How does fractional flow reserve (FFR) vary in the presence of microvascular dysfunction?

FFR Variation in Microvascular Dysfunction

FFR remains relatively stable and reliable even in the presence of microvascular dysfunction, with only modest elevation (approximately 4-5% increase) when microvascular resistance is significantly impaired. This limited impact allows FFR to maintain its diagnostic utility across diverse patient populations, including those with coronary microvascular dysfunction (CMD).

Core Physiological Principle

FFR is designed to assess epicardial stenosis severity and is fundamentally less susceptible to microvascular influences than flow-based indices 1. The measurement reflects the ratio of distal coronary pressure to aortic pressure during maximal hyperemia, which primarily captures the pressure drop across the epicardial stenosis rather than microvascular resistance 2.

Impact of Microvascular Dysfunction on FFR

Magnitude of Effect

The most direct evidence comes from a 2001 study demonstrating that:

• A 25% reduction in hyperemic flow (as seen in severe microvascular dysfunction, such as post-MI patients) causes only a 5% increase in FFR values 3
• In matched comparisons, FFR was 79% ± 12% in patients without MI versus 83% ± 12% in MI patients with documented microvascular dysfunction (p = 0.3) 3
• This modest change maintains FFR within clinically acceptable ranges for decision-making

Mechanistic Understanding

The key mechanism is flow-dependent pressure gradient changes:

• When hyperemic flow decreases due to microvascular dysfunction, the translesional pressure gradient across an epicardial stenosis decreases proportionally 3
• This results in a higher distal-to-aortic pressure ratio (FFR), but the magnitude is clinically limited
• The relationship remains predictable: lower flow → lower pressure gradient → slightly higher FFR 4

Clinical Implications and Diagnostic Performance

Diagnostic Accuracy Considerations

Recent evidence reveals nuanced effects on FFR-based diagnostics:

• FFR maintains high diagnostic performance even with CMD present, though some degradation occurs with severe microvascular impairment 5
• The diagnostic concordance of FFR-derived indices (like FFRangio) shows reduced accuracy in high-IMR patients (AUC 0.83 vs 0.98 in low-IMR patients, p = 0.04) 5
• Standard FFR cutoffs (0.75-0.80) remain valid across most clinical scenarios 1

Discordance with Non-Hyperemic Indices

The difference between resting indices (RFR) and FFR provides insight into microvascular status:

• RFR is less affected by microvascular dysfunction than FFR 6
• When microvascular dysfunction is present, FFR tends to be higher while RFR remains relatively stable 6
• The RFR-FFR gradient (delta) correlates with microvascular dysfunction severity 7, 8, 9
• Patients with CMD show lower CFR and RRR values but similar RFR, while FFR is elevated 7

Contrast with CFR

CFR is far more susceptible to microvascular dysfunction than FFR:

• CFR represents the entire coronary compensatory capacity including both epicardial and microvascular components 2
• CFR is significantly influenced by hemodynamics, baseline flow conditions, and microvascular resistance 2, 10
• In patients with elevated IMR, CFR is markedly reduced while FFR shows only modest elevation 6, 4

Practical Clinical Algorithm

When Interpreting FFR in Suspected CMD:

1. Recognize that FFR may be falsely reassuring (slightly elevated) in severe microvascular dysfunction

   • The effect is modest (~5% elevation) but may shift borderline values 3

2. Consider complementary indices:

   • Measure IMR or CFR alongside FFR when CMD is suspected 6, 7
   • Calculate RFR-FFR gradient: larger negative values suggest microvascular dysfunction 8, 9
   • IMR ≥25 or CFR ≤2.0 indicates CMD 7

3. Adjust interpretation in high-risk scenarios:

   • Post-MI patients (especially recent infarction) 3
   • Patients with diabetes, hypertension, or heart failure 7
   • When RFR-FFR gradient is markedly negative (<-0.06) 8

4. Use conservative cutoffs when CMD is present:

   • Consider the gray zone (0.75-0.80) more critically 1
   • Values just above 0.80 may warrant additional assessment with CFR or IMR

Important Caveats

• FFR was validated in stable patients with single-vessel disease and normal LV function 1; extrapolation to complex scenarios requires caution
• Acute MI settings have limited validation for FFR accuracy 1
• Diffuse disease creates continuous pressure gradients that confound FFR interpretation 1
• The presence of CMD predicts worse outcomes even after FFR-guided revascularization 8, 9