Fig. 2. Concept of fractional flow reserve.

Fractional flow reserve (FFR) is defined as ratio of maximal coronary blood flow in diseased artery (${\text{Q}}_{\text{s}}^{\text{max}}$) to normal maximal coronary blood flow in same artery (${\text{Q}}_{\text{n}}^{\text{max}}$). As venous pressure (Pv) is negligible compared to aortic (Pa) and distal coronary pressure (Pd), FFR can be calculated as ratio of Pd and Pa.