What is the most common cause of bradycardia?

Medication. A slow heart rate can be normal in fit individuals at rest. Many medications, notably beta blockers, slow the heart rate. A slow heart rate can also be due to disease of the av node, which conducts impulses from the atria to the ventricles. This can be due to scarring from heart attack or aging, and is seen in some infections such as lyme disease and chagas disease.