Infra-Hisian Block

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor-In-Chief: Sara Mohsin, M.D.[2]

Overview

Infra-Hisian blocks are defined as impaired conduction in the electrical system of the heart that occur below the AV node.

Classification

Infrahisian block describes block of the distal conduction system. Types of infrahisian block include:

• Type 2 second degree heart block (Mobitz II)
• Left bundle branch block
  • Left anterior fascicular block
  • Left posterior fascicular block
• Right bundle branch block

Of these types of infrahisian block, Mobitz II heart block is considered most important because of the possible progression to complete heart block.

Pathophysiology

Mobitz type II second degree AV block Mobitz type II second degree AV block, in which the PR interval remains unchanged prior to a P wave that fails to conduct to the ventricles. It almost always results from conduction system disease below the level of the AV node, occurring in the bundle of His in approximately 20 percent of cases and in the bundle branches in the remainder. Patients with bundle branch involvement also have axis shifts and QRS widening depending upon the location of the block. In addition, at least two-thirds of patients with this disorder also have bifascicular or even trifascicular disease. Mobitz type I and Mobitz type II second degree AV block cannot be differentiated from the ECG when 2:1 AV block is present. In this situation, every other P wave is non-conducted and there is no opportunity to observe for the constant PR interval that is characteristic of Mobitz type II second degree AV block.

Causes

The potential etiologies of Mobitz type II second degree AV block include reversible (both pathologic and iatrogenic) and idiopathic causes that are similar to other degrees of AV block (table 1). Common potentially reversible causes include:

●Pathologic – Myocardial ischemia (acute or chronic) involving the conduction system, cardiomyopathy (eg, amyloidosis, sarcoidosis), myocarditis (eg, Lyme disease), endocarditis with abscess formation, hyperkalemia, and hypervagotonia.

●Iatrogenic – Medication-related (AV nodal blocking medications), post-cardiac surgery, post-catheter ablation, post-transcatheter aortic valve implantation.

Mobitz type II second degree AV block is rarely seen in patients without underlying heart disease. When identifiable, the reversible causes most commonly associated with Mobitz type II second degree AV block are myocardial infarction with ischemia of the AV node and medications that alter conduction through the AV node (eg, digoxin, beta blockers, calcium channel blockers). When no specific reversible cause is identified, the block is often felt to be related to idiopathic progressive cardiac conduction disease with myocardial fibrosis and/or sclerosis that affects the conduction system.

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