Quantitative flow ratio as a new tool for angiography-based physiological evaluation of coronary artery disease: a review.

The functional evaluation of coronary stenoses has obtained important clinical results in recent years, resulting in strong guideline recommendations. Nonetheless, the use of coronary wire-based functional evaluation has not yet become part of the routine in catheterization laboratories for several reasons, including the need to advance a wire into the coronary vessel to interrogate the stenosis. Angiography-derived indexes have been introduced to expand the current use of physiology to estimate the functional meaning of a stenosis on the basis of angiographic data only. The most studied and validated angiography-derived index is certainly the quantitative flow ratio. This article will summarize the basics of the quantitative flow ratio, the related validation studies and its current and future applications. Lay abstract Coronary arteries are the main vessels that carry blood and oxygen to the heart muscle, ensuring its functionality. In cardiology, coronary stenoses are defined as pathological narrowing of one or more of these vessels, which can lead to a critical reduction in blood flow, ischemic problems and myocardial infarction. Currently, percutaneous coronary intervention is considered the first-line treatment of myocardial infarction. This technique consists of a procedure in which the stenosis is resolved and proper blood flow is restored thanks to balloon inflation and stent implantation through a percutaneous procedure performed under local anesthesia. Coronary angiography was, for many years, the only available tool to diagnose and assess the consequences of coronary atherosclerosis, becoming the standard reference in the study of ischemic heart disease, allowing us to routinely describe the severity of coronary stenosis or the severity of the disease based on the number of vessels affected. Subsequently, the introduction of invasive methods to assess coronary physiology allowed us to obtain a precise assessment regarding the physiological impact of stenoses located in main vessels while demonstrating the poor diagnostic performance of angiography to assess the functional impact of these stenoses. Among these coronary physiology techniques, quantitative flow ratio has recently been the object of many investigations. In contrast to invasive techniques that can be performed only by advancing a wire into the coronary artery, quantitative flow ratio allows us to perform a functional evaluation of coronary stenoses on the basis of angiographic data only. Given its potential, this work will summarize the basics of quantitative flow ratio, the main validation studies and its current and future applications in interventional cardiology.