Arrhythmia is a medical term that refers to a heart rate that is outside the normal range (60 to 100 beats per minute). An arrhythmia that is too slow is called a bradyarrhythmia or bradycardia, where the beats are less than 60 per minute. The two most common causes of bradycardia are diseases of the sinoatrial (SA) node (sick sinus syndrome), which is the heart's natural pacemaker or other problems with the heart's electrical conduction system (heart block).