The term bradycardia refers to an abnormally slow heart rate, of under 60 beats per minute (bpm). Bradycardia is often not a dangerous condition and may not require any treatment. In some cases, however, when the condition is caused by deteriorating heart health or medication, a patient may require medical assistance to restore a healthy heart rate.

Some examples of bradycardia causes include:

Intrinsic causes:

• Aging – As a person ages, the heart’s electrical system slowly deteriorates and may eventually lead to bradycardia.
• Myocardial infarction/ischemia – Ischemia occurs when a part of the heart muscle is deprived of oxygen, which becomes damaged and may die off. This can lead to myocardial infarction, commonly referred to as a heart attack. Once an area of the heart muscle is damaged, it fails to conduct the impulses at a normal rate, which can lead to abnormal heart rhythms such as bradycardia.
• Infections of the heart muscles such as endocarditis and Chagas disease
• Infiltrative diseases such as sarcoidosis, amyloidosis or hemochromatosis
• Autoimmune diseases such as systemic lupus erythematous (SLE), rheumatoid arthritis and scleroderma
• Heart transplant or heart surgery
• Myotonic muscular dystrophy

Extrinsic causes:

• Autonomic nervous system disorders such as neurocardiac disease and carotid-sinus hypersensitivity
• Medications such as beta blockers (such as atenolol, metoprolol and propranolol), calcium channel blockers (such as nifedipine, amlodipine, diltiazem, verapamil), clonidine and digoxin.
• Metabolic causes such as underactive thyroid function (hypothyroidism), electrolyte imbalance such as a high or low blood potassium level, hypothermia, raised intracranial pressure and obstructive sleep apnea.

Types of bradycardia

In many cases, bradycardia is not a dangerous condition and does not require treatment. For example, sinus bradycardia is a type of bradycardia that occurs in response to being deeply relaxed or extremely fit. Sinus bradycardia is commonly observed in athletes. On the other hand, another cause of bradycardia is sick sinus syndrome. This occurs when the SA node fails to generate the impulse needed for a heartbeat. Sick sinus syndrome can cause bradycardia, tachycardia (a fast heart rate) or a mixture of the two, referred to as brady–tachy syndrome. Patients with sick sinus syndrome may feel dizzy and tired and be prone to fainting.

Heart block is another cause of bradycardia that occurs when impulses flowing through the AV node to the ventricles are blocked. Also called AV block, this condition is divided into categories depending on its severity:

• In first-degree heart block, the impulses are slowed as they pass through the AV node but they still reach the ventricles. Treatment is not usually required.

• In second-degree heart block, some of the electrical impulses do not reach the ventricles and a pacemaker may be required to control the heart’s rhythm.
• Third-degree heart block is caused by signals from the SA node completely failing to reach the ventricles. This is usually the result of underlying disease and a pacemaker may be implanted to keep the heart pumping regularly.