What does a first-degree atrioventricular (AV) block on an electrocardiogram (EKG) mean?

What Does First-Degree AV Block on EKG Mean?

First-degree AV block is a misnomer—it represents a delay in conduction through the AV node rather than true blockage, defined as a PR interval >200 ms where every P wave is conducted to the ventricles. 1

Definition and ECG Characteristics

• PR interval is prolonged beyond 0.20 seconds (200 ms) with all atrial impulses successfully conducting to the ventricles 1
• The term "first-degree AV block" is technically inaccurate since no actual blockage occurs—more accurately termed "first-degree AV delay" 1
• The conduction delay typically occurs at the AV node level when the QRS complex is narrow 1
• When the QRS is wide, the delay may be in either the AV node or His-Purkinje system, requiring His bundle electrogram for precise localization 1

Clinical Significance and Prognosis

Most cases of isolated first-degree AV block are benign and have excellent prognosis, particularly when the PR interval is <300 ms 2, 3

However, emerging evidence challenges the universally benign view:

• Prolonged PR intervals are associated with increased risk of atrial fibrillation (RR 1.45), heart failure (RR 1.39), and all-cause mortality (RR 1.24) in population studies 4
• PR intervals >300 ms can cause symptoms resembling "pacemaker syndrome" due to loss of AV synchrony, resulting in decreased cardiac output and increased pulmonary capillary wedge pressure 1, 5
• Atrial contraction occurs before complete atrial filling when PR is markedly prolonged, compromising ventricular filling 1

When First-Degree AV Block Requires Attention

Symptomatic Patients (PR >300 ms)

• Permanent pacemaker implantation is reasonable (Class IIa) for patients with symptoms similar to pacemaker syndrome or hemodynamic compromise 1, 5
• Symptoms include fatigue, exercise intolerance, exertional dyspnea, or dizziness directly attributable to the prolonged PR interval 1, 2

Asymptomatic Patients

• No treatment is required for asymptomatic first-degree AV block with PR <300 ms 2, 5
• Permanent pacemaker implantation is NOT indicated for asymptomatic first-degree AV block 2, 5
• Regular follow-up with routine ECG monitoring is sufficient when QRS duration is normal 2

Evaluation Algorithm

For PR interval <300 ms and normal QRS:

• No further testing typically required 2
• Regular clinical follow-up with periodic ECG monitoring 2

For PR interval ≥300 ms or abnormal QRS:

• Echocardiogram to rule out structural heart disease 2, 5
• Exercise stress test to assess if PR interval shortens appropriately during exercise (should shorten in benign cases) 5
• 24-hour ambulatory monitoring to detect progression to higher-degree block 2

Important Clinical Pitfalls

Exercise-induced worsening of AV block (not due to ischemia) indicates His-Purkinje disease with poor prognosis and warrants pacing, even if baseline ECG shows only first-degree block 5

Monitor closely for progression in specific populations:

• Patients with coexisting bifascicular block 2, 5
• Neuromuscular diseases (myotonic dystrophy, Kearns-Sayre syndrome) due to unpredictable progression 1, 2, 5
• Structural heart disease or infiltrative diseases (sarcoidosis, amyloidosis) 5

Common Causes to Evaluate

Reversible causes that should be identified:

• Medications: beta-blockers, non-dihydropyridine calcium channel blockers (verapamil, diltiazem), digoxin, antiarrhythmic drugs 1, 5
• Electrolyte abnormalities, particularly potassium and magnesium 5
• Increased vagal tone (sleep, high-level athletic conditioning) 1
• Acute myocardial infarction, particularly inferior wall MI 5

Structural and infiltrative causes:

• Lyme carditis, bacterial endocarditis with perivalvar abscess 1
• Cardiac sarcoidosis, amyloidosis, myocarditis 1, 5
• Congenital heart disease (corrected transposition of great arteries) 5

Athletic Participation

Athletes with asymptomatic first-degree AV block can participate in all competitive sports unless excluded by underlying structural heart disease 2