Causes of Bradycardia
Bradycardia is caused by a variety of intrinsic cardiac conditions, extrinsic factors, and physiologic states, with the most common reversible causes being medication effects (21%), acute myocardial infarction (14%), intoxication (6%), and electrolyte disorders (4%) 1.
Definition and Classification
Bradycardia is defined as a heart rate below 60 beats per minute, though clinically significant bradycardia typically presents with a heart rate below 50 beats per minute accompanied by symptoms 1. Bradyarrhythmias can be broadly classified into two main categories:
Sinus Node Dysfunction (SND):
- Sinus bradycardia (sinus rate <50 bpm)
- Ectopic atrial bradycardia
- Sinoatrial exit block
- Sinus pause (>3 seconds)
- Sinus node arrest
- Tachycardia-bradycardia syndrome
- Chronotropic incompetence
Atrioventricular (AV) Conduction Disorders:
- First-degree AV block
- Second-degree AV block (Mobitz type I and II)
- Third-degree (complete) AV block
Intrinsic Causes
1. Cardiac Structural Disease
- Degenerative fibrosis of the conduction system (age-related)
- Ischemic heart disease/myocardial infarction
- Infiltrative cardiac diseases (amyloidosis, sarcoidosis)
- Congenital heart disease
- Post-cardiac surgery
2. Inflammatory/Infectious
- Myocarditis
- Lyme disease (periatrioventricular node inflammation)
- Viral infections (including HIV)
- Endocarditis
3. Genetic/Congenital
- Congenital AV block
- Genetic channelopathies
- Maternal connective tissue disease with anti-Ro/SSA and La-SSB antibodies causing neonatal AV block 1
Extrinsic (Reversible) Causes
1. Medication-Induced
- Beta-blockers (metoprolol, atenolol) 2, 3
- Calcium channel blockers
- Digoxin and other cardiac glycosides
- Antiarrhythmic drugs (amiodarone, sotalol)
- Clonidine
- Lithium
- Cholinergic agents
2. Metabolic/Endocrine
- Hypothyroidism (often associated with the "mosque sign" - dome-shaped symmetric T waves) 4
- Hypothermia
- Hypopituitarism
- Electrolyte disorders (particularly hyperkalemia)
- Obstructive jaundice
3. Neurologic
- Increased intracranial pressure
- Central nervous system disorders
- Meningitis
4. Autonomic Influences
- Vasovagal reactions
- Carotid sinus hypersensitivity
- Situational (micturition, defecation, coughing)
- Sleep (increased vagal tone)
5. Toxins
- Organophosphates
- Various intoxications
Physiologic Bradycardia
- Athletic conditioning (normal finding in trained athletes)
- Sleep-related (normal nocturnal bradycardia)
- Neonatal bradycardia (normal variant)
Special Considerations
Neonatal Bradycardia
In neonates, bradycardia is defined differently, with the lower normal limit being 91 beats/min during the first week and 107 beats/min in the first month of life 1. Specific causes in neonates include:
- Maternal connective tissue disease with anti-Ro/SSA antibodies
- Hypothyroidism
- Congenital heart defects
- CNS abnormalities
Extrinsic vs. Intrinsic Bradycardia
When evaluating bradycardia, it's crucial to distinguish between extrinsic (potentially reversible) and intrinsic causes. In an emergency department study, reversible causes accounted for 45% of significant bradycardias 1, highlighting the importance of identifying and treating these underlying conditions before considering permanent pacing.
Clinical Implications
The clinical significance of bradycardia depends on:
- The absolute heart rate
- The patient's age and baseline cardiovascular status
- The presence of symptoms (syncope, presyncope, dizziness, fatigue, heart failure)
- Whether the bradycardia is persistent or intermittent
- The underlying cause and its reversibility
When bradycardia leads to decreased cardiac output and cerebral hypoperfusion, it can result in significant morbidity and mortality, particularly in patients with limited cardiovascular reserve.