What is a trifascicular heart block?

What is Trifascicular Heart Block?

Trifascicular block refers to impaired conduction in all three main fascicles of the ventricular conduction system—the right bundle branch, left anterior fascicle, and left posterior fascicle—occurring either simultaneously or at different times. 1

Anatomic and Conceptual Framework

The ventricular conduction system is conceptualized as having three fascicles, though the anatomic basis in humans is less clearly defined than the electrocardiographic concept. 1 The three fascicles are:

• Right bundle branch 1
• Left anterior fascicle 1
• Left posterior fascicle 1

Electrocardiographic Definitions

There are two main ways the term "trifascicular block" is used clinically:

Complete Trifascicular Block

• Represents complete heart block with impaired conduction through all three fascicles simultaneously 1
• This is the true anatomic definition where all fascicles are blocked at the same time 2

Bifascicular Block Plus First-Degree AV Block

• The term is also commonly applied to bifascicular block combined with first-degree AV block (PR interval >200 ms) 2, 1
• This usage assumes the PR prolongation represents disease in the third fascicle, though this is not always accurate 3
• Common bifascicular patterns include:
  • Right bundle branch block + left anterior hemiblock 2
  • Right bundle branch block + left posterior hemiblock 2
  • Complete left bundle branch block alone 2

Alternating Bundle Branch Block

• Refers to electrocardiographically documented block of all three branches on the same or successive ECG recordings 2
• This pattern definitively proves disease in all three fascicles 3

Clinical Significance and Natural History

Progression Risk

• The annual incidence of progression to complete heart block is 2-3% in patients with bifascicular block 1
• Risk increases substantially when the HV interval exceeds 100 milliseconds, with 24% progressing to AV block at 4 years 1
• Progression accelerates with acute events such as drugs, electrolyte abnormalities, or ischemia 1

Mortality Considerations

• Sudden death in these patients is often due to ventricular tachyarrhythmias rather than progression to complete block 1, 3
• This is particularly true in patients with advanced underlying heart disease 1
• The HV interval has high sensitivity (82%) but low specificity (63%) for predicting complete block 1

Common Clinical Pitfalls

PR Interval Misinterpretation

• First-degree AV block in bifascicular block often represents AV nodal delay, not true trifascicular disease 3
• PR prolongation does not reliably correlate with progression to complete heart block 4

Cause of Death Attribution

• Do not assume bradycardia is the only life-threatening risk—electrophysiological evaluation should assess both bradyarrhythmias and tachyarrhythmias 1
• Ventricular arrhythmias may be the actual cause of syncope or sudden death 1

Population at Risk

• Most commonly occurs in patients with coronary artery disease and hypertensive heart disease 2, 5
• Prevalence increases with age, from 1% at age 17 to higher rates at age 80 2