Unveiling the Microvasculature: The Index of Microcirculatory Resistance and Its Expanding Role in Cardiovascular Care

Index of microcirculatory resistance (IMR) is a cutting-edge, wire-based tool that advances the capability assessment of coronary microvascular function. By utilizing distal coronary pressure and mean transit time under maximal hyperemia, IMR delivers consistent, reproducible insights into the microvasculature's dynamic health. IMR has been shown to be of particular use as a diagnostic tool in the assessment of coronary microvascular dysfunction (CMD) allowing for diagnostic clarification in conditions like ischemia with non-obstructive coronary arteries (INOCA), acute coronary syndrome (ACS), and post-percutaneous coronary intervention (PCI) complications. In addition to its ability to be used as a powerful diagnostic tool, it also serves as a prognostic tool, helping to predict which patients are at elevated risk of major adverse cardiovascular events (MACE) and mortality. With the increasing integration of artificial intelligence into the cath lab, previous sources of error in IMR, such as technical variability and hemodynamic influences, are no longer as significant of an issue and allow for greater precision in the use of this tool. This comprehensive review discusses the physiologic basis of IMR, measurement techniques, and diagnostic and prognostic capabilities of IMR, while highlighting its therapeutic potential and future applications.