Figure 1  .

Schematic illustration of the coronary artery and its dependent myocardial vascular bed. Myocardial blood flow equals the perfusion pressure across the myocardium, divided by myocardial resistance. Because at maximum arteriolar vasodilatation resistance is minimal and constant (R_min), maximum flow in the stenotic situation as a ratio to normal maximum flow equals the ratio of the myocardial perfusion pressure in the presence of the stenosis (P_d − P_v) to normal myocardial perfusion pressure (P_a − P_v), both measured after administration of a maximum hyperaemic stimulus. In other words, FFR equals (P_d − P_v)/(P_a − P_v ), which is generally very close to P_d/P_a . In this example, FFR equals 0.70. AO, aorta; P_a, P_d, and P_v, mean aortic, distal coronary, and central venous pressure measured at maximum coronary hyperaemia; Q_{max , normal}, maximum achievable myocardial flow if the coronary artery were normal; Q_{max, stenosis}, maximum achievable myocardial flow in the presence of a stenosis; R_min indicates minimal resistance of the myocardial vascular bed.