What is proximal isovelocity surface area (PISA) and how is it used to quantify valvular regurgitation?

What is PISA (Proximal Isovelocity Surface Area)?

PISA is a quantitative Doppler echocardiography method that measures the effective regurgitant orifice area (EROA) and regurgitant volume of valvular regurgitation by analyzing the hemispherical flow convergence zone that forms proximal to a regurgitant valve orifice. 1

Fundamental Principle

The PISA method is based on fluid dynamics: as blood flows toward a regurgitant orifice, it forms concentric hemispherical shells with progressively decreasing surface area and increasing velocity. 1 The flow rate through these hemispheric shells equals the flow rate through the regurgitant orifice by the conservation of mass principle.

Calculation Method

The basic formula is:

• Flow rate = 2πr² × aliasing velocity (where r = radius of the hemispheric shell)
• EROA = Flow rate / peak regurgitant velocity (by continuous wave Doppler)
• Regurgitant volume = EROA × time-velocity integral of the regurgitant jet 1

Technical Requirements for Accurate Measurement

To optimize PISA accuracy, you must: 1

• Magnify the PISA region with sector and depth optimized for color Doppler resolution
• Align measurement parallel to flow direction to optimize Doppler resolution
• Shift the Nyquist limit baseline toward the direction of regurgitant flow
• Use Nyquist limit ranges of 30-40 cm/s for optimal PISA radius measurement
• Measure at the appropriate time point during systole (for mitral/tricuspid regurgitation)

Critical Limitations You Must Know

PISA has substantial limitations that make it an imperfect tool: 2

Geometric Assumptions

• Assumes hemispheric flow convergence, which is rarely true in secondary (functional) mitral regurgitation where the PISA region is often hemielliptical 1
• Systematically underestimates EROA in functional mitral regurgitation because regurgitant orifices are typically crescentic rather than circular 2
• For elliptical orifices with 3:1 axis ratio, underestimation is approximately 8-10%, but can reach 35% for 10:1 ratios 3

Technical Challenges

• Errors in radius measurement are squared, resulting in large variance in calculated EROA 2
• Eccentric jets distort the PISA region due to constraint from atrial walls or valve leaflets, causing falsely increased radius and EROA overestimation 1
• Difficult to identify the precise location of the valve orifice where measurement should be taken 2
• High interobserver variability and poor agreement even among experienced observers 2

Physiologic Limitations

• Regurgitant flow changes during systole - particularly in secondary mitral regurgitation with biphasic flow patterns 2, 4
• Single-frame PISA measurements don't reflect mean EROA over the entire regurgitant period 2, 4
• Not validated for multiple jets 5, 2
• Limited feasibility in aortic regurgitation due to valve calcifications 5

Clinical Application by Valve Type

Mitral and Tricuspid Regurgitation

• PISA is measurable in 78% of mitral regurgitation cases and 69% of tricuspid regurgitation cases 6
• Most clinically useful for these atrioventricular valves 5, 6
• Different severity thresholds apply for primary vs. secondary mitral regurgitation:
  • Severe primary MR: EROA ≥0.4 cm², regurgitant volume ≥60 mL
  • Severe secondary MR: EROA ≥0.2 cm², regurgitant volume ≥30 mL 1

Aortic Regurgitation

• PISA is measurable in only 26% of aortic regurgitation cases 6
• When feasible, EROA ≥30 mm² or regurgitant volume ≥60 mL indicates severe AR 5
• Use parasternal long-axis view for eccentric jets 5

Critical Clinical Pitfalls to Avoid

1. Never rely on PISA alone - it should not be the sole criterion for determining regurgitation severity 2

2. Beware of timing errors in non-holosystolic regurgitation:

   • Measuring at peak early systolic flow overestimates severity
   • Measuring at mid-systolic nadir underestimates severity 2, 4

3. Consider 3D echocardiography when available - it allows direct measurement of vena contracta area and avoids the systematic underestimation inherent to 2D PISA, particularly in functional mitral regurgitation 1, 2

4. Integrate with other parameters - always use PISA together with vena contracta width, jet characteristics, and chamber dimensions to ensure internal consistency 5, 4

5. Account for flow constraints - adjacent structures (atrial walls, leaflets) affect PISA shape and accuracy 5, 2

When PISA Works Best

PISA is most accurate for: 7, 8, 6

• Central (non-eccentric) jets
• Circular or near-circular orifices
• Holosystolic regurgitation
• Moderate to severe regurgitation (large flow convergence zones are easier to measure)

The method becomes increasingly unreliable as jets become more eccentric, orifices more elliptical, and regurgitation milder.