Incremental value of coronary microcirculation resistive reserve ratio in predicting the extent of myocardial infarction in patients with STEMI. Insights from the Oxford Acute Myocardial Infarction (OxAMI) study.

•Coronary physiology may have an important role in discriminating STEMI patients with sub-optimal results after PPCI.•The best physiological index to predict the extent of myocardial injury after STEMI is still undetermined.•Resistive reserve ratio (RRR) is a novel index that expresses the microcirculation vasodilatory capacity.•Patients with impaired RRR showed lower myocardial reperfusion after PPCI, more MVO and larger infarct size.•RRR could be used in future trials to identify cases of suboptimal reperfusion, candidates to further therapeutic options.