Causes

Bradycardia can be caused by:

• Heart tissue damage related to aging
• Damage to heart tissues from heart disease or heart attack
• A heart disorder present at birth (congenital heart defect)
• Inflammation of heart tissue (myocarditis)
• A complication of heart surgery
• An underactive thyroid gland (hypothyroidism)
• Imbalance of chemicals in the blood, such as potassium or calcium
• Repeated pauses in breathing during sleep (obstructive sleep apnea)
• Inflammatory disease, such as rheumatic fever or lupus
• Medications, including sedatives, opioids, and drugs used to treat heart rhythm disorders, high blood pressure and certain mental health disorders

To better understand the causes of bradycardia, it may be helpful to know how the heart typically beats.

The typical heart has four chambers — two upper chambers (atria) and two lower chambers (ventricles). Within the upper right chamber of the heart (right atrium) is a group of cells called the sinus node. The sinus node is the heart's natural pacemaker. It produces the signal that starts each heartbeat.

Bradycardia occurs when these signals slow down or are blocked.

Sinus node problems

Bradycardia often starts in the area of the heart called the sinus node. In some people, sinus node problems cause alternating slow and fast heart rates (bradycardia-tachycardia syndrome).

Heart block (atrioventricular block)

Bradycardia can also occur if the heart's electrical signals don't move correctly from the upper chambers (atria) to the lower chambers (ventricles). If this occurs, the condition is called heart block, or atrioventricular block.

Heart blocks fall into three main groups.

• First-degree heart block. In the mildest form, all electrical signals from the atria reach the ventricles, but the signal is slowed. First-degree heart block rarely causes symptoms and usually needs no treatment if there's no other problem in electrical signaling.
• Second-degree heart block. Not all electrical signals reach the ventricles. Some beats are dropped, resulting in a slower and sometimes irregular heart rhythm.
• Third-degree (complete) heart block. None of the electrical signals from the atria reaches the ventricles. When this happens, the ventricles will usually beat on their own but at a very slow rate.