Causes of Bradycardia
Bradycardia can be broadly classified into two general categories: sinus node dysfunction (SND) and atrioventricular (AV) block, with multiple potential etiologies ranging from intrinsic cardiac disease to reversible extrinsic factors. 1
Primary Classifications
Sinus Node Dysfunction (SND)
- Sinus bradycardia: Sinus rate <50 bpm
- Ectopic atrial bradycardia: Atrial depolarization from non-sinus pacemaker with rate <50 bpm
- Sinoatrial exit block: Blocked conduction between sinus node and atrial tissue
- Sinus pause: Sinus node depolarizes >3 seconds after last atrial depolarization
- Sinus node arrest: No evidence of sinus node depolarization
- Tachycardia-bradycardia syndrome: Alternating bradycardia with periods of atrial tachycardia/flutter/fibrillation
- Chronotropic incompetence: Inability of heart to increase rate appropriately with activity 1
Atrioventricular Block
- First-degree AV block: PR interval >200 ms
- Second-degree AV block:
- Mobitz type I (Wenckebach): Progressive PR prolongation before blocked P wave
- Mobitz type II: Sudden block of P wave without PR prolongation
- Third-degree (complete) AV block: Complete dissociation between atria and ventricles 1
Reversible Causes
Medication-Related
- Beta-blockers (e.g., metoprolol) 2
- Non-dihydropyridine calcium channel blockers
- Digoxin
- Antiarrhythmic drugs (class I and III)
- Cholinergic agents 3
Metabolic/Endocrine
- Hypothyroidism
- Hypopituitarism
- Electrolyte abnormalities (particularly hyperkalemia)
- Hypothermia
- Acid-base disorders 3
Cardiac Causes
- Acute myocardial infarction (particularly inferior MI)
- Myocarditis
- Endocarditis
- Cardiac surgery
- Infiltrative diseases (amyloidosis, sarcoidosis)
- Congenital heart disease
- Heart transplantation (denervation) 1, 3
Increased Vagal Tone
- Vasovagal response
- Carotid sinus hypersensitivity
- Vomiting
- Defecation
- Sleep (physiologic)
- Athletes (physiologic) 1, 3
Neurologic
- Increased intracranial pressure
- Brain stem lesions
- Central nervous system disorders 3
Infectious
- Lyme disease
- Chagas disease
- Viral myocarditis
- Endocarditis 3
Other Systemic Conditions
Clinical Significance
The clinical manifestations of bradycardia range from asymptomatic to severe symptoms including:
- Syncope or presyncope
- Dizziness or lightheadedness
- Fatigue and exercise intolerance
- Confusion or altered mental status
- Heart failure symptoms
- Chest pain 1
Important Considerations
- Asymptomatic bradycardia, especially in young, healthy individuals or during sleep, often requires no treatment 4, 5
- Bradycardia in trained athletes is typically a normal physiologic adaptation 6
- The prognosis of untreated symptomatic bradycardia varies based on the underlying cause, with some forms associated with increased risk of systemic thromboembolism 1
- Evaluation of bradycardia should include assessment for potentially reversible causes before considering permanent pacing 3, 7
When evaluating bradycardia, it's essential to determine whether it's causing symptoms and to identify any underlying reversible causes before considering permanent interventions such as pacemaker implantation.