Patients with heart attack or heart injury are tested (angiogram) for blockages in their arteries. Many patients develop heart problems caused by damage to small (microvascular) blood vessels. These issues are also relevant to patients with coronarvirus-19 disease (COVID-19). Eplerenone reduces blood vessel injury and is used to treat heart failure. Aim: to test the use of eplerenone in patients with heart attack/heart injury who have small vessel disease, including patients with COVID-19 Patients referred to the Golden Jubilee hospital with a suspected heart attack heart / injury will be invited to participate into a registry-based clinical trial. Screening, enrolment and verbal, informed consent will be obtained during the angiogram then written consent on the ward. Small vessel disease will be assessed using a 'diagnostic' guidewire during the standard angiogram. People with small vessel problems will be allocated to a clinical trial of usual care or eplerenone. Coronary microvascular dysfunction is defined as an index of microvascular resistance ≥25. Coronary flow reserve (CFR abnormal \<2.0) and resistance reserve ratio (RRR abnormal \<2.0), measured simultaneously with IMR, are predefined parameters of interest. Patients will be allocated into one of the 3 groups: * Group 1: Patients without coronary microvascular dysfunction. No eplerenone * Group 2: Patient with coronary microvascular dysfunction. Usual care, no eplerenone. * Group 3: Small vessels abnormal. Eplerenone tablets. The primary outcome for the trial will be reduced heart injury (biomarkers) in patients with microvascular disease. We will also test heart function (MRI scan) at enrolment and at six months. All patients (Groups 1, 2 and 3) will have an angiogram. Standard blood tests will be collected during the hospital stay, and then again at 1 and 6 months. Other outcomes include questionnaires (health status). We will gather information on longer-term health outcomes (hospitalisation, death) using confidential electronic record linkage. We will ask for permission to store blood samples for future research. The research will improve scientific knowledge about eplerenone therapy in this patient group. The study will create a repository of clinical samples and images which will provide vital data for studies of COVID-19.