What are the causes of bradycardia?

Causes of Bradycardia

Bradycardia is defined as a heart rate less than 60 beats per minute, though clinically significant bradycardia typically occurs at rates below 50 beats per minute and is associated with symptoms or hemodynamic compromise. 1

Intrinsic Cardiac Causes

• Sinus Node Dysfunction (Sick Sinus Syndrome): Characterized by inappropriate sinus bradycardia, sinus arrest, sinoatrial block, or alternating bradycardia and tachycardia 1
• Atrioventricular (AV) Conduction Disorders: Including first-degree, second-degree (Mobitz type I and II), and third-degree (complete) heart block 1
• Congenital Heart Defects: Complex congenital heart malformations can lead to complete AV block 1
• Infiltrative Cardiac Disease: Conditions such as amyloidosis or lymphoma infiltrating the conduction system 1

Extrinsic/Reversible Causes

Medications

• Beta-blockers: Cause bradycardia through suppression of sinus node automaticity and AV conduction 2
• Calcium channel blockers: Particularly verapamil and diltiazem 1
• Digoxin: Can cause bradyarrhythmias at therapeutic doses in sensitive individuals or in overdose 1
• Antiarrhythmic drugs: Sotalol, amiodarone, and lidocaine can cause bradycardia as a side effect 1
• Chemotherapeutic agents: Cisplatin, paclitaxel, 5-fluorouracil, thalidomide, and arsenic trioxide have been associated with bradycardia 1

Electrolyte Abnormalities

• Hyperkalemia: Causes progressive depression of cardiac conduction, particularly when combined with renal failure and AV nodal blockers 3
• Hypokalemia: Can lead to bradyarrhythmias, especially in combination with digoxin 1
• Hypocalcemia: Can impair cardiac conduction 1

Autonomic Influences

• Increased vagal tone: Common in young athletes, during sleep, or with vagal maneuvers 1, 4
• Carotid sinus hypersensitivity: Exaggerated response to carotid sinus stimulation 5
• Vasovagal reflex: Triggered by pain, particularly abdominal pain 4

Systemic Conditions

• Hypothyroidism: Can cause bradycardia often with characteristic ECG changes ("mosque sign") 1
• Hypopituitarism: Can lead to bradycardia through multiple mechanisms 1
• Increased intracranial pressure: Causes bradycardia through the Cushing reflex 1
• Hypothermia: Progressive slowing of heart rate with decreasing core temperature 1
• Infectious diseases: Myocarditis, meningitis, typhoid fever 1
• Obstructive jaundice: Associated with bradycardia through unclear mechanisms 1

Acute Cardiac Conditions

• Acute myocardial infarction: Particularly inferior MI affecting the AV node blood supply 1
• Myocarditis: Can affect the conduction system 1

Other Causes

• Toxins: Certain herbal medicines, toad venom (similar to digoxin toxicity) 1
• Neoplasms: Vagal paraganglioma, catecholamine-secreting tumors, neck masses involving vagus nerves 1
• Maternal antibodies: Anti-Ro/SSA and La/SSB antibodies can cause congenital heart block in neonates 1

Clinical Significance and Evaluation

• Bradycardia may be physiologic, especially in trained athletes or during sleep 6, 7
• Symptoms suggesting pathologic bradycardia include syncope, dizziness, fatigue, reduced exercise capacity, dyspnea, and chest pain 1, 6
• Initial evaluation should focus on signs of increased work of breathing, oxygen saturation, blood pressure, and ECG findings 1
• Potentially reversible causes should be identified and treated before considering permanent interventions 1, 4

Management Considerations

• Asymptomatic bradycardia often requires no treatment 6
• For symptomatic bradycardia, treatment should address the underlying cause when possible 1, 8
• Acute management of symptomatic bradycardia may include atropine, especially for vagally-mediated bradycardia 4, 6
• Permanent pacemaker implantation is indicated for persistent symptomatic bradycardia, high-grade AV blocks, or sick sinus syndrome when reversible causes have been excluded 6, 8
• Avoid unnecessary permanent pacemaker implantation for reversible causes of bradycardia 4