Heart Failure (HF) is one of the most frequent diagnoses upon hospitalization and is a major cause of death. Cardiac resynchronization therapy (CRT) using an implantable device for synchronous bi-ventricular stimulation leads to improvement of hemodynamic parameters, physical capacity and quality of life. A continuous measurement of the left-ventricular volume would provide information about heart performance and could be used for both heart failure monitoring and optimization of CRT-therapy.