It is known that, at the end of the cardiac ejection period, potential energy is stored in elastic fibers of the heart, which promotes the suction of blood from the atria during early filling. The investigators have developed a new ultrasound-based method to quantify this suction effect. Here, it is necessary to reduce the complex 3-dimensional cardiac mechanics in a 1-dimensional (piston-like) pump system. In the study, several steps of model reduction will be tested. Each reduction is intended to allow non-invasive measurements to become increasingly simple and feasible at reduced echo quality. The reference method is the invasive data obtained from a pressure-volume conductance catheter. To increase the supply of potential energy in the elastic fibers, a substance (dobutamine) is administered for transient strengthening of the force of contraction.