The instantaneous wave-free ratio (iFR, sometimes referred to as the instant wave-free ratio or instant flow reserve) is a diagnostic tool used to assess whether a stenosis is causing a limitation of blood flow in coronary arteries with subsequent ischemia. iFR is performed during cardiac catheterisation (angiography) using invasive coronary pressure wires which are placed in the coronary arteries that are to be assessed. Pressure wires are commonly used by interventional cardiologists to guide decisions to perform revascularization, either by stenting or bypass surgery.
Assessment of coronary stenoses Coronary artery blockages or stenoses that limit blood flow to the heart muscle can cause angina and can be treated by stenting or bypass surgery. Relief of a stenosis by stenting aims to restore vessel patency with improvement in blood flow leading to a reduction in angina symptoms. However, if stenoses are not flow limiting, then they can be safely left alone without stenting or surgery and this help reduce patient's exposure to unnecessary procedures and potential complications. Identifying stenoses that cause flow limitation, or ischaemia, can be done in a variety of ways. Non-invasive tests can include stress testing such as exercise electrocardiograms, stress echocardiography, or perfusion imaging tests such as scintigraphy, SPECT, or PET. Alternatively, invasive tests can be performed at the time of angiography, and these include those that measure coronary flow velocity in the vessel, CFR or index flow against pressure gradients such as hyperaemic or basal stenosis resistance (HSR or BSR). More commonly coronary pressure measurements are used as a surrogate for flow measurement and techniques include iFR and fractional flow reserve (FFR). Cardiologists use a combination of these investigations together with the patient's history, symptoms and clinical risk factors to decide if a stenosis requires further treatment. An example of use in clinical practice is seen here.
iFR definition and calculation
Instantaneous wave-free ratio is performed using high fidelity pressure wires that are passed distal to the coronary stenosis. iFR isolates a specific period in diastole, called the wave-free period, and uses the ratio of distal coronary pressure (Pd) to the pressure observed in the aorta (Pa) over this period. During this wave-free period, the competing forces (waves) that affect coronary flow are quiescent meaning pressure and flow are linearly related as compared to the rest of the cardiac cycle. When stenoses are flow limiting, Pd and Pa pressures over the wave-free period diverge; a normal ratio is 1.0 and iFR values of below 0.90 suggest flow limitation. iFR can be calculated using dedicated consoles available for medical use and typically uses an average over 5 heart beats but can be performed using a single heart beat. iFR is measured at rest, without the need for pharmacological vasodilators or stressors and compares well to other invasive and non-invasive markers of ischemia or flow limitation.
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