Fig. 1. Concept of IMR. (a) Theoretical basis for the measurement of IMR. Reprinted from Lee et al. (2020), Copyright 2020, with permission from the authors. (b) Calculation method based on thermodilution curves for IMR values. Reprinted from Lee et al. (2018), Copyright 2018, with permission from the Korean Society of Cardiology and Measurement of IMR in the Clinic. (c) Clinical measurement of IMR. Reprinted from Li Y et al. (2020), Copyright 2020, with permission from the authors. In a state of maximal hyperemia, minimum microcirculatory resistance will be achieved. In clinical measurements, a pressure guide wire was used to obtain reproducible and consistent thermodilution curves at maximal hyperemia. The IMR is calculated by multiplying ${\mathit{P}}_{\mathbf{d}}$ by ${\mathit{T}}_{\mathbf{m}\mathbf{n}}$ . IMR: index of microcirculatory resistance; P _a: aortic pressure; ${\mathit{P}}_{\mathbf{d}}$ : distal coronary pressure; ${\mathit{T}}_{\mathbf{m}\mathbf{n}}$ : mean transit time; hyp: hyperemia; rest: resting; FFR: fractional flow reserve; CFR: coronary flow reserve.