Coronary physiology thresholds associated with microvascular obstruction in myocardial infarction

Objectives: To ascertain whether invasive assessment of coronary physiology soon after recanalisation of the culprit artery by primary percutaneous coronary intervention is associated with the development of microvascular obstruction by cardiac magnetic resonance in patients with ST-segment elevation myocardial infarction (STEMI). Methods: Between November 2020 and December 2021, 102 consecutive patients were prospectively enrolled in five tertiary centres in Italy. Coronary flow reserve (CFR) and index of microvascular resistance (IMR) were measured in the culprit vessel soon after successful primary percutaneous coronary intervention. Optimal cut-off points of IMR and CFR to predict the presence of microvascular obstruction were estimated, stratifying the population accordingly in four groups. A comparison with previously proposed stratification models was carried out. Results: IMR > 31 units and CFR <= 1.25 yielded the best accuracy. Patients with IMR > 31 and CFR <= 1.25 exhibited higher microvascular obstruction prevalence (83% vs 38%, p < 0.001) and lower left ventricular ejection fraction (45 +/- 9% vs 52 +/- 9%, p=0.043) compared with those with IMR <= 31 and CFR > 1.25, and lower left ventricular ejection fraction compared with patients with CFR <= 1.25 and IMR <= 31 (45 +/- 9% vs 54 +/- 7%, p=0.025). Infarct size and area at risk were larger in the former, compared with other groups. Conclusions: IMR and CFR are associated with the presence of microvascular obstruction in STEMI. Patients with an IMR>31 units and a CFR <= 1.25 have higher prevalence of microvascular obstruction, lower left ventricular ejection fraction, larger infarct size and area at risk.