The aim of the study is to determine whether increasing the heart rate by epicardial atrial pacing after surgery improves the contractile force of the myocardium. After complete weaning from CPB, several pressure-loops of the LV will be recorded at different levels of preload using transoesophageal echo, invasive blood pressure monitoring. The preload will be manipulated by adding fluid from the CPB reservoir or draining fluid into the CPB reservoir.