What is the clinical significance of the PISA (Proximal Isovelocity Surface Area) radius in assessing mitral regurgitation severity?

Clinical Significance of PISA Radius in Assessing Mitral Regurgitation Severity

The PISA (Proximal Isovelocity Surface Area) radius is a critical but technically challenging measurement that significantly impacts mitral regurgitation (MR) severity assessment, where even a 1mm difference in measurement (7mm vs 8mm) can change classification from mild to severe MR, directly affecting treatment decisions and patient outcomes. 1

Understanding PISA Radius and Its Clinical Importance

PISA radius is used to calculate:

• Effective Regurgitant Orifice Area (EROA)
• Regurgitant Volume (RVol)
• Regurgitant Fraction

These parameters determine MR severity classification, which guides clinical management decisions including:

• Medical therapy optimization
• Timing of surgical intervention
• Selection of appropriate procedural approaches

Technical Aspects and Measurement Considerations

Simplified PISA Formula

• EROA = 2πr² × (aliasing velocity ÷ peak MR velocity)
• With typical settings (aliasing velocity of 40 cm/s and peak velocity of 5 m/s), this simplifies to: EROA = r²/2 1

Critical Measurement Thresholds

• PISA radius ≥8 mm typically indicates severe MR (EROA ~0.32 cm²)
• PISA radius of 9 mm corresponds to EROA ~0.40 cm² (definitely severe)
• Small measurement errors have significant impact: a 1mm difference in radius measurement (7mm vs 8mm) can change EROA from 0.16 cm² to 0.26 cm², potentially changing classification from mild to severe MR 1

Limitations and Pitfalls of PISA Radius Measurement

Technical Challenges

• Difficulty identifying exact point of flow convergence on anatomical orifice 1
• Errors in radius measurement are squared in calculations, magnifying inaccuracies 1
• Poor interobserver agreement on MR severity classification using PISA 1

Geometric Assumptions

• PISA method assumes:
  1. Round orifice through a flat surface
  2. Hemispheric flow convergence region
  3. Single time point measurement represents entire systole 1

Common Sources of Error

1. Non-circular regurgitant orifices: EROA is underestimated in crescentic orifices common in secondary MR 1
2. Dynamic nature of MR: Secondary MR has a biphasic pattern during systole 1
3. Timing of measurement: Early systolic measurement may overestimate, mid-systolic may underestimate 1
4. Multiple jets: Separate PISAs may need to be added, but validation studies are lacking 1
5. Non-holosystolic regurgitation: Single-frame measurements don't reflect mean EROA 1

Improving Accuracy of MR Assessment

Integrated Approach

• Never rely solely on PISA measurements for determining MR severity 1
• Confirm with other parameters:
  • Mitral inflow patterns
  • Pulmonary vein flow patterns
  • Left atrial volume
  • Continuous-wave Doppler characteristics of MR jet 1

Advanced Techniques

• 3D echocardiography allows direct measurement of EROA without geometric assumptions 1
• 3D measurements typically yield larger EROA values than 2D PISA in secondary MR 1
• Volumetric methods may have greater prognostic significance than PISA in functional MR 2

Technical Tips

• Use zoom function to maximize pixels in radius measurement 1
• Toggle color on/off to ensure proper valve orifice level identification 1
• Consider angle correction for non-planar flow convergence 1, 3

Clinical Implications

• MR severity directly impacts mortality and morbidity risk stratification
• Dynamic nature of MR (especially secondary MR) means severity can change with loading conditions and medical therapy 1
• Small differences in PISA radius measurement can lead to significant differences in treatment decisions 1
• Volumetric methods may better predict outcomes in functional MR than PISA measurements 2

In conclusion, while PISA radius is a fundamental measurement in MR assessment, clinicians must understand its technical limitations and always integrate it with other echocardiographic parameters to accurately determine MR severity and guide appropriate clinical management decisions.