What are the causes of sinus bradycardia?

Etiology of Sinus Bradycardia

Sinus bradycardia results from either intrinsic cardiac dysfunction (primarily sinus node disease) or extrinsic reversible causes including medications, metabolic derangements, increased vagal tone, infections, and environmental factors—identifying and treating reversible causes is the critical first step before considering permanent interventions. 1

Intrinsic Cardiac Causes

• Sinus node dysfunction (sick sinus syndrome) represents the primary intrinsic cardiac cause of pathologic bradycardia, resulting from degenerative changes in the sinoatrial node 2, 3
• Acute myocardial infarction, particularly inferior wall infarction, can directly damage the sinus node or increase vagal tone 2, 4
• Structural heart disease including heart failure may be associated with bradycardia, though the relationship is complex 5

Medication-Induced Causes

• Beta-blockers, calcium channel blockers (non-dihydropyridine), and digoxin are the most common medication culprits and should be discontinued or dose-reduced when causing symptomatic bradycardia 1, 3
• Antiarrhythmic drugs including amiodarone can suppress sinus node function 1, 2
• Drugs transmitted transplacentally or through breast milk can cause bradycardia in neonates 6

Metabolic and Endocrine Causes

• Hypothyroidism is a key reversible metabolic cause, often presenting with the characteristic "mosque sign" on ECG (dome-shaped symmetric T wave without ST segment) in neonates, and requires thyroxine replacement 6, 1
• Electrolyte abnormalities including severe hypokalemia, hyperkalemia, and systemic acidosis must be corrected 1, 7
• Hypopituitarism can cause bradycardia, particularly in neonates 6

Neurologic Causes

• Increased intracranial pressure from any cause (trauma, hemorrhage, mass lesions) triggers reflex bradycardia through vagal stimulation 6, 1, 2
• Central nervous system abnormalities and meningitis are important causes, especially in neonates and acute presentations 6, 2

Infectious Causes

• Myocarditis (viral, HIV-related) can cause bradycardia through direct inflammation of the conduction system 6, 2
• Bacterial meningitis and systemic infections including typhoid fever are recognized causes 6, 2

Vagally-Mediated Causes

• Gastrointestinal distress and acute abdominal pain trigger vagal reflexes causing transient bradycardia 7, 8
• Increased vagal tone during feeding, sleep, defecation causes physiologic bradycardia in neonates 6
• Severe dehydration and hypovolemic shock paradoxically cause bradycardia through vagal stimulation rather than the expected tachycardia 7

Environmental and Physiologic Causes

• Hypothermia causes dose-dependent bradycardia and must be actively rewarmed 6, 1, 4
• Obstructive sleep apnea causes intermittent bradycardia through hypoxia and vagal stimulation 1, 2
• Athletic conditioning and sleep represent normal physiologic causes in healthy individuals 1, 2

Neonatal-Specific Causes

• Maternal anti-Ro/SSA antibodies in mothers with lupus erythematosus or connective tissue disease cause transient sinus bradycardia or complete heart block in neonates 6
• Obstructive jaundice is a recognized neonatal cause requiring investigation 6

Critical Clinical Pitfall

The most important clinical error is failing to identify reversible causes before considering permanent pacing—aggressive investigation for medications, hypothyroidism, electrolyte abnormalities, infections, and increased intracranial pressure must be completed first, as these account for the majority of treatable cases 1, 3. In acute presentations with hemodynamic compromise, atropine 0.5-1 mg IV can temporize while the underlying cause is addressed 1, 7, 8.