We plan to perform an extensive study of the sympathetic nervous system activity in heart failure patients with an ICD. The SNS activity will be measured directly by a recording of the muscle sympathetic nerve activity and indirectly by post processing of 24-hour Holter recording, catecholamine levels assessment, and spontaneous baroreflex sensitivity. A global assessment of the patients status will also be realised. Patients will be followed for one year. During follow up, this cohort will be divided into two groups according to the occurrence or not of an appropriate shock. We assume that an appropriate shock is a surrogate marker for sudden cardiac death. We plan to compare the autonomic nervous system activity in both groups of patients in order to identify a score able to detect heart failure patients at high risk for sudden cardiac death.