Third degree AV block pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Cafer Zorkun, M.D., Ph.D. [2]; Raviteja Guddeti, M.B.B.S. [3]

Pathophysiology

• In complete heart block because the impulse is blocked, an accessory pacemaker below the level of the block will typically activate the ventricles. This is known as an escape rhythm. Since this accessory pacemaker activates independently of the impulse generated at the SA node, two independent rhythms can be noted on the electrocardiogram (EKG).
  • One will activate the atria and create the P waves, typically with a regular P to P interval.
  • The second will activate the ventricles and produce the QRS complex, typically with a regular R to R interval. The PR interval will be variable, as the hallmark of complete heart block is no apparent relationship between P waves and QRS complexes.

Morphology of the QRS complex helps in determining the location at which the escape rhythms are occurring.

• If the site of complete heart block is at the level of AV node, two-thirds of the escape rhythms have a narrow QRS complex.
• If the site of block is the His bundle, typically a narrow QRS complex is seen.
• Patients with trifascicular block have a wide QRS complex (seen in 80% of the cases).

In short, if escape rhythm has a narrow QRS complex the level of block can be either AV node or His bundle and if the QRS duration is prolonged the level of block is in the fascicles or bundle branches.

Complete Heart Block in Myocardial Infarction

• An inferior wall myocardial infarction may cause damage to the AV node, causing third degree heart block. In this case, the damage is usually transitory, and the AV node may recover. Studies have shown that third degree heart block in the setting of an inferior wall myocardial infarction typically resolves within 2 weeks. The escape rhythm typically originates in the AV junction, producing a narrow complex escape rhythm.
• An anterior wall myocardial infarction may damage the distal conduction system of the heart, causing third degree heart block. This is typically extensive, permanent damage to the conduction system, necessitating a permanent pacemaker to be placed. The escape rhythm typically originates in the ventricles, producing a wide complex escape rhythm.

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