. Ziani AB, Latcu DG, Abadir S, Paranon S, Dulac Y, Guerrero F, Acar P. Assessment of proximal isovelocity surface area (PISA) shape using three-dimensional echocardiography in a paediatric population with mitral regurgitation or ventricular shunt. Arch Cardiovasc Dis. 2009 Mar;102(3):185-91.