Sinoatrial node (SA node) is the normal pacemaker of the heart which gives regular electrical pulses to the heart muscles to beat. It is part of the electrical system of the heart. If the sinoatrial node fails, in a normal heart, the atrioventricular node (AV node) should take over the pacemaker function. But the spontaneous rate of the atrioventricular node is lower than that of the sinoatrial node. Hence the heart rate will be lower. Moreover, impulses from the atrioventricular node is transmitted simultaneously to atria and ventricles. Hence both atria and ventricles contract simultaneously. This prevents atrial contraction from helping the ventricular filling. This can lover the stroke volume, especially in those with stiff ventricles (diastolic dysfunction).