Influence of the amount of myocardium subtended to a coronary stenosis on the index of microcirculatory resistance. Implications for the invasive assessment of microcirculatory function in ischaemic heart disease.

__Aims:__ The index of microcirculatory resistance (IMR) is increasingly used to quantify microcirculatory function. However, in normal coronary arteries, resistance increases with the branching structure of the coronary tree, which suggests that IMR could be influenced by the amount of downstream myocardial mass (MM). We aimed to evaluate the influence of the amount of MM subtended to an intermediate stenosis on the IMR. __Methods and results:__ IMR, fractional flow reserve and coronary flow reserve (CFR) were measured in 123 coronary arteries with intermediate stenosis. Jeopardised MM was estimated with the Myocardial Jeopardy Index (MJI). MM was inversely associated with IMR. Differently, CFR was MM-independent. Vessels with IMR ≥30 U subtended lower amounts of MM than vessels with IMR <30 and, at multivariate analyses, MM, aortic pressure, minimum lumen diameter and age were independent IMR predictors. Vessels with IMR ≥30 U and preserved CFR supplied the smallest MM amounts, suggesting an anatomically reduced but functionally preserved vascular bed. __Conclusions:__ The amount of myocardium subtending to a coronary stenosis is inversely associated with the IMR, while it is not associated with the CFR.