Absolute coronary blood flow measurement and the principle of microvascular resistance reserve.

Measurement of absolute coronary blood flow and myocardial resistance by continuous intracoronary thermodilution was recently introduced and facilitates assessment of the coronary microcirculation. In addition, Microvascular Resistance Reserve (MRR) is a new index that further permits the characterization of coronary microcirculatory function. It can be calculated from resting and hyperemic absolute coronary flow and resistance, measured during continuous thermodilution. By being independent of both epicardial disease and myocardial mass, MRR is highly specific for the microvascular compartment. Measurement of absolute flow, resistance, and MRR by continuous thermodilution has been shown to be accurate and reproducible, with minimal operator interference. In conjunction with the measurement of absolute flow and resistance, MRR could become the index of reference for the diagnosis of coronary microvascular disease and the evaluation of new therapies. In this paper, the basis and practicalities of these measurements are shortly reviewed and its characteristics, its advantages over existing indices, its limitations, and future perspectives are discussed. Word count abstract: 158. Word count text: 7954 (including references and legends).