What are the leading causes of bradycardia?

Leading Causes of Bradycardia

Bradycardia results from two primary intrinsic cardiac mechanisms—sinus node dysfunction and atrioventricular conduction disorders—alongside numerous extrinsic/reversible causes, with medications (particularly beta-blockers, calcium channel blockers, and digoxin) being the most common reversible etiology. 1

Intrinsic Cardiac Causes

Sinus Node Dysfunction (Sick Sinus Syndrome)

• Manifests as inappropriate sinus bradycardia, sinus arrest, sinoatrial block, or tachy-brady syndrome (alternating bradycardia and tachycardia). 1
• Syncope occurs in 50% of patients requiring pacemakers for this condition, with other symptoms ranging from mild fatigue to frank syncope depending on heart rate and pause duration. 2, 1
• The severity of clinical manifestations generally correlates with the heart rate or pause duration. 2

Atrioventricular Conduction Disorders

• Include first-degree, second-degree (Mobitz type I and II), and third-degree (complete) heart block. 1
• High second-degree or third-degree AV blocks require permanent pacemaker placement in symptomatic patients. 3

Other Intrinsic Causes

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

Extrinsic and Reversible Causes

Medications (Most Common Reversible Cause)

• Beta-blockers (e.g., metoprolol) cause bradycardia, including sinus pause, heart block, and cardiac arrest; patients with first-degree AV block, sinus node dysfunction, or conduction disorders are at increased risk. 4
• Non-dihydropyridine calcium channel blockers (verapamil and diltiazem) are significant bradycardia-inducing agents. 1
• Digoxin causes bradyarrhythmias at therapeutic doses in sensitive individuals or overdose, particularly when combined with hypokalemia. 1
• Antiarrhythmic drugs including sotalol, amiodarone, and lidocaine. 1
• Chemotherapeutic agents such as cisplatin, paclitaxel, 5-fluorouracil, thalidomide, and arsenic trioxide. 1

Electrolyte and Metabolic Abnormalities

• Hypokalemia leads to bradyarrhythmias, especially combined with digoxin. 1
• Hypocalcemia impairs cardiac conduction. 1
• Hypothyroidism causes bradycardia with characteristic ECG changes. 1
• Hypopituitarism through multiple mechanisms. 1

Acute Cardiac Conditions

• Acute myocardial infarction, particularly inferior MI affecting AV node blood supply. 2, 1
• Myocarditis affecting the conduction system. 1
• Transient injury during open heart surgery (valve replacement, maze procedure, coronary artery bypass graft). 2, 1

Neurologic and Autonomic Causes

• Increased vagal tone in young athletes, during sleep, or with vagal maneuvers. 1
• Vasovagal reflex triggered by pain, particularly abdominal pain. 1
• Increased intracranial pressure causing bradycardia through the Cushing reflex. 1

Infectious Diseases

• Lyme disease causes periatrioventricular node inflammation. 1
• Other infections including myocarditis, meningitis, typhoid fever, typhus, listeria, malaria, leptospirosis, Dengue fever, viral hemorrhagic fevers, legionella, psittacosis, and Guillain-Barré. 2, 1

Environmental and Toxic Causes

• Hypothermia (therapeutic post-cardiac arrest cooling or environmental exposure) causes progressive heart rate slowing. 2, 1
• Toxins including toluene, organophosphates, tetrodotoxin, cocaine, certain herbal medicines, and toad venom. 2, 1

Other Reversible Causes

• Sleep apnea manifesting as nocturnal bradycardia. 1
• Hypovolemic shock. 2
• Hypoxemia, hypercarbia, acidosis from respiratory insufficiency. 2
• Obstructive jaundice through unclear mechanisms. 1

Critical Clinical Pitfalls

• Do not confuse physiological sinus bradycardia with pathological bradyarrhythmias—a heart rate below 60 bpm can be normal in athletes or during sleep. 1, 5
• Always evaluate and treat reversible causes before considering permanent pacing—this is a Class I recommendation. 2, 1
• Establish symptom-rhythm correlation—this is the gold standard for diagnosis, though competing etiologies and monitoring limitations can make this challenging. 2
• Avoid unnecessary permanent pacemaker implantation for asymptomatic bradycardia or reversible causes. 1, 5