 This study will carefully examine electrical activity, using APD restitution, in the hearts of patients who are having ICDs fitted because of their high risk of sudden cardiac death and combine this with a detailed heart scan, assessment of autonomic nervous system and gene expression data. We will then follow these patients up to see who benefits from their ICD. This wide ranging information will give us as complete a picture as possible of the factors that cause sudden cardiac death. We hope to use this to identify better predictors of sudden cardiac death. 