href="#fb3_img_img_1b4f3f89-98e3-5906-8cf2-28fbd993c30f.jpg" alt="Schematic illustration of IMR calculation."/>
Figure 7.10 IMR calculation. A combined pressure/temperature guide wire is used to obtain mean Pd and mean distal coronary blood flow (based on the principles of thermodilution in response to a 3 ml hand‐ held intra‐coronary injection of room temperature saline). IMR is derived from the ratio of mean Pd (green circle) and mean distal coronary flow at maximal hyperemia. Distal coronary flow is inversely proportional to the mean transit time (Tmn) of the injectate. Therefore IMR = Pd : 1/Tmn = Pd × Tmn.
IMR correlates significantly with true microcirculatory resistance measured in an open‐chested pig model. In the presence of an epicardial stenosis, coronary wedge pressure should be included in the calculation (IMR = Pa x hyperemic mean transit time x [(Pd‐Pw)/(Pa‐Pw)]; this is particularly important in significant stenoses with an FFR≤0.60 where the likelihood of collateral flow is high. Mathematical derivation of Pw is also possible based upon the statistical relationship between FFRcorr and FFRmyo; IMR can therefore be calculated as IMRcalc = Pa × TmnHyp × ([1.35 × Pd / Pa‐0.32) [99]. This calculation is important when coronary arteries have stenoses that will alter the distal microcirculation. However, IMR is increasingly used in patients with unobstructed vessels to determine microvascular angina.
In general, IMR values below 25 are considered normal and were consistently found in validation cohorts of healthy populations with normal valves. Values over 25 reflect elevated microvascular resistance. After STEMI, patients with high IMR values (>40) are more likely to have raised cardiac enzymes, less cardiac recovery on non‐invasive imaging and evidence of microvascular obstruction on magnetic resonance imaging. In more stable patients, values over 25 suggest microvascular dysfunction and are associated with higher rates of cardiovascular presentation and suggest a microvascular cause of patient’s angina. IMR measurement can be combined with other vasoreactivity testing for a comprehensive testing in the cardiac catheter laboratory.
Interactive multiple choice questions are available for this chapter on www.wiley.com/go/dangas/cardiology
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