Background Despite the success of emergency PCI for acute STEMI, failed myocardial reperfusion (microvascular obstruction) is common and prognostically important, but it commonly passes undetected in clinical practice.

Purpose To investigate coronary flow reserve (CFR) and the index of microvascular resistance (IMR) in the culprit coronary artery in patients with acute reperfused STEMI.

Methods CFR and IMR were measured at the end of PCI using guidewire-based thermodilution (n = 283; mean age 60 (12) years, 73% male). CMR imaging was used to assess left ventricular (LV) function and infarct pathology 2 days and 6 months post-MI (n = 264). All-cause death or first heart failure hospitalisation (ACD) was a pre-specified outcome that was assessed during follow-up (median duration 845 days).

Results The median [IQR] CFR was 1.6 [1.1–2.1] and the median IMR was 25 [15–48]. Microcirculatory status was associated with Killip Heart Failure classification, ST-segment resolution, LV ejection fraction and infarct size and pathology revealed by CMR 2 days later. In multivariate analysis, the combination of a CFR <2.0 and an IMR >25 was associated with changes in both LV ejection fraction (-2.47 (95% CI: -4.45, -0.49); p = 0.015) and LV end-diastolic volume (8.49 (1.96, 15.92); p = 0.025), and a CFR < median and an IMR > median was a multivariable (2.96 (1.24, 7.08); p = 0.015) associate of ACD.

Conclusions The combination of a reduced CFR and an increased IMR immediately identifies patients who have a substantially increased risk of adverse LV outcomes and all-cause death or heart failure.