What labs should be checked for a patient with bradycardia?

Laboratory Testing for Bradycardia (Heart Rate 40)

In patients with bradycardia (heart rate 40), laboratory tests including thyroid function tests, electrolytes (particularly potassium), Lyme disease titer, and pH/acid-base status should be performed based on clinical suspicion for potential underlying causes. 1

Initial Laboratory Evaluation

The American College of Cardiology/American Heart Association/Heart Rhythm Society guidelines recommend targeted laboratory testing for patients with bradycardia. For a patient with a heart rate of 40 bpm, the following tests should be prioritized:

Essential Laboratory Tests:

• Electrolytes panel
  • Potassium (both hyper- and hypokalemia can cause bradycardia)
  • Sodium
  • Calcium
  • Magnesium
• Thyroid function tests (TSH, free T4)
• Renal function tests (BUN, creatinine)
• Acid-base status (arterial blood gas if clinically indicated)

Additional Tests Based on Clinical Context:

• Lyme disease titer (particularly in endemic areas or with relevant history)
• Drug levels (if on medications known to cause bradycardia)
• Inflammatory markers (ESR, CRP) if systemic inflammatory condition suspected
• Cardiac biomarkers (troponin, BNP) if myocardial injury or heart failure suspected

Diagnostic Algorithm

1. Assess hemodynamic stability

   • If unstable (hypotension, altered mental status, chest pain, dyspnea), prioritize stabilization while obtaining labs

2. Review medication history

   • Beta-blockers, non-dihydropyridine calcium channel blockers, digoxin, antiarrhythmics
   • Consider drug levels if appropriate

3. Evaluate for metabolic causes

   • Thyroid dysfunction (hypothyroidism)
   • Electrolyte abnormalities (particularly hyperkalemia)
   • Acid-base disturbances

4. Consider infectious etiologies

   • Lyme disease testing in endemic areas
   • Other infectious markers if clinically indicated

5. Assess for structural heart disease

   • Cardiac biomarkers if ischemia suspected

Common Reversible Causes to Consider

The guidelines highlight several reversible causes of bradycardia that laboratory testing can help identify:

• Metabolic/Endocrine

  • Hypothyroidism
  • Electrolyte abnormalities (hyperkalemia, hypokalemia)
  • Hypoglycemia

• Infectious

  • Lyme disease
  • Other systemic infections (legionella, typhoid, viral infections)

• Medication-related

  • Beta-blockers
  • Calcium channel blockers
  • Digoxin
  • Antiarrhythmic drugs

Important Considerations

• Timing matters: Obtain laboratory tests promptly, especially in symptomatic patients
• Medication review is crucial: Many cases of bradycardia are medication-induced
• Consider clinical context: Target testing based on patient history, symptoms, and physical examination findings
• Avoid unnecessary testing: In asymptomatic patients with mild sinus bradycardia, extensive laboratory testing may not be indicated

Pitfalls to Avoid

• Missing hyperkalemia: This is a potentially life-threatening and readily reversible cause of bradycardia
• Overlooking hypothyroidism: Can present subtly but is an important reversible cause
• Neglecting drug effects: Always review current medications and recent changes
• Failing to consider Lyme carditis: Particularly in young patients with new high-grade AV block in endemic areas

Laboratory testing should be part of a comprehensive evaluation that includes ECG, cardiac monitoring, and appropriate imaging studies based on the clinical presentation.