Postinfarction conduction abnormalities

Ischemia of the conduction system results in conduction abnormalities of the heart. Atrioventricular blocks are common in inferior infarcts and intraventricular blocks are common in anterior wall infarcts.

Conduction abnormalities post-infarction can be classified as atrioventricular blocks and intraventricular blocks.

• Atriventricular blocks:
  • Second degree blocks (Mobitz type I and II)
  • Third degree AV block

• Intraventricular block:
  • Bundle branch blocks
  • Bifascicular blocks

These blocks can be temporary or persistent.

• High grade AV blocks (second and third degree blocks) occur in up to 20% of patients with inferior MI.
• AV blocks are common in inferior infarcts than in anterior infarcts by three fold. The frequency is even more when the right ventricle is involved in the inferior infarcts than when not involved.
• In majority of inferior infarcts the blocks are transient.
• Presence of Av blocks in anterior infarcts indicates a larger infarction and there is a significant increase in short term mortality compared to patients without AV block.
• Mechanisms on AV block include:
  • In inferior MI:
    • Increased parasympathetic tone
    • Ischemic stunning of AV node
    • Increased local potassium due to infarction
    • Increased local release of adenosine

• Bundle branch blocks and fascicular blocks are markers for larger infarctions.
• Up to 22% of patients with new bundle branch block will progress to high grade AV block.
• New bifascicular block with or without PR prolongation has the highest likelihood of developing complete heart block.
• In approximately 25% of patients the conduction abnormalities are temporary.
• Mechanism of intraventricular blocks involves extensive necrosis of infra-His conduction system.

Patients with transient blocks have similar mortality compared to patients without blocks. Mortality rates for patients with bundle branch blocks (BBB) are significantly increased and patients who develop BBB in hospital have higher mortality rates than those who present with BBB.

• Transcutaneous:
  • New bundle branch block (BBB)
  • RBBB or LBBB with first degree AV block

• Transvenous:
  • Mobitz type II block
  • Alternating BBB
  • Bifascicular block with first degree AV block

• Permanent:
  • Persistent second degree AV block with bilateral bundle branch block
  • Complete heart block
  • Transient second degree or third degree block with bundle branch block
  • Symptomatic second or third degree AV block

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