 The surgery is then terminated as planned and postoperative care is performed according to the standard operation procedure SOP of the Medical Clinic I. Within the framework of a control visit routinely performed in the Medical Clinic I , an echocardiographic follow-up of heart failure, a history assessment and a laboratory-based blood analysis are performed 6 months after CRT implantation. In the context of this visit peripheral venous blood is collected again for the study. In addition, the 6-minute walk test is performed once more and the patient receives the same questionnaire again. 