What are the causes of bradycardia?

Causes of Bradycardia

Bradycardia, defined as a heart rate less than 60 beats per minute, can be caused by numerous intrinsic cardiac conditions and extrinsic factors, with clinically significant bradycardia typically occurring at rates below 50 beats per minute when 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 like amyloidosis or lymphoma infiltrating the conduction system 1
• Acute Myocardial Infarction: Particularly inferior MI affecting the AV node blood supply 1
• Myocarditis: Can affect the conduction system and cause bradycardia 1

Medication-Induced Bradycardia

• Calcium Channel Blockers: Particularly verapamil and diltiazem 1
• Beta Blockers: Can cause bradycardia, especially in combination with other factors like renal failure and hyperkalemia 2
• Digoxin: Can cause bradyarrhythmias at therapeutic doses in sensitive individuals or in overdose 1, 3
• Antiarrhythmic Drugs: Such as sotalol, amiodarone, and lidocaine 1
• Chemotherapeutic Agents: Including cisplatin, paclitaxel, 5-fluorouracil, thalidomide, and arsenic trioxide 1

Electrolyte and Metabolic Disturbances

• Hyperkalemia: Can cause bradycardia, especially in combination with other factors like beta blockers and renal failure 2
• Hypokalemia: Can lead to bradyarrhythmias, especially in combination with digoxin 1
• Hypocalcemia: Can impair cardiac conduction 1
• Hypomagnesemia: Can contribute to bradycardia 4
• Uremia: Can cause bradycardia through multiple mechanisms including electrolyte disturbances 4
• Hypothyroidism: Can cause bradycardia often with characteristic ECG changes 1
• Hypopituitarism: Can lead to bradycardia through multiple mechanisms 1

Neurological and Autonomic Causes

• Increased Vagal Tone: Common in young athletes, during sleep, or with vagal maneuvers 1
• Vasovagal Reflex: Triggered by pain, particularly abdominal pain 1
• Increased Intracranial Pressure: Can cause bradycardia through the Cushing reflex 1
• Carotid Sinus Hypersensitivity: Can lead to bradycardia 5

Other Causes

• Hypothermia: Can cause progressive slowing of heart rate with decreasing core temperature 1
• Infectious Diseases: Such as myocarditis, meningitis, and typhoid fever 1
• Obstructive Jaundice: Has been associated with bradycardia 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: Including anti-Ro/SSA and La/SSB antibodies can cause congenital heart block in neonates 1
• Sleep Apnea: May manifest as nocturnal bradycardia 6

Physiologic Bradycardia

• Athletic Training: Well-trained athletes commonly have resting bradycardia 7
• Sleep: Heart rate may decrease to 30 beats per minute during sleep with episodes of sinoatrial block and junctional rhythms that can be considered normal variants 7

Clinical Significance

• Bradycardia can be asymptomatic and physiologic, especially in trained athletes or during sleep 8
• Symptoms suggesting pathologic bradycardia include syncope, dizziness, fatigue, reduced exercise capacity, dyspnea, and chest pain 1
• In infants and children, the earliest and most frequent manifestation of bradycardia is cardiac arrhythmias, including sinus bradycardia, rather than other symptoms 3
• Sinus bradycardia may be a sign of impending digoxin intoxication, especially in infants, even in the absence of first-degree heart block 3

Understanding the various causes of bradycardia is essential for appropriate management, as treatment should address the underlying cause when possible rather than just the bradycardia itself 1.