The baseline instantaneous wave-free ratio as an index of coronary disease severity: relationship with fractional flow reserve and coronary flow reserve

Abstract

Over the last 30 years the development of invasive methods to directly measure the haemodynamic impact of individual coronary stenoses on blood flow has enabled the identification of vessel-specific and lesion-specific ischaemia. Fractional flow reserve (FFR) is the most commonly used technique, largely due to the simplification brought by its pressure-only methodology. Despite the evidence accumulated demonstrating the benefits of FFR-guided decisions, its adoption remains low worldwide (6-8%) and a large proportion of patients with coronary artery disease (CAD) still undergo percutaneous interventions without any objective evidence of myocardial ischaemia. This is partly due to FFR’s reliance on the induction of coronary hyperaemia, a methodological step which adds time, cost and inconvenience for patients and clinicians. Recently, our group presented a novel invasive pressure-only methodology, the instantaneous wave-free ratio (iFR), which differs from FFR as it can be calculated at baseline, without the need for vasodilator administration. In its initial validation studies, iFR demonstrated a close diagnostic agreement with FFR and with invasive coronary flow. In this thesis, I will present a series of studies which aim to further evaluate the utility of iFR as an index coronary stenosis severity. Firstly, I will explore its diagnostic relationship with FFR in details and present a novel methodology to measure classification agreement between methods of clinical measurement. Secondly, I will evaluate the merits of utilising iFR and FFR in a common diagnostic pathway and quantify the potential benefits of such a strategy to spare patients from the need for vasodilator administration. Finally, I will investigate the relationship between pressure-only indices (iFR and FFR) and coronary flow reserve, an extensively validated marker of prognosis in coronary disease.