Trifascicular Block

Definition and Anatomic Basis

Trifascicular block refers to impaired conduction in all three main fascicles of the ventricular conduction system—the right bundle branch, the left anterior fascicle, and the left posterior fascicle—occurring either simultaneously or at different times, as defined by the American College of Cardiology 1, 2
The intraventricular conduction system is conceptualized as trifascicular, consisting of the right bundle branch 3, and the left posterior fascicle 1, 4, 5
The anatomic basis for this trifascicular system in humans is less clearly defined than the electrocardiographic concept, according to the American College of Cardiology 1, 4

Complete trifascicular block represents complete heart block with impaired conduction through all three fascicles simultaneously, as described by the American College of Cardiology 6, 1
The term "trifascicular block" has also been applied to bifascicular block (such as right bundle branch block with left anterior hemiblock) combined with first-degree AV block, as noted by the European Heart Journal 2

Patients with bifascicular block and prolonged HV interval (>55 milliseconds) have a slightly increased risk of developing complete trifascicular block, according to the American College of Cardiology 6, 1, 3
The incidence of complete trifascicular block is low at 2-3% annually, but increases when the HV interval exceeds 100 milliseconds, as reported by the American College of Cardiology 6, 1, 4
The rate of progression is slow in the absence of acute intervening events such as drugs, electrolyte abnormalities, or ischemia, as stated by the American College of Cardiology 6, 1, 3
The HV interval has high sensitivity (82%) but low specificity (63%) for predicting development of complete trifascicular block, according to the American College of Cardiology 6, 1, 4

Sudden death in patients with bifascicular block may not be caused by progression to complete trifascicular block, but rather by ventricular tachyarrhythmias, as noted by the American College of Cardiology 6, 3, 7

Electrophysiological evaluation should assess both bradyarrhythmias and tachyarrhythmias, as ventricular arrhythmias may be the actual cause of symptoms or sudden death, according to the American College of Cardiology 6, 3, 7
Rapid atrial pacing during electrophysiological study can improve specificity for predicting progression to complete block, with an abnormal response being block distal to the His bundle during 1:1 AV nodal conduction, as described by the American College of Cardiology 6, 1

The American College of Cardiology suggests that the intraventricular conduction system is conceptualized as having three main fascicles: the right bundle branch, the left anterior fascicle, and the left posterior fascicle 8, 9
The European Heart Journal states that 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 10

The European Heart Journal indicates that the most common ECG pattern showing trifascicular disease consists of bifascicular block (either RBBB + left anterior fascicular block OR RBBB + left posterior fascicular block OR complete LBBB) plus first-degree AV block (PR interval >200 ms) 10

The American College of Cardiology recommends that complete left bundle branch block is characterized by a QRS duration ≥120 ms, monophasic notched or plateau-topped R waves in leads I, aVL, V5, and V6, and slurred predominant S wave in right precordial leads 11

The American College of Cardiology states that the HV interval on electrophysiologic study provides prognostic information, with an HV interval >55 ms indicating slightly increased risk of complete trifascicular block, and an HV interval >100 ms indicating higher risk with 24% progression to AV block at 4 years 8, 9, 10
The European Heart Journal reports that the annual incidence of complete trifascicular block is 2-3% in patients with bifascicular block, and the risk increases to 17% in patients with syncope 8, 9, 10
The American College of Cardiology notes that progression accelerates with acute events, such as drugs, electrolyte abnormalities, or ischemia, and that ventricular tachyarrhythmias may be the actual cause of sudden death, particularly in patients with advanced heart disease 9, 12

Complete trifascicular block is defined as a complete atrioventricular block caused by simultaneous impairment of conduction through the right bundle branch, left anterior fascicle, and left posterior fascicle. This reflects an anatomic blockage of all three fascicles at the same time. 13

Bifascicular block combined with first‑degree AV block (PR interval > 200 ms) is frequently termed “trifascicular block,” assuming that the PR prolongation indicates disease of the third fascicle, although this assumption may not always be accurate. 14
Typical bifascicular ECG patterns that may be classified as trifascicular when accompanied by first‑degree AV block include:
- Right bundle branch block + left anterior hemiblock 14
- Right bundle branch block + left posterior hemiblock 14
- Isolated complete left bundle branch block 14
Alternating bundle‑branch block is identified when electrocardiograms demonstrate block of all three branches either on the same recording or on successive recordings, providing definitive proof of disease in all three fascicles. 14

Patients with coronary artery disease and hypertensive heart disease are the most common population in which trifascicular block is observed. 14
The prevalence of trifascicular block rises with age, being about 1 % in adolescents (≈ 17 years) and increasing markedly in older adults (e.g., substantially higher rates by age 80). 14