What labs should be ordered for a patient presenting with bradycardia (abnormally slow heart rate)?

Laboratory Tests for Bradycardia Evaluation

For patients with bradycardia, targeted laboratory tests including thyroid function tests, electrolytes panel (especially potassium), Lyme disease titer, and acid-base status (pH) should be ordered based on clinical suspicion for potential underlying causes. 1, 2

Priority Laboratory Tests

First-Line Tests

• Electrolytes panel
  • Potassium (hyperkalemia is a common reversible cause)
  • Sodium
  • Calcium
  • Magnesium
• Thyroid function tests
  • TSH
  • Free T4
• Renal function tests
  • BUN
  • Creatinine

Additional Tests Based on Clinical Context

• Acid-base status (arterial blood gas if clinically indicated)
• Lyme disease titer (particularly in endemic areas or with relevant history)
• Drug levels (digoxin, beta-blockers, calcium channel blockers)

Clinical Rationale for Testing

Metabolic/Endocrine Causes

• Hypothyroidism: Can cause profound bradycardia that resolves with thyroid replacement 3
• Electrolyte abnormalities: Hyperkalemia can cause severe bradycardia and conduction disorders
• Acid-base disturbances: Acidosis can contribute to bradycardia and conduction abnormalities

Infectious Causes

• Lyme disease: Can cause high-grade AV block, particularly in young patients in endemic areas
• Other infections: Systemic infections can cause transient bradycardia

Medication-Related Causes

• Beta-blockers, calcium channel blockers, digoxin, and antiarrhythmic drugs are common culprits

Testing Algorithm Based on Presentation

For Symptomatic Bradycardia (HR <40 bpm)

1. Immediate testing: Complete electrolytes, renal function, thyroid panel
2. Based on history: Lyme titer, drug levels, arterial blood gas
3. If structural heart disease suspected: Consider cardiac imaging (echocardiography)

For Asymptomatic Bradycardia

• More selective testing based on specific clinical suspicion
• Routine cardiac imaging is not indicated for asymptomatic sinus bradycardia or first-degree AV block without clinical evidence of structural heart disease 1

Common Pitfalls to Avoid

• Missing hyperkalemia: A potentially life-threatening and readily reversible cause
• Overlooking hypothyroidism: May present subtly but is an important reversible cause
• Neglecting drug effects: Always review medications thoroughly
• Failing to consider Lyme carditis: Particularly in young patients with new high-grade AV block in endemic areas

Special Considerations

• Genetic testing: Not routinely indicated for most bradycardia cases, but should be considered in patients with inherited conduction disease to facilitate cascade screening of relatives 1
• Sleep apnea evaluation: Recommended in patients with documented or suspected bradycardia during sleep 1

By following this targeted approach to laboratory testing, clinicians can efficiently identify reversible causes of bradycardia and avoid unnecessary testing in patients with physiologic or chronic bradycardia.