What is the initial workup and differential diagnosis for a patient presenting with bradycardia (abnormally slow heart rate)?

Bradycardia Workup and Differential Diagnosis

The initial workup for bradycardia should include a 12-lead ECG, assessment of vital signs, oxygen saturation, and evaluation for signs of hemodynamic instability, followed by identification and treatment of underlying causes. 1, 2

Definition and Clinical Presentation

• Bradycardia is defined as a heart rate <60 beats per minute, though clinically significant bradycardia typically presents with a heart rate <50 beats per minute 1
• Common symptoms include:
  • Syncope or near-syncope
  • Dizziness
  • Fatigue
  • Dyspnea
  • Chest pain
  • Altered mental status
  • Exercise intolerance

Initial Assessment

Immediate Evaluation

1. Assess airway, breathing, and circulation
2. Provide supplementary oxygen if hypoxemic
3. Establish IV access
4. Attach cardiac monitor
5. Obtain 12-lead ECG
6. Check vital signs including blood pressure and pulse oximetry

Key History Elements

• Timing, duration, and triggers of symptoms
• Medication review (beta-blockers, calcium channel blockers, digoxin, amiodarone)
• Recent illness or infection
• Family history of cardiac disease or sudden death
• Exercise habits (athletes often have physiologic bradycardia)
• Sleep patterns and symptoms of sleep apnea

Diagnostic Testing Algorithm

First-Line Testing

1. 12-lead ECG - Essential first diagnostic test to identify:

   • Type of bradyarrhythmia (sinus bradycardia, AV block, etc.)
   • Conduction abnormalities
   • Signs of ischemia or prior infarction
   • QT interval abnormalities

2. Laboratory studies:

   • Electrolytes (particularly potassium, magnesium, calcium)
   • Thyroid function tests
   • Complete blood count
   • Cardiac enzymes if ischemia suspected
   • Drug levels (digoxin, etc.) if applicable

Second-Line Testing (Based on Initial Findings)

1. For intermittent symptoms:

   • 24-48 hour Holter monitoring
   • Event recorders
   • Mobile cardiac telemetry

2. For infrequent symptoms (>30 days between episodes):

   • Implantable cardiac monitor if initial evaluation is nondiagnostic 1, 2

3. Additional testing as indicated:

   • Transthoracic echocardiography if structural heart disease suspected
   • Exercise electrocardiographic testing for exercise-related symptoms or suspected chronotropic incompetence
   • Sleep study if sleep apnea suspected
   • Electrophysiology study in selected patients when noninvasive evaluation is nondiagnostic 1

Differential Diagnosis

Physiologic Bradycardia

• Athletic conditioning
• Sleep-related bradycardia
• Vasovagal response

Sinus Node Dysfunction

• Sick sinus syndrome
• Sinoatrial exit block
• Sinus arrest
• Chronotropic incompetence

Atrioventricular Conduction Disorders

• First-degree AV block
• Second-degree AV block (Mobitz type I/Wenckebach or Mobitz type II)
• Third-degree (complete) AV block
• Bundle branch blocks

Medication-Induced Bradycardia

• Beta-blockers
• Non-dihydropyridine calcium channel blockers (diltiazem, verapamil)
• Digoxin
• Amiodarone
• Ivabradine
• Clonidine
• Lithium

Metabolic/Systemic Causes

• Hypothyroidism
• Electrolyte abnormalities (hyperkalemia, hypokalemia, hypercalcemia)
• Hypothermia
• Increased intracranial pressure
• Hypoxemia

Cardiac Causes

• Acute myocardial ischemia/infarction (especially inferior MI)
• Myocarditis
• Endocarditis
• Cardiac surgery complications
• Infiltrative diseases (amyloidosis, sarcoidosis)
• Congenital heart disease

Other Causes

• Obstructive sleep apnea 1
• Increased vagal tone
• Infectious diseases (Lyme disease, endocarditis)
• Neuromuscular disorders
• Trauma

Management Considerations

• Asymptomatic bradycardia generally does not require intervention 2
• For symptomatic patients in the acute setting, atropine is the first-line pharmacologic therapy 3, 4
• Atropine prevents or abolishes bradycardia by blocking vagal activity and may lessen the degree of partial heart block when vagal activity is a factor 3
• Approximately 20% of patients with compromising bradycardia may require temporary emergency pacing for initial stabilization 5
• About 50% of patients with iatrogenic or potentially reversible bradyarrhythmia may still need permanent pacemaker implantation despite correction of the underlying cause 6

Important Pitfalls to Avoid

1. Failure to identify reversible causes before considering permanent pacing
2. Overlooking medication-induced bradycardia - review all medications carefully
3. Missing sleep apnea as a potential cause - up to 59% of patients receiving pacemakers may have undiagnosed sleep apnea 1
4. Confusing pseudobradycardia (slow peripheral pulse due to frequent non-conducted atrial premature beats or ventricular bigeminy) with true bradycardia 7
5. Unnecessary pacemaker implantation in asymptomatic patients with bradycardia 2
6. Inadequate monitoring in patients with AV node disease, who have higher risk of recurrence even after addressing reversible causes 6

Remember that the correlation between symptoms and bradyarrhythmia is essential when determining the need for intervention, and observation may be the most appropriate approach for asymptomatic patients.