What causes sinus bradycardia?

Causes of Sinus Bradycardia

Sinus bradycardia is primarily caused by medications, physiological conditions, and underlying pathologies that affect the sinus node function, with negative chronotropic medications being the most common reversible cause. 1

Medication-Related Causes

• Negative chronotropic drugs:
  • Beta blockers (e.g., metoprolol) - can cause significant bradycardia, especially in patients with pre-existing conduction disorders 2
  • Calcium channel blockers (non-dihydropyridine) - particularly verapamil and diltiazem 3
  • Digoxin - frequently prescribed medication that can decrease sinus rate 1
  • Antiarrhythmic drugs - sodium-channel and potassium-channel blockers can exacerbate bradycardia 1
  • Combination therapy - using beta-blockers with calcium channel blockers or digoxin significantly increases bradycardia risk 4, 3

Pathological Causes

• Endocrine disorders:

  • Hypothyroidism - often associated with the "mosque sign" (dome-shaped symmetric T wave without ST segment) 1
  • Hypopituitarism 1

• Neurological conditions:

  • Increased intracranial pressure 1, 5
  • Central nervous system abnormalities 1
  • Meningitis 1, 5

• Cardiac conditions:

  • Sinus node dysfunction (sick sinus syndrome) 5
  • Inferior myocardial infarction 1, 5
  • Myocarditis 5

• Other pathological conditions:

  • Obstructive jaundice 1
  • Typhoid fever 1
  • Severe systemic acidosis 1
  • Hypokalemia 1
  • Severe hypothermia 1
  • Obstructive sleep apnea 1

Physiological/Normal Causes

• Athletic conditioning (common in trained athletes) 4, 5
• Sleep (nocturnal sinus bradycardia is a common physiological phenomenon) 1
• Increased vagal tone during activities like feeding, defecation 1
• Pregnancy 5

Immunological Causes

• Transient sinus bradycardia in newborns from anti-Ro/SSA positive mothers (especially women with lupus erythematosus or other connective diseases) 1
• Steroid pulse therapy (high-dose intravenous methylprednisolone) can cause acute symptomatic sinus bradycardia 6

Management Considerations

When evaluating sinus bradycardia, it's crucial to distinguish between pathological and physiological causes:

1. For medication-induced bradycardia:

   • First-line approach should be eliminating or mitigating the offending medication 1
   • Consider switching to medications without negative chronotropic effects (e.g., ACE inhibitors, ARBs, dihydropyridine CCBs for hypertension) 4
   • Dose reduction may be sufficient if complete discontinuation isn't possible 1

2. For symptomatic bradycardia:

   • Atropine (0.5-1mg IV) is first-line treatment for acute symptomatic bradycardia 4
   • Consider temporary pacing for severe symptomatic bradycardia due to reversible causes 7
   • Permanent pacing should be considered only for symptomatic bradycardia with no reversible cause 4

3. For asymptomatic bradycardia:

   • Generally requires observation only, especially in athletes or during sleep 4
   • Electrophysiological studies should not be performed unless other indications exist 1

Important Clinical Pitfalls

• Failing to identify and address reversible causes before considering permanent pacing
• Overlooking medication combinations that can synergistically cause bradycardia (beta-blockers + calcium channel blockers)
• Misinterpreting physiological bradycardia (e.g., in athletes or during sleep) as pathological
• Not recognizing that severe bradycardia can lead to heart failure, particularly in elderly patients with organic heart disease and chronotropic incompetence 8
• Underestimating the potential for steroid pulse therapy to cause acute symptomatic bradycardia 6

Remember that sinus bradycardia is often a manifestation of an underlying condition rather than a primary disorder, making identification of the root cause essential for appropriate management.