How is patient-prosthesis mismatch assessed using aortic annulus size indexed to body surface area?

Assessment of Patient-Prosthesis Mismatch Using Aortic Annulus Size Indexed to BSA

Patient-prosthesis mismatch (PPM) is assessed by calculating the effective orifice area (EOA) of the prosthetic valve indexed to the patient's body surface area (BSA), with severe PPM defined as an indexed EOA <0.65 cm²/m² for non-obese patients (BMI <30 kg/m²) and <0.55 cm²/m² for obese patients (BMI ≥30 kg/m²). 1, 2

How PPM Assessment Works

The fundamental calculation involves:

• Indexed EOA = EOA (cm²) / BSA (m²) 2

This indexing accounts for the patient's body size and metabolic demands, recognizing that a valve with adequate flow for a small patient may be insufficient for a larger patient.

Standardized Definitions

According to the 2017 AHA/ACC guidelines and the 2021 EACTS-STS-AATS consensus:

Severe PPM thresholds:

• Non-obese patients (BMI <30 kg/m²): indexed EOA <0.65 cm²/m² 1, 2
• Obese patients (BMI ≥30 kg/m²): indexed EOA <0.55 cm²/m² 2

Moderate PPM thresholds:

• Non-obese patients: indexed EOA 0.65-0.85 cm²/m² 1
• Obese patients: indexed EOA 0.55-0.70 cm²/m² 2

Clinical Significance

Severe PPM is particularly detrimental in patients with reduced left ventricular ejection fraction and predicts:

• High transvalvular gradients
• Persistent LV hypertrophy
• Increased rate of cardiac events after AVR
• Decreased likelihood of symptom resolution 1

Prevention Strategy

PPM can be avoided by selecting a valve prosthesis that will provide an adequate indexed orifice area based on:

1. Patient's body surface area
2. Aortic annular dimensions measured pre-operatively
3. Known reference EOA values for each valve model and size 1, 2

The 2021 expert consensus recommends manufacturers provide standardized PPM probability charts showing the percentage risk of severe PPM for each valve size across different patient BSA ranges (1.3-2.6 m² in 0.1 m² increments) 2. These charts should use high-quality reference EOA values from prospective multicenter studies with at least 30 patients per valve size.

Alternative Approach: Indexed Annulus Size

Recent research has explored using CT-measured aortic annulus size indexed to BSA as a predictor of PPM risk 3, 4. This approach categorizes patients into:

• Small indexed annulus: 9-12 mm/m²
• Medium indexed annulus: >12-14 mm/m²
• Large indexed annulus: >14-18 mm/m²

Indexed annulus size is an independent predictor of PPM after both TAVR and SAVR, with PPM rates declining as indexed annulus size increases 3. This method allows pre-procedural risk stratification before valve selection.

Critical Distinction: Measured vs. Predicted PPM

Important caveat: PPM can be assessed using either:

1. Measured EOA from post-procedural echocardiography (higher PPM rates)
2. Predicted EOA from manufacturer reference values (lower PPM rates)

Studies show the incidence of severe PPM is markedly higher when using measured versus predicted EOA (6-9% vs. 0.1-1.2%) 5, 6. The clinical significance of this discrepancy remains debated, with some studies showing no association between either definition and long-term mortality after TAVR 5, 7.

Avoiding Common Pitfalls

Do not mistake PPM for prosthetic valve stenosis. With bileaflet mechanical valves, significant pressure recovery may result in high velocities that should not be confused with true stenosis or PPM, particularly in patients with small aortic diameters 1.

When severe PPM is anticipated, consider annular enlargement procedures or alternative approaches (including TAVR over SAVR) to allow implantation of an appropriately sized valve 1.