Microvascular Resistance Predicts Outcome After PCI in STEMI

It is now well known that even if a coronary artery is opened well after a myocardial infarction, with good flow in the epicardial coronary arteries, there could be impaired myocardial perfusion. That is due to impaired flow through the coronary microcirculation and is associated with higher mortality as well as chance for left ventricular dysunction and heart failure. In the initial era of primary PCI, angiographic myocardial blush was taken as a surrogate of effective myocardial reperfusion. It was shown that even with TIMI 3 flow after PCI, 50-60% had impaired tissue perfusion in the infarct zone. TIMI myocardial perfusion grade was noted as an important correlate of mortality and poor recovery of left ventricular function [1].

Later it was shown that index of microvascular resistance which quantified the inability of microcirculation to vasodilate and improve coronary flow after primary PCI, was associated with worst outcomes independent of epicardial coronary flow. Microvascular resistance evaluated whether the vasodilatory reserve capacity of coronary microcirculation was restored in the infarcted territory, regardless of concomitant epicardial coronary artery disease and aortic pressure. An article published in the Journal of American College of Cardiology used pooled individual patient data from 6 cohorts that measured the index of microvascular resistance directly after primary PCI in 1265 patients with ST elevation myocardial infaction. A subgroup analysis was performed in patients in whom both microvascular resistance reserve and index of microcirculatory resistance was available. Both microvascular resistance reserve and index of microcirculatory resistance were shown to be independent predictors for the composite of all-cause mortality or hospitalization for heart failure [2].

Microvascular function and dysfunction can mentioned in terms of minimal microvascular resistance and vasodilatory capacity. Minimal microvascular resistance is studied invasively using the index of microcirculatory resistance. It can be measured directly with continous thermodilution technique which has good reproducibility. Vasodilatory capacity was earlier measured using coronary flow reserve, but it can be influenced by disease of epicardial coronary arteries. Microvascular resistance reserve is a novel index to assess vasodilatory capacity of microcirculation, which is not influenced by epicardial coronary artery disease.

Index of microvascular resistance is the mean distal pressure during maximal hyperemia multiplied by mean hyperemic transit time. Transit time is measured by saline bolus injection using a thermodilution technique. Hyperemia can be induced by either intravenous adenosine infusion or intracoronary papaverine injection. Coronary flow reserve can be calculated by dividing the mean resting transit time by the mean hyperemic transit time. Fractional flow reserve is calculated as the mean distal coronary pressure divided by the mean aortic pressure during maximal hyperemia. Microvascular resistance reserve is calculated using a formula, from the ratio of coronary flow reserve to fractional flow reserve with correction for the impact of changes in aortic pressure from resting to hyperemic conditions.

MRR = (CFR/FFR) × (P_a,rest/P_a,hyper)

References

1. Michel Zeitouni, Ghilas Rahoual, Gilles Montalescot, and the ACTION study group. Immediate Microvascular Physiology After Mechanical Coronary Reperfusion of STEMI. J Am Coll Cardiol. 2024 May, 83 (21) 2077–2079.
2. Rob Eerdekens, Mohamed El Farissi, Giovanni Luigi De Maria, Niels van Royen, Marcel van ‘t Veer, Maarten A.H. van Leeuwen, Stephen P. Hoole, Federico Marin, David Carrick, Pim A.L. Tonino, Nico H.J. Pijls, Massimo Fineschi, Keith G. Oldroyd, Colin Berry, Adrian P. Banning, William F. Fearon, and Frederik M. Zimmermann. Prognostic Value of Microvascular Resistance Reserve After Percutaneous Coronary Intervention in Patients With Myocardial Infarction. J Am Coll Cardiol. 2024 May, 83 (21) 2066–2076.