What is the management of second-degree atrioventricular (AV) block?

Management of Second-Degree Atrioventricular Block

Permanent pacemaker implantation is recommended for all patients with acquired second-degree Mobitz type II atrioventricular block, high-grade atrioventricular block, or third-degree atrioventricular block not attributable to reversible causes, regardless of symptoms. 1

Classification and Assessment

Second-degree AV block is classified into two main types:

1. Mobitz Type I (Wenckebach):

   • Progressive PR interval prolongation before a blocked P wave
   • Usually located in the AV node
   • Generally has better prognosis
   • Responds to atropine

2. Mobitz Type II:

   • Sudden block of P wave without PR prolongation
   • Usually located below the AV node (infranodal)
   • Higher risk of progression to complete heart block
   • Does not respond well to atropine

3. 2:1 AV Block:

   • Cannot be classified as Type I or II based on ECG alone
   • Requires additional testing to determine location

Initial Management Algorithm

1. Assess for Symptoms and Hemodynamic Stability

• If symptomatic (lightheadedness, syncope, dyspnea, chest pain) or hemodynamically unstable:
  • Provide immediate intervention

2. Acute Pharmacological Management

• For symptomatic bradycardia:
  • Atropine: 0.5-1 mg IV every 3-5 minutes, maximum 3 mg 2
  • Important: Atropine is ineffective for infranodal AV block (including most Mobitz type II) 3
  • If atropine ineffective, consider:
    • Epinephrine 2-10 μg/min IV infusion
    • Dopamine 2-10 μg/kg/min IV infusion 2

3. Temporary Pacing for Unstable Patients

• Transcutaneous pacing: For persistent symptomatic bradycardia refractory to medical therapy 2
• Transvenous temporary pacing: If symptoms or hemodynamic compromise persist despite transcutaneous pacing 2

Definitive Management Based on AV Block Type

For Mobitz Type I (Wenckebach):

• If asymptomatic: Generally observation is appropriate
• If symptomatic:
  • Ambulatory electrocardiographic monitoring to establish correlation between symptoms and rhythm abnormalities 1
  • Exercise testing for patients with exertional symptoms 1
  • Permanent pacing if symptoms correlate with bradycardia 1

For Mobitz Type II:

• Permanent pacing is recommended regardless of symptoms 1
• Higher risk of sudden progression to complete heart block 4
• All correctly defined type II blocks are infranodal and require pacing 4

For 2:1 AV Block:

• Cannot be classified as Type I or II based on ECG alone
• Consider:
  • Electrophysiological study to determine level of block 1
  • Carotid sinus massage and/or pharmacological challenge with atropine, isoproterenol, or procainamide 1
• If determined to be infranodal (especially with wide QRS), permanent pacing is recommended 2

Special Considerations

Potentially Reversible Causes

• Identify and treat potentially reversible causes:

  • Medication effects (beta-blockers, calcium channel blockers, digoxin)
  • Electrolyte abnormalities
  • Acute myocardial ischemia/infarction
  • Increased vagal tone 2

• Important: If AV block persists despite treatment of reversible cause, permanent pacing is recommended 1

QRS Width as a Prognostic Indicator

• Wide QRS: Suggests infranodal block with higher risk of progression to complete heart block
• Narrow QRS: More likely to be AV nodal (better prognosis) but can still be infranodal in 60-70% of cases with bundle branch block 4

Perioperative Considerations

• Patients with second-degree AV block may progress to complete AV block during anesthesia
• Consider temporary pacemaker placement before surgery, even in asymptomatic patients 5

Long-term Follow-up

• Regular evaluation of pacemaker function
• Monitoring for progression of conduction disease
• Avoidance of medications that can worsen AV block (digoxin, beta-blockers, calcium channel blockers) 2

Pitfalls and Caveats

• Misdiagnosis: Mobitz type II is commonly overdiagnosed; an unchanged PR interval after the block is essential for diagnosis 6
• Pseudo-AV Block: Concealed His bundle or ventricular extrasystoles may mimic both type I and type II block 4
• Natural History: Chronic second-degree AV nodal block (Mobitz type I) has a relatively benign course in patients without organic heart disease 7
• Vagal Surge: Can cause simultaneous sinus slowing and AV nodal block, which may superficially resemble type II block 6