Normal Index of Microcirculatory Resistance (IMR) Measurement
An IMR value less than 25 units is considered normal, with values of 25 units or greater indicating microvascular dysfunction. 1
Diagnostic Threshold
- IMR <25 units = Normal microcirculatory function 1, 2
- IMR ≥25 units = Abnormal, indicating structural microvascular disease 1, 2, 3
The 2019 ESC Guidelines for Chronic Coronary Syndromes explicitly state that IMR ≥25 units is the threshold for diagnosing microvascular dysfunction and guiding treatment decisions. 1
Measurement Technique
IMR is calculated during maximal hyperemia (induced by intravenous adenosine) using the formula: IMR = Pd × Tmn, where:
- Pd = distal coronary pressure measured by pressure wire 1
- Tmn = mean transit time of a 3 mL bolus of room temperature saline measured by thermodilution 1
Clinical Context and Nuances
IMR is independent of hemodynamic variations because it is measured during hyperemia, making it more reproducible than coronary flow reserve (CFR). 1 This is a critical advantage over CFR, which varies with blood pressure, heart rate, and other hemodynamic factors.
Alternative Cut-off Values in Research
While the guideline-endorsed threshold is 25 units, some research studies have used slightly different cut-offs:
- One study in hypertrophic cardiomyopathy used IMR ≤22.0 as normal 4
- An angiography-derived IMR study found 25.1 as the optimal cut-off for predicting adverse outcomes 5
However, for clinical practice, use IMR <25 units as the normal threshold as recommended by the European Society of Cardiology guidelines. 1, 2
Comparison with Coronary Flow Reserve
Unlike CFR (which has a normal value ≥2.0-2.5), IMR specifically measures structural microvascular resistance rather than functional reserve capacity. 1, 2 A patient can have:
- Normal IMR (<25) but abnormal CFR (<2.0): suggests functional microvascular dysfunction or diffuse epicardial disease 1
- Abnormal IMR (≥25) with abnormal CFR (<2.0): indicates structural microvascular disease 1
Clinical Application
When IMR ≥25 units is documented with abnormal CFR <2.0 and negative acetylcholine provocation testing, initiate treatment with beta-blockers, ACE inhibitors, statins, and lifestyle modifications. 1 This represents guideline-directed therapy for structural microvascular disease.
Target IMR reduction to <25 units as an objective treatment endpoint when optimizing medical therapy for microvascular dysfunction. 3
Important Caveats
- IMR requires a pressure-temperature sensor guidewire and hyperemic agents, which limits routine use 6, 7
- IMR is measured in specific coronary territories, so values may differ between vessels in the same patient 7
- Angiography-derived IMR (wire-free methods) show 83% accuracy compared to invasive IMR, with a similar threshold of approximately 25 units 5, 6