What is the management of heart block, particularly second- or third-degree (complete) heart block?

Management of Heart Block: Second-Degree and Third-Degree Heart Block

For patients with symptomatic second-degree or third-degree atrioventricular block, temporary medical therapy followed by permanent pacing is the definitive management strategy to reduce mortality and improve quality of life. 1

Types of Heart Block

Heart blocks are classified based on the degree of conduction abnormality:

• First-degree AV block: PR interval >0.20 seconds (prolonged conduction)
• Second-degree AV block:
  • Type I (Mobitz I/Wenckebach): Progressive PR prolongation before a blocked beat
  • Type II (Mobitz II): Sudden block without PR prolongation
  • Advanced: Block of two or more consecutive P waves
• Third-degree AV block (Complete): No impulses pass between atria and ventricles

Assessment of Heart Block

Clinical Evaluation

• Assess for symptoms: syncope, pre-syncope, dizziness, fatigue, dyspnea, confusion
• Evaluate hemodynamic status: blood pressure, signs of heart failure
• Obtain 12-lead ECG to define the rhythm and type of block
• Consider the anatomic level of block (supra-His, intra-His, infra-His)

Risk Stratification

• Type II second-degree AV block carries worse prognosis than Type I 1
• Complete heart block with wide QRS indicates infranodal block with poorer prognosis
• Patients with syncope and complete heart block have improved survival with pacing 1

Acute Management Algorithm

1. Identify and Treat Reversible Causes

• Class I recommendation: Patients with transient/reversible causes (Lyme carditis, drug toxicity) should receive medical therapy and supportive care, including temporary pacing if necessary, before determining need for permanent pacing 1

2. Medical Therapy for Symptomatic Bradycardia

• For AV nodal block with symptoms/hemodynamic compromise:
  • Atropine is reasonable (Class IIa) to improve conduction and increase heart rate 1
  • For blocks refractory to atropine:
    • Beta-adrenergic agonists (isoproterenol, dopamine, dobutamine, epinephrine) may be considered (Class IIb) in patients with low likelihood of coronary ischemia 1
    • For inferior MI with AV block: IV aminophylline may be considered (Class IIb) 1

3. Temporary Pacing

• For symptomatic bradycardia refractory to medical therapy:
  • Temporary transvenous pacing is reasonable (Class IIa) 1
  • For prolonged temporary pacing, externalized permanent active fixation leads are preferred over standard temporary leads (Class IIa) 1
  • Transcutaneous pacing may be considered (Class IIb) until transvenous pacing is established 1

Permanent Pacing Indications

Definite Indications (Class I)

1. Third-degree and advanced second-degree AV block with:

   • Symptoms (including heart failure) or ventricular arrhythmias 1
   • Need for medications causing symptomatic bradycardia 1
   • Asystole ≥3.0 seconds or escape rate <40 bpm 1
   • AF with bradycardia and pauses ≥5 seconds 1

2. Second-degree AV block with symptomatic bradycardia regardless of type 1

3. Asymptomatic persistent third-degree AV block with:

   • Average ventricular rates ≥40 bpm if cardiomegaly/LV dysfunction present 1
   • Block below the AV node 1

4. Second/third-degree AV block during exercise (non-ischemic) 1

Special Scenarios

• Drug-induced AV block: About 28% of patients experience recurrence of AV block despite drug discontinuation, with approximately half requiring permanent pacing 2
• Post-MI AV block: Higher in-hospital mortality (RR 1.41), particularly with inferior MI receiving reperfusion therapy 3
• Cardiac sarcoidosis with AV block: Permanent pacing reasonable without waiting for reversibility (Class IIa) 1

Pitfalls and Caveats

1. Don't assume Type I second-degree AV block is always benign:

   • Can be infranodal even with narrow QRS
   • May require electrophysiological study to determine level of block 1

2. First-degree AV block is not entirely benign:

   • 40.5% of patients with first-degree AV block monitored with insertable cardiac monitors progressed to higher-grade block or developed bradycardia requiring pacemaker implantation 4
   • Extreme first-degree AV block (PR >0.30s) can cause pacemaker syndrome-like symptoms 5

3. Temporary pacing challenges:

   • Transcutaneous pacing can be painful and may have inconsistent capture
   • Transvenous temporary pacing carries risks of infection, lead dislodgement, and perforation

4. Drug-specific considerations:

   • Carvedilol-induced AV block typically resolves after discontinuation (21/24 cases)
   • Metoprolol-induced AV block often persists or recurs (24/36 cases) 2

By following this evidence-based approach to heart block management, clinicians can optimize outcomes by providing appropriate temporary support while determining the need for permanent pacing based on block characteristics, symptoms, and underlying etiology.