If you have bradycardia, your brain and other organs might not get enough oxygen, possibly causing these symptoms:

• Near-fainting or fainting (syncope)
• Dizziness or lightheadedness
• Fatigue
• Shortness of breath
• Chest pains
• Confusion or memory problems
• Easily tiring during physical activity

Bradycardia can be caused by:

• Heart tissue damage related to aging
• Damage to heart tissues from heart disease or heart attack
• Heart disorder present at birth (congenital heart defect)
• Infection of heart tissue (myocarditis)
• A complication of heart surgery
• Underactive thyroid gland (hypothyroidism)
• Imbalance of chemicals in the blood, such as potassium or calcium
• Repeated disruption of breathing during sleep (obstructive sleep apnea)
• Inflammatory disease, such as rheumatic fever or lupus
• Medications, including some drugs for other heart rhythm disorders, high blood pressure and psychosis

Electrical circuitry of the heart

Your heart comprises four chambers — two upper (atria) and two lower (ventricles). A natural pacemaker (the sinus node), situated in the right atrium, normally controls your heart rhythm by producing electrical impulses that initiate each heartbeat.

These electrical impulses travel across the atria, causing them to contract and pump blood into the ventricles. Then these impulses arrive at a cluster of cells called the atrioventricular (AV) node.

The AV node transmits the signal to a collection of cells called the bundle of His. These cells transmit the signal down a left branch serving the left ventricle and a right branch serving the right ventricle, which causes the ventricles to contract and pump blood — the right ventricle sending oxygen-poor blood to the lungs and the left ventricle sending oxygen-rich blood to the body.

Bradycardia occurs when electrical signals slow down or are blocked.

Sinus node problems

Bradycardia often starts in the sinus node. A slow heart rate might occur because the sinus node:

• Discharges electrical impulses slower than is normal
• Pauses or fails to discharge at a regular rate
• Discharges an electrical impulse that's blocked before causing the atria to contract

In some people, the sinus node problems result in alternating slow and fast heart rates (bradycardia-tachycardia syndrome).

Heart block (atrioventricular block)

Bradycardia can also occur because electrical signals transmitted through the atria aren't transmitted to the ventricles (heart block, or atrioventricular block).

Heart blocks are classified based on the degree to which signals from the atria reach your heart's main pumping chambers (ventricles).

• 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 abnormality in electrical signal conduction.
• Second-degree heart block. Not all electrical signals reach the ventricles. Some beats are "dropped," resulting in a slower and sometimes irregular rhythm.
• Third-degree (complete) heart block. None of the electrical impulses from the atria reaches the ventricles. When this happens, a natural pacemaker takes over, but this results in slow and sometimes unreliable electrical impulses to control the beat of the ventricles.

Age

A key risk factor for bradycardia is age. Heart problems, which are often associated with bradycardia, are more common in older adults.

Risk factors related to heart disease

Bradycardia is often associated with damage to heart tissue from some type of heart disease.

Therefore, factors that increase your risk of heart disease can also increase the risk of bradycardia. Lifestyle changes or medical treatment might decrease the risk of heart disease associated with the following factors:

• High blood pressure
• Smoking
• Heavy alcohol use
• Recreational drug use
• Psychological stress or anxiety

If bradycardia causes symptoms, possible complications can include:

• Frequent fainting spells
• Inability of the heart to pump enough blood (heart failure)
• Sudden cardiac arrest or sudden death