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Instantaneous Wave-Free Ratio (iFR)

Summary

The Instantaneous Wave-Free Ratio (iFR) is a wire-based coronary physiological index used to assess the severity of coronary artery stenoses without the administration of vasodilator agents. It is measured under resting conditions during a specific period of diastole, known as the wave-free period, when microvascular resistance is naturally minimized and stable. Like FFR, iFR is used to guide clinical decisions regarding percutaneous coronary intervention (PCI).

Definition and Calculation

iFR is calculated as the ratio of distal coronary pressure ($P_d$) to aortic pressure ($P_a$) during the "wave-free period" of diastole.

$$\text{iFR} = \frac{P_d}{P_a} \quad (\text{during the wave-free period})$$

Unlike Fractional Flow Reserve (FFR), which requires a pharmacological vasodilator (e.g., adenosine) to achieve maximal hyperemia and minimize microvascular resistance across the entire cardiac cycle, iFR exploits the natural physiology of the diastolic wave-free period. During this specific window, microvascular resistance is low and constant, allowing for a reliable assessment of stenosis severity under resting conditions.

Clinical Interpretation

  • iFR > 0.89: Typically indicates a non-ischemic stenosis.
  • iFR ≤ 0.89: Typically indicates a hemodynamically significant stenosis where revascularization may be beneficial (as of 2026-07-19).
  • Clinical trials (e.g., DEFINE-FLAIR and iFR-SWEDEHEART) have shown that iFR-guided revascularization is noninferior to FFR-guided strategies.

Citations

  • Randles A. Physiological Assessment of Coronary Artery Disease. N Engl J Med 2026;395:80-83. NEJMe2602134.pdf
  • Davies JE, Sen S, Dehbi H-M, et al. Use of the instantaneous wave-free ratio or fractional flow reserve in PCI. N Engl J Med 2017;376:1824-34.