The PROTECT-ICD trial is a physician-led, multi-centre randomised controlled trial targeting prevention of sudden cardiac death in patients who have poor cardiac function following a myocardial infarct (MI). The trial aims to assess the role of electrophysiology study (EPS) in guiding implantable cardioverter-defibrillator (ICD) implantation, in patients early following MI (first 40 days). The secondary aim is to assess the utility of cardiac MRI (CMR) in analysing cardiac function and viability as well as predicting inducible and spontaneous ventricular tachyarrhythmia when performed early post MI. Following a MI patients are at high risk of sudden cardiac death (SCD). The risk is highest in the first 40 days; however, current guidelines exclude patients from receiving an ICD during this time. This limitation is based largely on a single study, The Defibrillator in Acute Myocardial Infarction Trial (DINAMIT), which failed to demonstrate a benefit of early ICD implantation. However, this study was underpowered and used non-invasive tests to identify patients at high risk. EPS identifies patients with the substrate for re-entrant tachyarrhythmia, and has been found in multiple studies to predict patients at risk of SCD. Contrast-enhanced CMR is a non-invasive test without radiation exposure which can be used to assess left ventricular function. In addition, it provides information on myocardial viability, scar size and tissue heterogeneity. It has an emerging role as a predictor of mortality and spontaneous ventricular arrhythmia in patients with a previous MI. A total of 1,058 patients who are at high risk of SCD based on poor cardiac function (left ventricular ejection fraction (LVEF) ≤40%) following a ST-elevation or non-STE myocardial infarct will be enrolled in the trial. Patients will be randomised 1:1 to either the intervention or control arm. In the intervention arm all patients undergo early EPS. Patients with a positive study (inducible ventricular tachycardia cycle length ≥200ms) receive an ICD, while patients with a negative study (inducible ventricular fibrillation or no inducible VT) are discharged without an ICD, regardless of the LVEF. In the control arm patients are treated according to standard local practice. This involves early discharge and repeat assessment of cardiac function after 40 days or after 90 days following revascularisation (PCI or CABG). ICD implantation after 40 days according to current guidelines (LVEF≤30%, or ≤35% with New York Heart Association (NYHA) class II/III symptoms) could be considered, if part of local standard practice, however the ICD is not funded by the trial. A proportion of trial patients from both the intervention and control arms at \>48 hours following MI will undergo CMR to enable correlation with (1) inducible VT at EPS and (2) SCD and non-fatal arrhythmia on follow up. It will be used to simultaneously assess left ventricular function, ventricular strain, myocardial infarction size, and peri-infarction injury. The size of the infarct core, infarct gray zone (as a measure of tissue heterogeneity) and total infarct size will be quantified for each patient. All patients will be followed for 2 years with a combined primary endpoint of non-fatal arrhythmia and SCD. Non-fatal arrhythmia includes resuscitated cardiac arrest, sustained ventricular tachycardia (VT) and ventricular fibrillation (VF) in participants without an ICD. Secondary endpoints will include all-cause mortality, non-sudden cardiovascular death, non-fatal repeat MI, heart failure and inappropriate ICD denial. Secondary endpoints for CMR correlation will include (1) the presence or absence of inducible VT at EP study, and (2) combined endpoint of appropriate ICD activation or SCD at follow up. It is anticipated that the intervention arm will reduce the primary endpoint as a result of prevention of a) early sudden cardiac deaths/cardiac arrest, and b) sudden cardiac death/cardiac arrest in patients with a LVEF of 31-40%. It is expected that the 2-year primary endpoint rate will be reduced from 6.7% in the control arm to 2.8% in the intervention arm with a relative risk reduction (RRR) of 68%. A two-group chi-squared test with a 0.05 two-sided significance level will have 80% power to detect the difference between a Group 1 proportion of 0.028 experiencing the primary endpoint and a Group 2 proportion of 0.067 experiencing the primary endpoint when the sample size in each group is 470. Assuming 1% crossover and 10% loss to follow up the required sample size is 1,058 (n=529 patients per arm). To test the hypothesis that tissue heterogeneity at CMR predicts both inducible and spontaneous ventricular tachyarrhythmias will require a sample size of 400 patients to undergo CMR. It is anticipated that the use of EPS will select a group of patients who will benefit from an ICD soon after a MI. This has the potential to change clinical guidelines and save a large number of lives.