What is a bifascicular block?

Bifascicular Block: Definition and Clinical Significance

Bifascicular block refers to ECG evidence of impaired conduction below the AV node in two of the three fascicles of the right and left bundles. 1 This conduction disorder involves blockage in two of the three major fascicles of the ventricular conduction system.

Types of Bifascicular Block

The most common presentations of bifascicular block include:

• Right bundle branch block (RBBB) with left anterior fascicular block
• Right bundle branch block (RBBB) with left posterior fascicular block
• Left bundle branch block (LBBB) alone (considered bifascicular as it affects both the anterior and posterior fascicles of the left bundle)

Anatomical Basis

The ventricular conduction system consists of three main fascicles:

1. Right bundle branch
2. Left anterior fascicle
3. Left posterior fascicle

When conduction is impaired in any two of these fascicles, it constitutes a bifascicular block.

Clinical Significance and Progression

• The rate of progression from bifascicular block to complete (third-degree) AV block is generally slow 1
• No single clinical or laboratory variable, including bifascicular block alone, identifies patients at high risk of death due to future bradyarrhythmia 1
• Although third-degree AV block is most often preceded by bifascicular block, the overall risk of progression is low 1

Associated Conditions

Bifascicular block is commonly associated with:

• Coronary artery disease (particularly affecting the left anterior descending artery) 2
• Hypertension 3
• Diabetes mellitus 3
• Cardiomyopathies
• Degenerative conduction system disease

Clinical Management

Asymptomatic Patients

• Permanent pacemaker implantation is NOT indicated for asymptomatic patients with bifascicular block without AV block 1
• Permanent pacemaker implantation is NOT indicated for patients with bifascicular block with first-degree AV block without symptoms 1
• Regular follow-up with ECG monitoring is recommended for surveillance of potential progression 4

Indications for Permanent Pacing

Permanent pacemaker implantation is indicated in the following scenarios:

1. Alternating bundle-branch block (evidence of block in all 3 fascicles on successive ECGs) 1
2. Syncope not demonstrated to be due to AV block when other likely causes have been excluded 1
3. Incidental finding of markedly prolonged HV interval (≥100 milliseconds) in asymptomatic patients 1
4. Incidental finding of pacing-induced infra-His block that is not physiological 1
5. Bifascicular block with intermittent complete heart block associated with symptomatic bradycardia 1

Important Considerations

• Syncope is common in patients with bifascicular block but is not necessarily associated with an increased incidence of sudden death 1
• While pacing can relieve neurological symptoms in patients with syncope and bifascicular block, it does not reduce the occurrence of sudden death 1
• In patients with permanent or transient third-degree AV block, syncope is associated with an increased incidence of sudden death 1
• PR interval prolongation is common in bifascicular block but often occurs at the level of the AV node and does not correlate with progression to complete heart block 1

Special Circumstances

• In the perioperative setting, prophylactic temporary pacing is generally not necessary in asymptomatic patients with bifascicular block, even with prolonged PR interval 5
• In acute myocardial infarction, bifascicular block may indicate a poorer prognosis due to its association with more extensive myocardial damage 2
• In neuromuscular diseases (myotonic dystrophy, limb-girdle muscular dystrophy, peroneal muscular atrophy), permanent pacing may be considered with bifascicular block even without symptoms 1

Bifascicular block requires clinical vigilance and appropriate follow-up, but in the absence of symptoms or higher-degree AV block, it generally does not require immediate intervention with permanent pacing.