Test Findings
 

Trifascicular Block

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Overview

  • When the right bundle branch and the left bundle branch (or both left-sided fascicles) are blocked, complete heart block occurs. However, several patterns suggest ‘incomplete' or ‘impending' trifascicular block.
    • Look For

    • Bifascicular block (RBBB and LAFB or LPFB) with first degree AV block
    • Alternating LBBB and RBBB
    • Fixed RBBB with alternating LAFB and LPFB
    • Complete heart block

Bifascicular Block with 1st Degree AV block

  • The combination of right bundle branch block, left anterior or posterior fascicular block and first degree AV block is often referred to as trifascicular block.
  • It is difficult to determine from ECG whether this pattern represents true bifascicular block and impending failure of the remaining fascicle, or whether coexistant bifascicular block and first degree AV block are two distinct pathologies.
    • Look For

    • Right bundle branch block - wide QRS (≥120ms), RSR’ pattern in V1 or V2, slurred S wave in I, V5, V6
    • Either left or right axis deviation
    • First degree AV block - prolonged PR interval >200ms

Alternating Bundle Branch Block

  • The presence of alternating bundle branch block suggests that a patient is at high risk of developing complete heart block.
    • Look For

    • Left and right bundle branch block in one patient, either on separate ECGs or even alternating on the same ECG.
    • Alternating Bundle Branch Block
       
    • Alternating Bundle Branch Block
       

RBBB with Alternating Fascicular Block

  • Fixed right bundle branch block with alternating left anterior and left posterior fascicular block suggests that a patient is at high risk of developing complete heart block.
    • Look For

    • Right bundle branch block
    • Left axis deviation and right axis deviation in one patient, either on separate ECGs or even alternating on the same ECG

Complete Trifascicular Block

  • When there is block of the right bundle branch and either the left bundle branch or both left anterior and posterior fascicles, complete AV block occurs.
    • Complete Trifascicular Block
       
    • Look For

    • Complete AV block, as manifested by complete dissociation of P waves and QRS complexes.
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