The functional severity of atherosclerotic coronary lesions is the single most important prognostic factor in patients with documented coronary artery disease (CAD). Assessment of the haemodynamic significance of coronary artery lesions by invasive fractional flow reserve (FFR) measurement now has an I-A indication by the European Society of Cardiology (ESC) to identify haemodynamically relevant coronary lesions when evidence of ischaemia is not available.1 FFR represents the extent to which maximal myocardial blood flow is limited by the presence of a coronary stenosis and in clinical practice FFR is defined as the ratio of distal coronary to aortic pressure at maximal vasodilation.2 FFR provides a physiologic adjunct to invasive coronary angiography, challenging the notion of coronary revascularisation need on the basis of anatomic coronary stenosis alone.3 In the Fractional Flow Reserve Versus Angiography for Multivessel Evaluation (FAME) study, of 1005 patients with multivessel CAD, those who underwent FFR-guided revascularisation experienced lower rates of adverse events with fewer coronary stents and lower healthcare costs, than patients undergoing angiogram-guided revascularisation.4,5
The results from FAME are in accordance with the five-year follow-up of individuals from the Deferral Versus Performance of PTCA in Patients Without Documented Ischemia (DEFER)6 study which demonstrated that amongst lesions judged angiographically “obstructive,” >50 % were haemodynamically insignificant by FFR and no benefit was observed by revascularisation. In patients with stable CAD and functionally significant stenoses, FFR-guided percutaneous coronary intervention (PCI) in combination with medical therapy, as compared with medical therapy alone, decreased the need for urgent revascularisation.7 Despite these benefits, less than 10 % of PCI procedures in the UK use adjunctive intracoronary measurements, and even fewer diagnostic cases employ FFR to guide management. This is due to the various drawbacks associated with the measurement of FFR, such as the requirement of invasive cardiac catheterisation, an expensive coronary pressure wire and intracoronary or intravenous adenosine infusion which is associated with adverse effects such as AV block, bronchial hyper-reactivity, and chest pain.8,9 A clinical implication is the inadvertent revascularisation of patients with stable CAD and “innocent” lesions, who clearly do not benefit from intervention.10 Thus, a tool that could accurately and rapidly calculate FFR without the need of a pressure wire would make this physiologic index become available to a wider population. Recent advances in coronary imaging and computational fluid dynamics (CFD) enable calculation of coronary flow and pressure fields from anatomic image data.11 Novel techniques of FFR calculation have been developed based on coronary image analysis and CFD techniques which aim to provide an alternate to interventional FFR measurement by abolishing some or most of its limitations.