The Role of Fractional Flow Reserve and Instantaneous Wave-Free Ratio Measurements in Patients with Acute Coronary Syndrome.

PURPOSE OF REVIEW: The role of fractional flow reserve to guide revascularization in patients with stable angina is well established. The instantaneous wave-free ratio (iFR) is an emerging adenosine-free resting index that is non-inferior to FFR and has potential to streamline the functional evaluation of coronary artery disease. The feasibility and utility of intracoronary physiology in patients with acute coronary syndrome (ACS) is unclear. This review will discuss the physiological principles and validity of using FFR and iFR in patients presenting with ACS. We will also provide an overview of the available evidence for their role in guiding revascularization in this patient group. RECENT FINDINGS: The use of intracoronary physiology in culprit lesions of patients presenting with STEMI is not recommended and its accuracy is uncertain in patients with NSTEMI. In contrast, the physiological assessment of non-culprit vessels with FFR and IFR is a reliable measure of lesion-specific ischemia. Recent studies have demonstrated that FFR-guided revascularization of non-culprit lesions improves clinical outcomes although the role of iFR in this patient cohort is unknown. Physiology-guided revascularization of non-culprit ACS lesions improves clinical outcomes. Future studies investigating the complementary role of plaque morphology, biomechanics, and systemic inflammation may provide clinicians with a more comprehensive framework to guide treatment decisions.