Bradycardia refers to heart rates that are too slow (less than 60 beats/min.). Bradycardias often result from irregular electrical activity of the sinus node, the pacemaker tissue of the heart. The sinus node signal can be either too slow, termed sinus bradycardia, or temporary paused (sinus arrest). Other bradycardias are related to the lack of proper electrical signal transmission from the atria (top heart chambers) to the ventricles (bottom heart chambers).