Fractional flow reserve (FFR) is a measure used during coronary angiography to assess the risk of coronary stenosis causing myocardial ischemia. It quantifies reduced blood flow due to coronary disease and guides treatment decisions. Clinical trials support using percutaneous coronary intervention (PCI) for ischemic lesions (FFR ≤0.80) over medical therapy. For nonischaemic FFR lesions (FFR >0.80), PCI isn't recommended. FFR calculates maximal blood flow with a stenosis compared to normal flow. It's easily derived from pressure ratios during vasodilation, unaffected by systemic changes, and considers collateral blood supply. Normal FFR is 1.0, and FFR below 0.75 indicates significant stenoses associated with myocardial ischemia.   References Pijls N. et al., Measurement of Fractional Flow Reserve to Assess the Functional Severity of Coronary-Artery Stenoses. N Engl J Med 1996; 334:1703-1708. DOI: 10.1056/NEJM199606273342604 https://jamanetwork.com/journals/jama/fullarticle/2773127 https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2796612