Proximal isovelocity surface area
The proximal isovelocity surface area (PISA) of a regurgitant color flow jet is used in echocardiography to estimate the severity of valvular insufficiency, mainly mitral regurgitation.
Principle of PISA
When fluid passes through a small circular orifice in a flat plate, there is flow acceleration just proximal to the orifice (Flow Convergence Zone). The flow converges on the orifice in hemispheric layers of equal velocity. The radius of this 3-dimensional surface area is called PISA (proximal isovelocity surface area) radius and can be used to calculate the severity of valvular regurgitation. The principle is that if the regurgitation is mild, only the blood nearer to the valve orifice accelerates towards the orifice while in severe regurgitation, blood farther away from the valve moves back. From PISA radius it is possible to estimate the regurgitant flow, the effective regurgitant orifice area (EROA) and the regurgitant volume (R Vol).
A simple method to determine severity of valvular, especially mitral regurgitation is to measure the PISA radius. In case of mitral regurgitation is mild if radius is <0.4 cm and severe if >1 cm.
Instantaneous Regurgitant flow
Setting the aliasing velocity in order to obtain an hemiseferic convergence zone, the instantaneous regurgitant flow (RF) can be calculated as:
RF = 2π * r2 * Va
where r is the PISA radius of the convergence zone and Va is the aliasing velocity. It can be used to assess the severity of mitral, tricuspid and aortic regurgitation.
The equation above assumes that the regurgitant orifice is in a flat plane and that the proximal isovelocity surface is a complete hemisphere. The orifice however is typically at the apex of a conical structure. A correction term of θ°/180° can be used to adjust for this, where θ is the apical angle of the cone:
RF = (θ°/180°) * 2π * r2 * Va
Effective regurgitant orifice area (EROA)
The EROA is calculated using the instantaneous regurgitant flow and derives from the following formula:
Area of the orifice = Regurgitant Flow / velocity through the orifice
EROA = RF / Vmax = (θ°/180°) * 2π * r2 * Va / Vmax
where r is the PISA radius, Va is the aliasing velocity and Vmax is the peak velocity of the regurgitant jet assesed by Continous-Wave Doppler.
Values for assesing severity of mitral regurgitation are: Mild = 0-0.2 cm2, moderate = 0.2 - 0.4 cm2, severe = >0.4 cm2
Regurgitant volume (R Vol)
If: Regurgitant Flow = Area of the orifice x velocity through the orifice
and: Regurgitant Volume = Regurgitant Flow x Time of regurgitation
then: Regurgitant Volume = Area of the orifice x Velocity through the orifice x Time of regurgitation
Therefore Regurgitant volume (R vol) is estimated as follows:
R Vol = EROA x VTI
where VTI is the time-velocity integral of the regurgitant jet obtained bt the Continous-Wave Doppler
Advantages and Limitations
- It seems to be an estimation of valvular regurgitation quite independent of hemodinamic factors, etiology of the disease and presence of multiple valve alterations.
- It can be used in central as well as in eccentric jets (altough less accurate).
- It is a quantitative estimation of lesion severity and volume overload with an accetable reproduciblity.
- If the valve orifice is not flat or circular, the flow convergence zone will not be hemisferic, thus PISA radius cannot be used for the calculation of the regurgitant flow.
- Systolic changes of regurgitant flow are not taken into account.
- In assesment of regurgitant flow/volume, errors in calculation of PISA radius are squared.