The following are key points to remember about the evolving future of instantaneous wave-free ratio (iFR) and fractional flow reserve (FFR):

1. The use of coronary physiology to guide revascularization has been found to improve patient outcomes and defer stenting of nonischemic lesions compared with angiographic assessment.
2. FFR is defined as ratio of the pressure distal to a stenosis (Pd) relative to the pressure proximal to the stenosis (Pa) during hyperemia induced by a vasodilating agent, typically adenosine.
3. FFR carries a Class 1a recommendation for guiding revascularization in angiographically intermediate coronary stenoses in patients with stable angina. However, despite this, uptake of FFR in coronary catheter laboratories worldwide has remained low.
4. Potential reasons for the low adoption rate of FFR despite demonstrated clinical benefit of its use may include time consumption to perform FFR measurements, costs associated with adenosine, or, in certain countries, no availability of adenosine, patient-related discomfort, contraindications, or lack of reimbursement.
5. Recently, there has been renewed interest and development in the field of coronary physiology, driven by the introduction of a new, nonhyperemic pressure-based index of stenosis severity: the iFR.
6. By means of the distal pressure value obtained during the wave-free period of diastole, iFR measures the physiological impact of a coronary stenosis on the distal coronary bed.
7. Emerging data suggest that iFR may be a superior prognostic tool in comparison with FFR for deferring nonculprit lesions in patients with acute coronary syndrome.
8. The DEFINE-FLAIR and iFR-SWEDEHEART trials suggest that iFR may be the new evidence-based standard for invasive evaluation of intermediate-severity coronary lesions. Furthermore, imperative for the continued development of iFR-based physiological applications, the trials also validated the use of a single iFR 0.89 cutoff value, without resorting to a diagnostic gray zone or hybrid approach.
9. It is now also possible to generate a fully-integrated physiological map of any coronary vessel acquired under manual iFR pullback that is coregistered with the angiogram in real time.
10. Real-time coregistered iFR pressure mapping with virtual percutaneous coronary intervention (PCI) capability may herald a new paradigm for functional lesion assessment, where physiology is used to both justify and guide optimal coronary intervention.

Keywords: Acute Coronary Syndrome, Adenosine, Angina, Stable, Constriction, Pathologic, Coronary Angiography, Coronary Stenosis, Diastole, Fractional Flow Reserve, Myocardial, Hyperemia, Ischemia, Myocardial Revascularization, Percutaneous Coronary Intervention, Vasodilator Agents

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