What is instantaneous wave-free ratio (iFR) and its clinical use?

• A. A resting index measured during a specific diastolic period to assess stenosis without hyperemia; iFR ≤ ~0.89 indicates significant lesion (threshold variable by protocol).
• B. A hyperemia-based index used during adenosine administration.
• C. A method for measuring blood oxygen levels.
• D. An imaging technique to assess myocardial perfusion.

Answer: (A)

Instantaneous wave-free ratio is a resting physiologic index that uses a pressure wire to compare distal coronary pressure to aortic pressure during a diastolic wave-free period when microvascular resistance is stable. Because resistance is relatively constant in that window, the pressure ratio reflects the effect of the stenosis on blood flow without needing vasodilator-induced hyperemia. A value around 0.89 or lower is typically labeled as indicating a functionally significant lesion, though exact thresholds can vary by protocol. Clinically, this allows assessment and decision-making about revascularization without adenosine, unlike hyperemia-based indices. It is not a measure of blood oxygen levels, nor an imaging technique for perfusion.