The aim of this study is to analyze whether the sympathetic tone, measured indirectly and directly by muscle sympathetic nerve activity recording, is elevated in patients with heart failure receiving an appropriate shock from their implantable cardiac defibrillator (ICD) compared to heart failure patients not receiving a shock from their ICD. All parameters measured in this study will be used to build a risk algorithm able to identify heart failure patients at high risk for sudden cardiac death who could receive an ICD.