The use of coronary physiology to guide myocardial revascularisation was shown to improve clinical outcomes and reduce costs in patients with coronary artery disease. Fractional flow reserve (FFR) is the most commonly used pressure-derived physiological index for coronary lesion assessment, being supported by a body of compelling randomised evidence, but its uptake into clinical practice remains unacceptably low.

The instantaneous wave-free ratio (iFR) is a novel adenosine-free pressure-derived index of coronary stenosis severity that was recently introduced to circumvent the limitations of existing hyperaemic pressure-derived indices with the aim of increasing the widespread adoption of coronary physiology assessment. Following the recent publication of two large-scale, randomised, patient outcome clinical trials, iFR has emerged as a simpler, safe and effective alternative to FFR to guide coronary revascularisation. This has led to renewed interest in the field of coronary physiology and challenges current paradigms supporting the need for pharmacologically-induced maximal hyperaemia as an essential requirement for coronary stenosis assessment.

This review aims at addressing the physiological concepts and patient outcome evidence supporting the use of iFR and discuss the recent development of novel iFR-based applications allowing full integration of invasive coronary physiology in percutaneous coronary intervention planning strategy.