What is the definition and treatment of a 1st degree atrioventricular (AV) block?

First-Degree AV Block: Definition and Management

Definition

First-degree AV block is defined as prolongation of the PR interval beyond 200 milliseconds on ECG, where every atrial impulse is conducted to the ventricles but with delayed conduction. 1, 2

• The American College of Cardiology notes this is more accurately termed "first-degree AV delay" rather than true block, since all impulses ultimately conduct to the ventricles 2, 3
• The conduction delay may occur at the AV node level or within the His-Purkinje system 1, 2
• When the QRS complex is narrow, the delay is usually in the AV node; when wide, it may be either nodal or infranodal, requiring His bundle electrogram for precise localization 1

Clinical Significance and Risk Stratification

Most isolated first-degree AV block is benign, but profound prolongation (PR >300 ms) can cause hemodynamic symptoms similar to pacemaker syndrome. 1, 3

• Marked first-degree AV block causes atrial contraction to occur before complete atrial filling, compromising ventricular filling and increasing pulmonary capillary wedge pressure while decreasing cardiac output 1
• In patients with stable coronary artery disease or heart failure, first-degree AV block is associated with increased risk of heart failure hospitalization, cardiovascular mortality, and all-cause mortality 3, 4
• Research shows that 40.5% of patients with first-degree AV block may progress to higher-grade block or develop more severe bradycardia requiring pacemaker implantation 5

Treatment Algorithm

For Asymptomatic Patients with PR <300 ms:

• No treatment is required and permanent pacemaker implantation is NOT indicated (Class III recommendation). 1, 3, 6
• Regular follow-up with routine ECG monitoring is sufficient if QRS duration is normal 3
• Athletes can participate in all competitive sports unless excluded by underlying structural heart disease 3

For Patients with PR ≥300 ms or Symptoms:

Evaluate for:

• Symptoms of fatigue, exercise intolerance, or pacemaker syndrome-like symptoms (dyspnea, presyncope) 3, 6
• Signs of hemodynamic compromise including hypotension or increased wedge pressure 6
• Structural heart disease via echocardiography if QRS is abnormal 3, 6

Diagnostic testing should include:

• 24-48 hour ambulatory monitoring to detect intermittent progression to higher-grade block 3
• Exercise stress testing to assess whether PR interval shortens appropriately (normal response) or worsens (suggests infranodal disease) 3, 6

Indications for Permanent Pacing:

• Permanent pacemaker implantation is reasonable (Class IIa) for symptomatic patients with PR >300 ms causing hemodynamic compromise or pacemaker syndrome-like symptoms. 1, 3, 6
• Small uncontrolled trials suggest symptomatic and functional improvement with pacing in these patients 1
• Consider biventricular pacing in patients with left ventricular dysfunction and heart failure symptoms rather than conventional right ventricular pacing 3

High-Risk Features Requiring Close Monitoring

Refer to cardiology for:

• Coexisting bundle branch block or bifascicular block (significantly increases risk of progression to complete heart block) 3
• Neuromuscular diseases including myotonic dystrophy, Kearns-Sayre syndrome, or Emery-Dreifuss muscular dystrophy (unpredictable progression risk) 3, 6
• Evidence of structural heart disease 3, 6
• Exercise-induced worsening of AV block (indicates His-Purkinje disease with poor prognosis) 6

Reversible Causes to Address

Identify and treat underlying causes before considering permanent pacing:

• Medications: beta-blockers, non-dihydropyridine calcium channel blockers (verapamil, diltiazem), digoxin, amiodarone, and antiarrhythmic drugs 3, 6
• Electrolyte abnormalities, particularly potassium and magnesium 6
• Acute myocardial infarction (particularly inferior MI) 1, 6
• Infectious diseases (Lyme disease) and infiltrative diseases (sarcoidosis, amyloidosis) 6

Critical Pitfalls to Avoid

• Do NOT implant pacemakers for isolated, asymptomatic first-degree AV block—this is a Class III recommendation (not indicated). 3, 6
• Do NOT use atropine doses <0.5 mg, as this may paradoxically slow heart rate further 6
• Exercise caution with AV nodal blocking agents in patients with pre-existing first-degree AV block 3
• Recognize that bifascicular block with first-degree AV block is high-risk and can progress to complete heart block, particularly during anesthesia or acute illness 3
• AV block during sleep apnea is reversible and does not require pacing unless symptomatic 6