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ECG Findings

  • RBBB with left anterior fascicular block and/or left posterior fascicular block and first- or second-degree AV block.

  • Somewhat of a misnomer, this is not a true blocking of three fascicles; rather, two fascicles are blocked coupled with PR interval prolongation.

  • LBBB plus first-degree AV block is sometimes referred to as trifascicular block as well, but again, only two fascicles are truly blocked in this scenario.


  1. A complete trifascicular block, which includes involvement of all three fascicles, is actually a complete or third-degree heart block. An incomplete trifascicular block technically is a bifascicular block (plus AV block) and may be a precursor to complete heart block. Observation and pacemaker evaluation are warranted, especially if symptomatic.

  2. AV-blocking agents can potentiate degree of block.

  3. Trifascicular block is usually indicative of primary advanced conduction system abnormalities secondary to coronary artery disease.


Trifascicular Block. (ECG contributor: R. Jason Thurman, MD.)


The QRS is wide with RBBB pattern (blue double arrow). Left anterior fascicular block is present (diagonal arrows) along with a prolonged PR interval and first-degree AV block (black double arrow).

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