A heart rate below 60 bpm is medically termed brachycardia. There are individuals that have a heart rate that is lower than normal, and these individuals are in very good physical condition.
Brachycardia can be caused by
Changes in the heart that are the result of aging.
Diseases that damage the heart's electrical system. These include coronary artery disease, heart attack, and infections such as endocarditis and myocarditis.
Conditions that can slow electrical impulses through the heart. Examples include having a low thyroid level (hypothyroidism) or an electrolyte imbalance, such as too much potassium in the blood.
Some medicines for treating heart problems or high blood pressure, such as beta-blockers, antiarrhythmics, and digoxin.