Microvascular Resistance Reserve (MRR): a new concept to understand the coronary microcirculation

In recent years, there have been numerous developments in the field of coronary physiology with the addition of new indices for the evaluation of the coronary microcirculation, such as invasive measurement of absolute coronary blood flow and microvascular resistance. This has resulted in more accurate diagnostic tools for patients with angina and no obstructive coronary artery disease (ANOCA), with the possibility to distinguish different endotypes of coronary microvascular dysfunction (CMD). Because of the growing recognition of ANOCA and, along with that, the increasing application of coronary function testing due to the latest guideline recommendations, it is timely and important to further explain the concept of absolute coronary flow, microvascular resistance, and the latest addition to the field: Microvascular Resistance Reserve (MRR). Differentiation between specific endotypes of CMD will help to develop tailored therapy for ANOCA patients.