Instantaneous wave-free ratio

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.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 or SPECT. 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.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.A number of studies using the Imperial College developed iFR algorithm have been conducted.Two outcome studies, DEFINE-FLAIR and the iFR-SWEDEHEART will assess whether patient outcomes differ if stenoses are treated according to iFR or FFR classification of stenosis severity. DEFINE-FLAIR, which aims to recruit 2500 patients, will be the largest physiological study to date and will provide the first randomised data of the use of iFR and FFR in patients with both stable coronary disease and acute coronary syndromes. It will also provide the first randomised data of the clinical utility of FFR in guiding and deferring coronary intervention outside of the FAME and DEFER studies. Both DEFINE-FLAIR and iFR-SWEDEHEART have harmonised clinical endpoints which will enable combined analysis of over 4500 patients.The instantaneous wave-free ratio can also be calculated on a beat-to-beat basis. If performed during the slow withdrawal of a pressure wire (referred to as 'pullback') under resting conditions, it is possible to plot the change of iFR or trans-stenotic gradient over the wave-free period throughout the vessel. This can help identify focal and diffuse coronary disease potentially amenable to revascularisation. An example of pullback in clinical use can be seen in this video. The pressure wire data can be co-registered with the angiographic findings to aid ease of interpretation. With mechanised pullback approaches which move the pressure wire at a fixed speed, the physiological length of a stenosis can be determined. With advanced live co-registration this is also feasible with during manual pullback.Alternative iFR computation systems have been proposed, for example incorporating part of systole into the definition of diastole and optionally different time-shifts between Pd and Pa signals, yielding unsatisfactory results. The same datasets reanalysed using the standard algorithms confirm the mainstream findings. Various explanations have been proposed.