What is trifascicular block?

What is Trifascicular Block?

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. 1

Anatomic and Electrocardiographic Basis

The intraventricular conduction system is conceptualized as trifascicular, consisting of:

• The right bundle branch
• The left anterior fascicle
• The left posterior fascicle 1

The anatomic basis for this trifascicular system in humans is less clearly defined than the electrocardiographic concept. 1

Electrocardiographic Manifestations

The term "trifascicular block" has been used in two distinct ways:

Complete Trifascicular Block

• Represents complete heart block with impaired conduction through all three fascicles simultaneously 1
• Manifests as complete AV dissociation with a wide QRS escape rhythm 2, 3
• May progress from bifascicular block when the remaining functioning fascicle fails 1

Bifascicular Block with First-Degree AV Block

• The term has also been applied to bifascicular block (such as right bundle branch block with left anterior hemiblock) combined with first-degree AV block 1
• This usage is somewhat imprecise, as the PR prolongation often reflects AV nodal delay rather than disease in the third fascicle 4

Clinical Significance and Prognosis

Risk of Progression

• Patients with bifascicular block and prolonged HV interval (>55 milliseconds) have a slightly increased risk of developing complete trifascicular block 1
• The incidence of complete trifascicular block is low at 2-3% annually, but increases when the HV interval exceeds 100 milliseconds 1
• The rate of progression is slow in the absence of acute intervening events such as drugs, electrolyte abnormalities, or ischemia 1

Predictive Value of HV Interval

• The HV interval in patients with bifascicular block measures conduction time through the remaining functioning fascicle 1
• The HV interval has high sensitivity (82%) but low specificity (63%) for predicting development of complete trifascicular block 1

Mortality Considerations

• Sudden death in patients with bifascicular block may not be caused by progression to complete trifascicular block, but rather by ventricular tachyarrhythmias 1
• This is particularly important in patients with advanced heart disease 1

Clinical Presentations

Trifascicular block may present as:

• Intermittent complete heart block with symptomatic bradycardia, syncope, or Adams-Stokes attacks 2, 5, 3
• Asymptomatic electrocardiographic findings in patients with bifascicular block and PR prolongation 1
• Acute transient forms (associated with myocarditis or acute myocardial infarction) that may resolve with medical treatment 5
• Chronic progressive forms requiring permanent pacemaker implantation 5, 6

Common Etiologies

The most frequent causes include:

• Coronary artery disease (particularly with anteroseptal myocardial infarction) 7
• Arterial hypertension 7
• Cardiomyopathies 7
• Lev and Lenègre diseases (progressive degenerative conduction system disease) 7
• Chagas disease in endemic areas 3
• Acute myocarditis 2

Important Clinical Pitfalls

• Do not assume that PR interval prolongation in bifascicular block indicates disease in the third fascicle—the delay is often at the AV node level 4
• Electrophysiological evaluation should assess both bradyarrhythmias and tachyarrhythmias, as ventricular arrhythmias may be the actual cause of symptoms or sudden death 1
• 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 1