 Baseline ECG markers of electrical instability will be measured in patients with cardiomyopathy. These ECG markers will be evaluated from a clinical Holter recording. All patients will have a primary implantable cardioverter defibrillator ICD as part of their clinical care. Patients will be followed prospectively for 2 years to evaluate the primary outcome of appropriate ICD shocks. Multivariable modelling will be performed to determine whether the ECG markers of electrical instability independently predict the primary clinical outcome. 