Table 2.

Summary of Coronary Flow and Microcirculatory Resistance Indices and Formulas

Flow Index	Formula	Explanation
Index of microcirculatory resistance (IMR)	$P_{dHyp}\times {Tmn}_{Hyp}$	Hyperemic microcirculatory resistance
Corrected index of microcirculatory resistance (IMRC)	$P_{aHyp}\times {Tmn}_{Hyp}\times \left(1.35\times \frac{P_{dHyp}}{P_{aHyp}}-0.32\right)$	Hyperemic microcirculatory resistance corrected for collateral flow
Baseline resistance index (BRI)	$P_{dRest}\times {Tmn}_{Rest}$	Resting microcirculatory resistance
Resting flow velocity (RFV)	$\frac{1}{{Tmn}_{Rest}}$	Estimated resting flow velocity.
Hyperemic flow velocity (HFV)	$\frac{1}{{Tmn}_{Hyp}}$	Estimated hyperemic flow velocity
Coronary flow reserve (CFR)	$\frac{{Tmn}_{Rest}}{{Tmn}_{Hyp}}$	Hyperemic coronary flow velocity in relation to resting flow velocity
Microvascular resistance reserve (MRR)	$\frac{CFR}{FFR}\times \frac{P_{aRest}}{P_{aHyp}}$	Resting microcirculatory resistance in relation to hyperemic microcirculatory resistance, adjusted for the presence of epicardial lesions
Fractional flow reserve (FFR)	$\frac{P_{dHyp}}{P_{aHyp}}$	Distal coronary pressure in relation to aortic pressure during hyperemia

P_aHyp = aortic pressure during hyperemia; P_aRest = aortic pressure during rest; P_dHyp = distal coronary pressure during hyperemia; P_dRest = distal coronary pressure during rest; Tmn_Hyp = mean transit time of a saline bolus during hyperemia; Tmn_Rest = mean transit time of a saline bolus during rest.