causes

Types of sick sinus syndrome and their causes include:

+Sinoatrial block. Electrical signals move too slowly through the sinus node, causing an abnormally slow heart rate.
+Sinus arrest. The sinus node activity pauses.
+Bradycardia-tachycardia syndrome. The heart rate alternates between abnormally fast and slow rhythms, usually with a long pause between heartbeats.

running a lot, exercising to much.