Patients who need temporary pacing following open heart surgery and who are willing to participate receive a phrenic nerve electrode during the operation. Heart function is postoperatively assessed by acoustic cardiography (Audicor, Inovise Medical Inc., Portland, USA) during different forms of pacing (right ventricle, left ventricle, biventricular; AAI, DDD) and with or without phrenic nerve stimulation. Acoustic cardiography simultaneously integrates heart sounds and single-channel electrocardiography input to generate multiple parameters that correlate to established hemodynamic measures. Effects of phrenic nerve stimulation on heart function will be analyzed.