23-23: Trifascicular Block

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.

Pearls

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.