What are the common causes of a first-degree AV block?

Common Causes of First-Degree AV Block

First-degree AV block is primarily caused by delayed conduction in the AV node due to medications, electrolyte abnormalities, structural heart disease, or increased vagal tone. 1

Definition and Characteristics

• First-degree AV block is defined as a prolongation of the PR interval beyond 0.20 seconds on ECG 1
• It represents a delay in conduction through the AV conduction system, typically at the level of the AV node 2
• Traditionally considered benign, especially when PR interval is between 0.20-0.30 seconds 1

Common Causes of First-Degree AV Block

Medications

• Drugs that slow AV nodal conduction are common causes 1:
  • Beta-blockers
  • Non-dihydropyridine calcium channel blockers (diltiazem, verapamil)
  • Digoxin
  • Antiarrhythmic medications (particularly class I and III)

Electrolyte Abnormalities

• Hyperkalemia can cause progressive AV conduction delay 1
• Hypokalemia may also affect cardiac conduction 1

Structural Heart Disease

• Myocardial infarction (particularly inferior MI affecting AV nodal blood supply) 1
• Myocarditis 1
• Cardiomyopathies 1
• Congenital heart disease 3
• Congenitally corrected transposition of the great arteries (can present with first-degree AV block with minimal other physical findings) 3

Increased Vagal Tone

• Common in well-trained athletes 3, 4
• May be more pronounced during sleep 3

Degenerative Conduction System Disease

• Age-related fibrosis and sclerosis of the conduction system 3
• May be a marker for more severe intermittent conduction disease 5

Other Causes

• Infectious diseases affecting the cardiac conduction system (e.g., Lyme disease) 3
• Infiltrative diseases (sarcoidosis, amyloidosis) 3
• Neuromuscular diseases 1
• Connective tissue disorders 1

Clinical Significance and Progression

• First-degree AV block was traditionally considered entirely benign, but recent evidence suggests it may be a marker for more severe conduction disease 5
• In a study of patients with first-degree AV block who received insertable cardiac monitors, 40.5% eventually required pacemaker implantation due to progression to higher-grade block or detection of more severe bradycardia 5
• Marked first-degree AV block (PR interval >0.30 seconds) can cause symptoms similar to pacemaker syndrome due to suboptimal timing of atrial and ventricular contractions 3, 2
• In athletes, profound first-degree AV block (PR interval >0.40 seconds) may require additional evaluation to rule out structural cardiac abnormalities 4

Evaluation Considerations

• Asymptomatic first-degree AV block with PR interval <0.30 seconds generally requires no specific treatment 3, 1
• For PR intervals ≥0.30 seconds, assess for symptoms similar to pacemaker syndrome (fatigue, exercise intolerance) 3, 1
• Consider echocardiography if there are signs of structural heart disease or if the QRS complex is abnormal 3
• Exercise testing may be helpful as the PR interval will typically shorten during exercise in benign cases 3

Management Implications

• Identify and treat reversible causes (medications, electrolyte abnormalities) 1
• Permanent pacing is reasonable for symptomatic patients with PR interval >0.30 seconds causing hemodynamic compromise 3, 1
• Patients with neuromuscular diseases and first-degree AV block may warrant closer monitoring due to unpredictable progression 1
• First-degree AV block may be a risk marker for future development of higher-grade AV block 5

Understanding these causes helps guide appropriate evaluation and management decisions for patients with first-degree AV block.