Lecture Notes: Heart Block and the Cardiac Conduction System

The Heart's Electrical System

• Sinoatrial Node (SA Node):
  • Located in the right atrium.
  • Initiates the electrical impulse.
• Impulse Pathway:
  • Travels through atria, causing atrial contraction.
  • Moves via Bachmann's bundle to the left atrium.
  • Reaches Atrioventricular Node (AV Node) - impulse is momentarily delayed.
  • Proceeds to the Bundle of His, divides into left and right bundle branches.
  • Ends at Purkinje fibers, causing ventricular contraction.
• Left Bundle Branches:
  • Divides into anterior, posterior, and sometimes septal fascicles.
• Right Bundle Branch:
  • No further division.

Types of Heart Block

First Degree AV Block

• Definition:
  • Delay through the AV node with PR interval > 200 ms.
• Causes:
  • Increased vagal tone (younger patients, athletes), fibrosis (elderly).
  • Drugs (beta blockers, calcium channel blockers, digoxin).
  • Coronary artery disease, mitral valve surgery, electrolyte imbalances.
• Symptoms and Treatment:
  • Often asymptomatic; no treatment needed.
  • Increased risk of atrial fibrillation; follow-up recommended.
  • Pacemakers considered if symptomatic and PR intervals > 300 ms.

Second Degree AV Block

• Mobitz 1 (Wenkebach):

  • Characteristics:
    • Progressive PR lengthening, eventual non-conducted beat.
    • Cyclical (e.g., 4:3 P waves to QRS complexes).
  • Causes:
    • High vagal tone, normal variant, inferior myocardial ischemia.
    • AV blocking agents, mitral valve surgery, hyperkalemia.
  • Treatment:
    • Often not needed; atropine for bradycardia/hypotension.
    • Reduce AV blocking drugs; pacing if atropine ineffective.

• Mobitz 2 (Hay Block):

  • Characteristics:
    • Intermittent non-conducted P wave without progressive PR lengthening.
    • Fixed P:QRS ratio (e.g., 2:1).
  • Causes:
    • Structural heart disease, myocardial ischemia, fibrosis.
    • 75% below Bundle of His (broad QRS), 25% in the bundle (narrow QRS).
  • Symptoms and Treatment:
    • Often symptomatic: syncope, fatigue, chest pain, risk of sudden death.
    • High progression to complete block.
    • Requires pacing; atropine can worsen condition to 3rd degree block.

Third Degree AV Block (Complete Heart Block)

• Characteristics:
  • No association between atrial and ventricular contractions.
  • Occurrence of junctional/ventricular escape rhythm.
  • ECG: More P waves than QRS complexes.
• Causes:
  • End stage of second degree block.
  • Inferior myocardial infarction, AV blocking agents, conduction system degeneration.
• Symptoms and Treatment:
  • Syncope, fatigue, chest pain, shortness of breath, risk of sudden death.
  • Atropine rarely effective; dopamine and adrenaline may be used.
  • Requires transcutaneous/transvenous pacing before permanent pacemaker.

Key Takeaways

• Follow up: Essential for patients with first-degree block due to risk of atrial fibrillation.
• Pacing: Often necessary for Mobitz 2 and third degree block due to symptomatic risk and progression.
• Medications: Watch for AV node blocking agents; careful with atropine in Mobitz 2.