No, CRBBB with First-Degree AV Block Alone is NOT a Bifascicular Block
According to ACC/AHA/HRS guidelines, bifascicular block refers specifically to ECG evidence of impaired conduction below the AV node in BOTH the right and left bundles—meaning complete right bundle branch block (CRBBB) must be combined with EITHER left anterior fascicular block OR left posterior fascicular block. 1, 2 First-degree AV block represents delay at the AV node level, not fascicular involvement, and therefore does not constitute the second "fascicle" required for bifascicular block.
Understanding the Terminology
Bifascicular block requires:
- CRBBB (affecting the right bundle), PLUS
- Left anterior fascicular block (LAFB) OR left posterior fascicular block (LPFB)
Alternatively, isolated left bundle branch block (LBBB) is also considered bifascicular block since it affects both the anterior and posterior fascicles of the left bundle 1, 3.
The Role of First-Degree AV Block
When first-degree AV block occurs in addition to true bifascicular block (e.g., CRBBB + LAFB + prolonged PR interval), this combination raises concern for more extensive conduction system disease. The guidelines explicitly state that "patients with first-degree AV block in association with bifascicular block and symptomatic, advanced AV block have a high mortality rate and a substantial incidence of sudden death" 2.
However, the PR prolongation in bifascicular block patients often reflects delay at the AV node level rather than additional fascicular involvement 3. Importantly, the guidelines specify that permanent pacemaker implantation is NOT indicated for fascicular block with first-degree AV block without symptoms (Class III recommendation, Level of Evidence B) 1.
Clinical Implications
The distinction matters because:
- True bifascicular block (CRBBB + LAFB or LPFB) indicates two-fascicle involvement with only one remaining functional fascicle
- CRBBB + first-degree AV block alone indicates right bundle disease plus AV nodal delay, but both left fascicles remain intact
- The risk of progression to complete heart block differs significantly between these patterns
Common Pitfall to Avoid
Do not confuse "bifascicular block with first-degree AV block" (which describes CRBBB + LAFB/LPFB + prolonged PR) with "CRBBB and first-degree AV block" (which lacks the left fascicular component). The former represents trifascicular involvement and carries higher risk; the latter is not bifascicular block 4, 5.
Research confirms that perioperative progression to complete heart block in patients with bifascicular block or LBBB is rare, and an additional first-degree AV block does not significantly increase the incidence of severe bradyarrhythmias 4, 5.