IFR

Instantaneous wave-free ratio (iFR) is a resting physiological index used during invasive coronary angiography to assess the functional significance of a coronary artery stenosis. It is defined as the ratio of distal coronary pressure (Pd) to aortic pressure (Pa) during the wave-free period of diastole, a phase in which microvascular resistance is naturally minimized and hyperemia is not induced.

Measurement and interpretation are performed with a pressure wire. An iFR value is typically reported as a number between 0 and 1. A commonly used threshold is 0.89; lesions with an iFR at or below about 0.89 are considered likely to be haemodynamically significant, while higher values suggest non-significance. Some protocols use slightly different cutoffs (for example, 0.90 or region-specific thresholds). iFR can be assessed as a single value or during pullback to evaluate lesion length and focality.

Clinical evidence and use. iFR was developed as an alternative to fractional flow reserve (FFR), which requires inducing maximal hyperemia with a vasodilator such as adenosine. Trials comparing iFR-guided strategies with FFR-guided strategies, including DEFINE-FLAIR and iFR-SWEDEHEART, showed noninferiority in major adverse cardiac events at 12 months, supporting the use of iFR for revascularization decision-making. iFR offers potential advantages such as avoiding vasodilators, shorter procedure times, and improved patient comfort, but its accuracy can vary in certain clinical contexts.

Indications and limitations. iFR is used to determine whether a coronary lesion is functionally significant and thus whether revascularization is beneficial. Limitations include possible variability in serial lesions, complex microvascular states, and certain acute or hemodynamically unstable conditions where FFR may still be preferred. Overall, iFR is considered a validated alternative to FFR in appropriate patients and settings.