The primary aim of the current study is a better understanding of the role of Type 2 Diabetes Mellitus (T2DM) in heart failure and, in particular, changes in cardiac metabolism, which may contribute to heart failure. Various biomarkers in the coronary artery blood, as well as in the arterial and peripheral venous blood, are to be identified for this purpose. Included are patients with and without T2DM and with or without heart failure (HFpEF, HFmrEF, HFrEF), who have a clinically indicated and guidance-appropriate Cardiac Resynchronisation Therapy (CRT) implantation or pulmonary vein ablation/electrophysiological examination. Not all patients currently benefit from the implantation of a CRT system (so-called non-responder). Despite narrow inclusion criteria, these non-responders cannot be unmasked in advance of the implantation. A further aim of this study is to identify biomarkers, which can be determined in advance of implantation to differentiate between responders and non-responders.