Causes of Bradycardia
Bradycardia is primarily caused by sinus node dysfunction, atrioventricular conduction abnormalities, increased vagal tone, medication effects, and systemic conditions affecting the cardiac conduction system. 1, 2
Definition and Classification
Bradycardia is defined as a heart rate below 60 beats per minute. In neonates, the lower normal limit (2nd percentile) is 91 beats/min during the first week and 107 beats/min in the first month of life 1.
Physiologic vs. Pathologic Bradycardia
Physiologic bradycardia:
- Occurs in trained athletes
- During sleep
- In pregnancy
- Normal variant in some individuals
Pathologic bradycardia: Requires investigation and potential treatment when symptomatic
Common Causes of Bradycardia
1. Cardiac Causes
Sinus Node Dysfunction:
- Sick sinus syndrome
- Sinoatrial conduction block
- Age-related degenerative changes 2
- Infiltrative diseases (amyloidosis)
Atrioventricular Conduction Disorders:
- First-degree AV block
- Second-degree AV block (Mobitz type I and II)
- Third-degree (complete) AV block
- Infiltration of AV nodes by lymphoma or amyloidosis 1
2. Increased Vagal Tone
- Vasovagal response
- Carotid sinus hypersensitivity
- Vomiting
- Defecation
- Sleep 1
- Vagal reflexes during procedures
3. Medication-Induced Bradycardia
- Beta-blockers
- Non-dihydropyridine calcium channel blockers
- Digoxin toxicity 1
- Antiarrhythmic drugs (amiodarone, sotalol)
- Chemotherapeutic agents:
- Cisplatin
- Irinotecan
- Paclitaxel
- Mitoxantrone
- Thalidomide
- Methotrexate
- 5-fluorouracil
- Arsenic trioxide 1
4. Neurological Causes
- Increased intracranial pressure
- Central nervous system abnormalities
- Meningitis 1
- Brain tumors
- Stroke affecting brainstem centers
5. Metabolic and Endocrine Disorders
- Hypothyroidism (often with "mosque sign" - dome-shaped symmetric T waves) 1
- Hypopituitarism
- Electrolyte abnormalities (particularly hyperkalemia)
- Hypothermia
- Obstructive jaundice 1
6. Infectious Causes
- Myocarditis
- Endocarditis
- Typhoid fever 1
- Lyme disease (periatrioventricular node inflammation) 1
- HIV infection 1
- Viral myocarditis
7. Autoimmune Disorders
- Maternal lupus erythematosus (in newborns from anti-Ro/SSA positive mothers) 1
- Other connective tissue diseases
8. Structural Heart Disease
- Inferior wall myocardial infarction
- Congenital heart defects
- Cardiac tumors 1
- Cardiac surgery complications
9. Miscellaneous Causes
- Obstructive sleep apnea
- Neck masses affecting vagus nerves 1
- Catecholamine-secreting tumors (10% of patients) 1
- Ethanol injection for hepatocellular carcinoma treatment 1
Risk Factors for Bradycardia
- Advanced age (>70 years)
- Pre-existing cardiac disease
- Renal or hepatic dysfunction
- Electrolyte abnormalities
- Concomitant use of multiple bradycardia-inducing medications 2
Clinical Implications
Symptomatic bradycardia may present with:
- Syncope or near-syncope
- Dizziness or lightheadedness
- Confusion due to cerebral hypoperfusion
- Fatigue
- Exercise intolerance
- Congestive heart failure 1
Management Considerations
For symptomatic bradycardia:
- First-line treatment: Atropine 0.5 mg IV (can be repeated up to 3 mg total) 2, 3
- If persistent: Consider IV infusion of β-adrenergic agonists (dopamine or epinephrine) 2
- Transcutaneous or transvenous pacing for refractory cases 2
- Permanent pacemaker for persistent symptomatic bradycardia not due to reversible causes 2
Atropine works by inhibiting the muscarinic actions of acetylcholine, preventing or abolishing vagal cardiac slowing or asystole 3.
Important Caveats
- Always investigate for potentially reversible causes before considering permanent pacing 1
- Asymptomatic bradycardia generally doesn't require treatment
- Beta blockers should be started at very low doses and titrated gradually to minimize bradycardia risk 2
- Prophylactic measures like adequate hydration and careful adjustment of antihypertensive medications can help prevent bradycardia in at-risk patients 2