What are the causes of bradycardia?

Causes of Bradycardia

Intrinsic Cardiac Causes

Bradycardia results from either intrinsic cardiac dysfunction or extrinsic reversible causes, and identifying reversible causes is the critical first step before considering permanent interventions. 1, 2

Sinus Node Dysfunction (Sick Sinus Syndrome)

• Characterized by inappropriate sinus bradycardia, sinus arrest, sinoatrial block, or alternating bradycardia and tachycardia (tachy-brady syndrome) 3, 1
• Represents failure of impulse formation or sinoatrial conduction block 4
• Patients may manifest symptoms ranging from mild fatigue to frank syncope, with syncope present in 50% of patients who received pacemakers for this condition 3

Atrioventricular Conduction Disorders

• First-degree, second-degree (Mobitz type I and II), and third-degree (complete) heart block can all cause bradycardia 1
• High second-degree or third-degree AV blocks require permanent pacemaker placement in symptomatic patients 5
• Infranodal block and atrioventricular nodal block are additional mechanisms 4

Structural and Infiltrative Disease

• Congenital heart defects, particularly complex malformations, can lead to complete AV block 1
• Infiltrative cardiac diseases including amyloidosis or lymphoma infiltrating the conduction system 1

Extrinsic and Reversible Causes

Medications (Most Common Reversible Cause)

• Beta-blockers, calcium channel blockers (particularly verapamil and diltiazem), and digoxin are the most common medication culprits and should be discontinued or dose-reduced when causing symptomatic bradycardia 1, 2
• Antiarrhythmic drugs including sotalol, amiodarone, and lidocaine 1
• Chemotherapeutic agents: cisplatin, paclitaxel, 5-fluorouracil, thalidomide, and arsenic trioxide 1
• Drugs transmitted transplacentally or through breast milk can cause neonatal bradycardia 2

Metabolic and Endocrine Disorders

• Hypothyroidism is a key reversible metabolic cause, often presenting with characteristic ECG changes, and requires thyroxine replacement 1, 2
• Hypopituitarism can lead to bradycardia through multiple mechanisms 1
• Electrolyte abnormalities: hypokalemia (especially in combination with digoxin), hypocalcemia, hyperkalemia, and systemic acidosis must be corrected 1, 2

Vagally-Mediated Causes

• Increased vagal tone is common in young athletes, during sleep, or with vagal maneuvers 1
• Gastrointestinal distress and acute abdominal pain trigger vagal reflexes causing transient bradycardia 2, 6
• Vasovagal reflex triggered by pain 1
• Increased vagal tone during feeding, sleep, and defecation causes physiologic bradycardia in neonates 2
• Carotid sinus hypersensitivity 4

Neurologic Causes

• Increased intracranial pressure from any cause triggers reflex bradycardia through vagal stimulation (Cushing reflex) 1, 2

Environmental Causes

• Hypothermia causes dose-dependent progressive slowing of heart rate with decreasing core temperature and requires active rewarming 1, 2

Infectious Causes

• Myocarditis can affect the conduction system through direct inflammation 1, 2
• Meningitis and typhoid fever 1
• Lyme disease can cause periatrioventricular node inflammation 3

Cardiac Ischemia and Injury

• Acute myocardial infarction, particularly inferior MI affecting the AV node blood supply, can cause bradycardia 1
• Transient injury to the conduction system at the time of open heart surgery 3

Other Causes

• Obstructive jaundice (through unclear mechanisms) 1
• Toxins including certain herbal medicines and toad venom 1
• Neoplasms such as vagal paraganglioma and catecholamine-secreting tumors 1
• Maternal antibodies (anti-Ro/SSA and La/SSB) can cause congenital heart block in neonates 1
• Sleep apnea may manifest as nocturnal bradycardia 1

Critical Clinical Pitfall

The most important clinical error is failing to identify reversible causes before considering permanent pacing—aggressive investigation for medications, hypothyroidism, electrolyte abnormalities, infections, and increased intracranial pressure must be completed first. 2 Avoid unnecessary permanent pacemaker implantation for reversible causes of bradycardia 1, 6

Clinical Significance

• Symptoms suggesting pathologic bradycardia include syncope, dizziness, fatigue, reduced exercise capacity, dyspnea, and chest pain 1
• Symptomatic bradycardia is defined as a documented bradyarrhythmia directly responsible for syncope, near syncope, transient dizziness, lightheadedness, or confusional states from cerebral hypoperfusion attributable to slow heart rate 3
• Fatigue, exercise intolerance, and congestive heart failure may also result from bradycardia 3
• Definite correlation of symptoms with a bradyarrhythmia is required to fulfill criteria for symptomatic bradycardia 3
• Caution should be exercised not to confuse physiological sinus bradycardia (as occurs in highly trained athletes) with pathological bradyarrhythmias 3