What are the causes and management of complete heart block?

Causes and Management of Complete Heart Block

Complete heart block is most commonly caused by degenerative fibrosis of the conduction system, myocardial ischemia/infarction, or iatrogenic factors following cardiac procedures. 1

Etiology

Complete heart block (CHB), also known as third-degree atrioventricular block, occurs when no atrial impulses reach the ventricles. The main causes include:

Common Causes:

• Degenerative fibrosis of the conduction system (age-related)
• Myocardial ischemia/infarction (seen in up to 8% of post-MI patients) 2
• Iatrogenic factors:
  • Cardiac surgery
  • Alcohol septal ablation for hypertrophic cardiomyopathy
  • Transcatheter aortic valve replacement
  • Endomyocardial biopsy
  • Ventricular septal defect complications 1

Other Causes:

• Cardiac disease:
  • Myocarditis
  • Infectious endocarditis
  • Infiltrative cardiac diseases
  • Non-ischemic cardiomyopathy 2
• Metabolic/Pharmacologic:
  • Electrolyte disturbances
  • Drug side effects (e.g., beta-blockers, calcium channel blockers, digoxin) 2
• Congenital AV blocks 2

Clinical Presentation

Patients with CHB may present with:

• Bradycardia (typically 20-60 bpm depending on escape rhythm)
• Fatigue
• Dizziness
• Syncope (Adams-Stokes attacks)
• Shortness of breath
• Chest pain
• Hypotension
• Altered mental status in severe cases

The severity of symptoms depends on:

• Ventricular rate
• Location of the escape rhythm
• Presence of underlying cardiac disease

Diagnosis

Diagnosis is made by ECG showing:

• Complete dissociation between P waves and QRS complexes
• Atrial rate faster than ventricular rate
• Regular ventricular rhythm (escape rhythm)
• QRS morphology:
  • Narrow QRS (40-60 bpm): AV nodal or high His-Purkinje escape
  • Wide QRS (20-40 bpm): Ventricular escape rhythm 2

Management

Acute Management:

• For hemodynamically unstable patients:
  • Transvenous temporary pacing for persistent instability 1
  • IV atropine (may be attempted but often ineffective)
  • Transcutaneous pacing as bridge to transvenous pacing 2

Definitive Management:

• Permanent pacemaker implantation is indicated for:
  • Symptomatic complete heart block
  • Complete heart block with wide QRS escape rhythm
  • Complete heart block with mean daytime heart rate below 50 bpm
  • Complete heart block with complex ventricular ectopy
  • Complete heart block with ventricular dysfunction 1

Special Considerations in Acute MI:

• In acute MI with complete heart block:
  • Temporary pacing is indicated if not responsive to medical therapy
  • Permanent pacing is indicated if high-degree AV block persists >72 hours 1
  • Higher risk of cardiogenic shock and mortality 1

Prognosis

• Prognosis varies based on underlying etiology and management
• Untreated CHB with Adams-Stokes attacks has poorer survival compared to asymptomatic CHB 3
• Complicating conditions like ischemic heart disease, hypertension, diabetes, or cardiac enlargement worsen prognosis 3
• Most patients with isolated CHB without structural heart disease who receive appropriate pacing can lead normal lives with good functional capacity 4

Special Populations

Pregnancy:

• Pregnant women with CHB require multidisciplinary management
• Asymptomatic patients with adequate heart rates (50-60 bpm) may be managed conservatively
• Temporary or permanent pacing may be required for symptomatic patients
• Careful anesthetic planning for delivery is essential 5

Clinical Pearls and Pitfalls

• Don't miss the diagnosis: Always consider CHB in patients with unexplained bradycardia
• Watch for deterioration: Patients with ventricular escape rhythms can rapidly destabilize
• Be prepared: Have transcutaneous pacing capability immediately available for unstable patients
• Consider underlying causes: Always investigate for reversible etiologies (ischemia, drug effects, electrolyte abnormalities)
• Monitor closely: Even asymptomatic patients require close observation as they may suddenly develop symptoms