What causes bradycardia in adults?

Causes of Bradycardia in Adults

Bradycardia in adults is primarily caused by sinus node dysfunction, atrioventricular conduction disorders, or extrinsic factors including medication effects, electrolyte abnormalities, and underlying medical conditions. 1

Intrinsic Causes

Sinus Node Dysfunction

• Degenerative fibrosis - Most common cause, particularly in elderly patients in their 70s and 80s 1
• Sick sinus syndrome - Constellation of bradyarrhythmias including:
  • Sinus bradycardia
  • Sinus pauses/arrest
  • Sinoatrial exit block
  • Chronotropic incompetence (inadequate heart rate response to exercise)
  • Tachy-brady syndrome (alternating bradycardia and tachyarrhythmias) 2

Atrioventricular (AV) Conduction Disorders

• First-degree AV block - Prolonged PR interval (>0.20 second)
• Second-degree AV block:
  • Mobitz type I (Wenckebach) - Progressive PR prolongation until a beat is dropped
  • Mobitz type II - Sudden failure of conduction without PR prolongation
• Third-degree (complete) AV block - No impulses pass between atria and ventricles 1
• Bundle branch blocks - Conduction delay or block in the His-Purkinje system 1

Extrinsic (Reversible) Causes

Medication Effects

• Cardiac medications:
  • Beta-blockers
  • Calcium channel blockers (non-dihydropyridine)
  • Antiarrhythmic drugs (especially class I and III)
  • Digoxin 2
• Other medications:
  • Clonidine
  • Lithium
  • Amiodarone
  • Ivabradine 2

Electrolyte Abnormalities

• Hyperkalemia
• Hypokalemia
• Hypercalcemia
• Hypermagnesemia 2

Underlying Medical Conditions

• Cardiac conditions:
  • Acute myocardial infarction/ischemia (especially inferior MI)
  • Myocarditis
  • Endocarditis
  • Cardiac surgery trauma 1
• Systemic conditions:
  • Hypothyroidism
  • Increased intracranial pressure
  • Hypothermia
  • Sleep apnea
  • Infectious diseases (Lyme disease, diphtheria)
  • Infiltrative diseases (amyloidosis, sarcoidosis, hemochromatosis)
  • Collagen vascular diseases 1, 2

Autonomic Effects

• Increased vagal tone:
  • Vasovagal reactions
  • Carotid sinus hypersensitivity
  • Vomiting
  • Defecation
  • Pain 1, 2
• Athletic training - Physiologic bradycardia in well-trained athletes 1

Diagnostic Considerations

When evaluating bradycardia, it's important to distinguish between:

1. Physiologic bradycardia - Common in athletes and during sleep, typically asymptomatic 3
2. Pathologic bradycardia - Associated with symptoms such as:
   • Syncope or pre-syncope
   • Dizziness
   • Fatigue
   • Reduced exercise capacity
   • Chest pain
   • Dyspnea
   • Heart failure symptoms 4

Clinical Pearls

• In the emergency setting, approximately 21% of bradycardias are caused by adverse drug effects, 14% by acute myocardial infarction, 6% by intoxication, and 4% by electrolyte disorders 1
• The same degenerative fibrosis that causes sinus node dysfunction can also affect the AV conduction system, explaining why some patients with sinus node disease later develop AV block 1
• Bradycardia-related cardiomyopathy and heart failure can develop with chronic, untreated bradyarrhythmias 5
• Bradycardia can both trigger and be caused by atrial fibrillation in the tachy-brady syndrome 5

Remember that bradycardia should be considered pathologic primarily when it causes symptoms or when it occurs in the setting of underlying heart disease. The threshold for intervention should be based on symptom severity and risk of progression rather than heart rate alone.