Risk Factors for First-Degree AV Block
First-degree AV block is caused by medications that slow AV nodal conduction (beta-blockers, non-dihydropyridine calcium channel blockers, digoxin, antiarrhythmics), structural heart disease (myocardial infarction, congenital heart disease), infectious diseases (Lyme disease), infiltrative diseases (sarcoidosis, amyloidosis), electrolyte disturbances, and neuromuscular diseases. 1, 2
Medication-Related Risk Factors
Beta-blockers are a major cause of first-degree AV block by slowing AV nodal conduction, and patients with pre-existing first-degree AV block may be at increased risk for severe bradycardia when receiving metoprolol or other beta-blocking agents 1, 2
Non-dihydropyridine calcium channel blockers (verapamil, diltiazem) slow AV conduction and can precipitate or worsen first-degree AV block 1
Digoxin and antiarrhythmic medications commonly cause first-degree AV block through their effects on the AV node 1
Amiodarone should be used with caution as it slows AV conduction 3
Structural and Ischemic Heart Disease
Myocardial infarction, particularly inferior wall MI, is commonly associated with first-degree AV block, with the site of AV delay usually located above the bundle of His 3
Congenital heart disease is a significant risk factor, with first-degree AV block occurring in conditions such as repaired tetralogy of Fallot, ventricular septal defects, and congenitally corrected transposition of the great arteries 3, 1
Structural heart disease of any etiology increases risk, and patients with evidence of structural abnormalities should be considered for more intensive monitoring 1, 4
Infectious and Infiltrative Diseases
Lyme disease can affect the cardiac conduction system and cause first-degree AV block 1
Infiltrative diseases including sarcoidosis and amyloidosis can damage the conduction system and lead to AV block 1
Neuromuscular Diseases
- Myotonic dystrophy and Kearns-Sayre syndrome are associated with unpredictable progression of conduction disease, and patients with these conditions require close monitoring even with first-degree AV block 1, 4
Electrolyte and Metabolic Disturbances
- Electrolyte abnormalities can precipitate first-degree AV block and should be identified and corrected in symptomatic patients 1
Pregnancy-Related Considerations
Pregnancy itself can unmask first-degree AV block in the absence of underlying heart disease, though it typically has a favorable outcome and the site of delay is usually above the bundle of His without progression to complete heart block 3
Thirty percent of congenital AV blocks remain undiscovered until adulthood and may present during pregnancy due to increased hemodynamic demands 3
Common Pitfalls
Do not assume all first-degree AV block is benign—PR intervals ≥0.30 seconds can cause symptoms similar to pacemaker syndrome due to inadequate timing of atrial and ventricular contractions 1, 4, 5
Coexisting bifascicular block with first-degree AV block increases risk of progression to higher-degree block and warrants closer monitoring 4
Recent evidence suggests that first-degree AV block may be a risk marker for more severe intermittent conduction disease, with up to 40% of monitored patients showing progression to higher-grade block requiring pacemaker implantation 6