Causes of Prolonged PR Interval (First-Degree AV Block)
The most common causes of prolonged PR interval include medications (especially beta-blockers, calcium channel blockers, and digoxin), increased vagal tone, structural heart disease, conduction system fibrosis, and electrolyte abnormalities. 1
Definition and Characteristics
- First-degree AV block is defined as PR interval prolongation beyond 0.20 seconds (200 ms) 1
- Represents a delay in conduction through the AV conduction system, most commonly at the AV node
- May occur transiently or become permanent
Common Causes
Medications
- Cardiac medications:
- Digoxin (can cause PR prolongation at therapeutic doses) 2
- Beta-blockers (metoprolol, propranolol, etc.)
- Calcium channel blockers (verapamil, diltiazem)
- Antiarrhythmic drugs (amiodarone, propafenone)
Physiological Factors
- Increased vagal tone (common in athletes and during sleep)
- Aging (degenerative changes in the conduction system)
- Electrolyte abnormalities (hyperkalemia, hypercalcemia)
Cardiac Conditions
- Coronary artery disease (CAD), especially affecting AV nodal blood supply 1
- Acute myocardial infarction (particularly inferior MI affecting AV node) 1
- Myocarditis or other inflammatory cardiac disorders 1
- Cardiomyopathies (hypertrophic, dilated, infiltrative)
- Congenital heart disease
- Valvular heart disease (especially aortic valve disease)
Degenerative Conditions
- Fibrosis of the conduction system (Lev's disease, Lenègre's disease)
- Age-related sclerosis of the cardiac skeleton
Anatomical Variations
- Intra-atrial conduction delay (accounts for 39-63% of first-degree AV block in patients with atrial fibrillation or flutter) 3
Clinical Significance
While first-degree AV block was historically considered benign, recent evidence suggests it may have important clinical implications:
- Associated with increased risk of atrial fibrillation (2-fold), pacemaker implantation (3-fold), and all-cause mortality (1.4-fold) 4
- Each 20 ms increase in PR interval is associated with 11% increased risk of atrial fibrillation 4
- Extreme PR prolongation (>300 ms) can cause symptoms similar to pacemaker syndrome due to poor AV synchrony 5, 6
Progression Risk
- First-degree AV block with bifascicular block carries increased risk of progression to high-grade AV block 1
- PR interval >300 ms may warrant closer monitoring due to higher risk of progression 6
- Presence of structural heart disease increases risk of progression to higher-degree AV block 7
Special Considerations
- In patients with atrial fibrillation/flutter, a significant proportion of first-degree AV block is due to intra-atrial conduction delay rather than AV nodal disease 3
- First-degree AV block that occurs during exercise or recovery phase may indicate underlying conduction system disease 1
- Medications that prolong AV conduction should be used cautiously in patients with pre-existing first-degree AV block 2
Diagnostic Approach
When evaluating a patient with first-degree AV block, consider:
- Medication review (identify potential causative agents)
- Evaluation for structural heart disease
- Assessment of electrolyte abnormalities
- Consideration of exercise testing to evaluate dynamic changes in AV conduction
- Evaluation for symptoms of hemodynamic compromise in cases of extreme PR prolongation
First-degree AV block may be the first sign of more serious conduction system disease and warrants appropriate clinical evaluation, especially when the PR interval is markedly prolonged or associated with other conduction abnormalities.