ECG Criteria for Trifascicular Block
Trifascicular block on ECG manifests as either complete heart block (third-degree AV block) representing simultaneous impairment of all three fascicles, or as bifascicular block (RBBB with left anterior or posterior fascicular block) combined with first-degree AV block, indicating disease in all three fascicles occurring at different times. 1
Understanding the Trifascicular Concept
The intraventricular conduction system is conceptualized as having three main fascicles: the right bundle branch, the left anterior fascicle, and the left posterior fascicle. 2, 1 While the anatomic basis for this trifascicular system in humans is less clearly defined than the electrocardiographic concept, this framework remains the most clinically useful approach. 2, 1
ECG Manifestations of Trifascicular Block
Complete (Simultaneous) Trifascicular Block
- Complete heart block (third-degree AV block) with impaired conduction through all three fascicles simultaneously 1
- This represents the most severe form where no atrial impulses conduct to the ventricles 3
Incomplete (Sequential) Trifascicular Block
The most common ECG pattern showing trifascicular disease consists of: 2
- Bifascicular block (either RBBB + left anterior fascicular block OR RBBB + left posterior fascicular block OR complete LBBB) 4, 5
- PLUS first-degree AV block (PR interval >200 ms) 2
This combination indicates that while two fascicles are completely blocked, the third fascicle has delayed conduction, demonstrating disease in all three pathways. 2
Specific ECG Criteria for Bifascicular Blocks
Right Bundle Branch Block + Left Anterior Fascicular Block
- RBBB pattern: rsr', rsR', or rSR' in V1-V2 with QRS ≥120 ms 6
- Left anterior fascicular block: Left axis deviation between -45° to -90°, qR pattern in aVL, R peak time in aVL ≥45 ms 7
- S wave duration >R wave or >40 ms in leads I and V6 6
Right Bundle Branch Block + Left Posterior Fascicular Block
- RBBB pattern as above 6
- Left posterior fascicular block: Right axis deviation, small q waves in inferior leads (II, III, aVF), small r waves in lateral leads (I, aVL) 4
Complete Left Bundle Branch Block
- QRS duration ≥120 ms 2
- Monophasic notched or plateau-topped R waves in leads I, aVL, V5, and V6 2
- Slurred predominant S wave in right precordial leads 2
The Most Alarming ECG Pattern
Alternating complete left and right bundle branch block, or alternating RBBB with left anterior or posterior fascicular block, represents the most concerning finding, suggesting imminent high-degree AV block. 2 This pattern demonstrates intermittent failure of different fascicles and carries the highest risk for progression to complete heart block. 2
Critical Clinical Context
Risk Stratification Beyond Surface ECG
While surface ECG identifies the pattern, the HV interval on electrophysiologic study provides prognostic information: 2
- HV interval >55 ms: Slightly increased risk of complete trifascicular block 2, 1
- HV interval >100 ms: Higher risk with 24% progression to AV block at 4 years 2
- Sensitivity 82%, specificity only 63% for predicting progression 2, 1
Progression Rates
- Annual incidence of complete trifascicular block: 2-3% in patients with bifascicular block 2, 1
- Risk increases to 17% in patients with syncope versus 2% without syncope 2
- Progression accelerates with acute events: drugs, electrolyte abnormalities, or ischemia 2, 1
Important Clinical Pitfall
Do not assume syncope in bifascicular block patients is solely due to progression to complete heart block—ventricular tachyarrhythmias may be the actual cause of sudden death, particularly in patients with advanced heart disease. 2, 1 Electrophysiologic evaluation should assess both bradyarrhythmias and tachyarrhythmias. 2, 1
Additional ECG Pattern: Quadrifascicular Concept
Some evidence suggests that bifascicular block (RBBB + LAFB or LPFB) combined with an incomplete LBBB pattern may represent trifascicular block involving the septal fascicle of the left bundle. 5 This pattern carries significantly higher risk: