Coronary physiology: From basic concepts to FFR and IFR

Coronary angiography (CAG) complemented with invasive fractional flow reserve (FFR) assessment in assessing stenosis severity improves clinical outcomes compared to CAG alone in intermediate stenoses in stable coronary artery disease (CAD) [Tonino et al. N Engl J Med. 360(3):213-24, 2009; van Nunen et al. Lancet (London, England). 386(10006):1853-60, 2015; Xaplanteris et al. N Engl J Med. 379(3):250-9, 2018]. The advance of invasive coronary physiology measurements aiding clinical decision-making in the catheterization laboratory is ongoing but focuses merely on the consequences of epicardial narrowing on flow impairment rather than the entire coronary circulation, including the coronary microcirculation. Coronary flow reserve (CFR) is a flow-derived parameter which interrogates the entire coronary circulation and has strong prognostic value for clinical outcomes in stable CAD [Lee et al., JACC Cardiovasc Interv. 2018;11(15):1423-33, 2018; van de Hoef et al. Circ Cardiovasc Interv. 7(3):301-11, 2014]. Combined FFR and CFR assessment improves diagnosis and prognosis in ischemic heart disease, but FFR and CFR disagree in 30-40% of all patients due to the impact of microvascular resistance on both indices. It has been recognized that both epicardial narrowing and microcirculatory disease impact on IHD, but clinical implementation is hampered due to required operator's experience. The relatively novel pressure-derived index instantaneous wave-free ratio (iFR), a feasible resting vasodilator-free index, recently has been introduced and led to renewed interest in coronary physiology measurements. This chapter focuses on elaborating the coronary physiological parameters and potential of implementing these parameters in daily clinical practice.