What is the workup for a patient with 1st degree Atrioventricular (AV) block?

Workup for First-Degree AV Block

First-degree AV block generally requires minimal workup in asymptomatic patients without structural heart disease, but a focused evaluation should be performed to identify underlying causes and assess risk of progression to higher-grade blocks. 1

Initial Assessment

• 12-lead ECG: Confirm first-degree AV block (PR interval >0.20 seconds) 1

• History: Focus on:

  • Symptoms (syncope, pre-syncope, fatigue, exercise intolerance)
  • Medication use (beta-blockers, calcium channel blockers, digoxin, antiarrhythmics)
  • Underlying conditions (coronary artery disease, cardiomyopathy, myocarditis)
  • Family history of conduction disorders or sudden cardiac death

• Physical examination: Assess for signs of structural heart disease

Risk Stratification

Low Risk (Minimal Workup Needed)

• Asymptomatic patients with:
  • PR interval <0.30 seconds
  • Normal QRS duration
  • No structural heart disease
  • No concerning symptoms

Higher Risk (More Extensive Workup Needed)

• PR interval ≥0.30 seconds (profound first-degree AV block) 1, 2
• Presence of symptoms (syncope, pre-syncope)
• Abnormal QRS complex
• Known structural heart disease
• Evidence of progression to higher-grade block

Additional Testing Based on Risk

For Higher Risk Patients:

1. Echocardiogram: To assess for structural heart disease 1

2. Exercise stress test: Particularly important when:

   • Patient reports exercise-related symptoms
   • Need to assess if PR interval shortens appropriately with exercise
   • To detect exercise-induced higher-degree AV block 1, 3

3. 24-hour Holter monitoring or extended ambulatory monitoring: To detect:

   • Intermittent progression to higher-degree AV block
   • Bradycardia episodes
   • Other arrhythmias 4

4. Laboratory tests: To identify reversible causes

   • Electrolytes (particularly potassium, magnesium)
   • Thyroid function tests
   • Drug levels if applicable (digoxin)

5. Electrophysiologic study (EPS): Consider only in specific situations:

   • Symptomatic patients where the relationship between symptoms and AV block is unclear
   • When type II second-degree AV block is suspected
   • When knowledge of the site of block may guide therapy 1

Important Considerations

• First-degree AV block is not entirely benign as previously thought; recent evidence shows association with increased risk of progression to higher-grade blocks, heart failure hospitalization, and mortality in patients with coronary artery disease 4, 5

• Insertable cardiac monitors have revealed that 40.5% of patients with first-degree AV block either progress to higher-grade block or have intermittent more severe bradycardia requiring pacemaker implantation 4

• Profound first-degree AV block (PR interval ≥0.30 seconds) may cause symptoms similar to pacemaker syndrome due to loss of AV synchrony and requires more thorough evaluation 2, 3

• Athletes may have first-degree AV block as a normal finding related to high vagal tone, but profound first-degree AV block or progression to Mobitz type I warrants cardiology consultation 6

When to Consider Referral to Cardiology

• PR interval ≥0.30 seconds
• Symptomatic patients
• Evidence of progression to higher-degree block
• Presence of structural heart disease
• Abnormal QRS complex

Remember that first-degree AV block is often drug-related and reversible, but can also be a marker for more advanced conduction system disease requiring vigilant monitoring and follow-up.