What are the features of bifascicular and trifascicular (tri-fascicular) blocks?

From the Guidelines

Bifascicular and trifascicular blocks are conduction abnormalities affecting the bundle branches of the heart's electrical system, with bifascicular block involving two of the three fascicles and trifascicular block affecting all three fascicles, and management depends on symptom severity, with asymptomatic patients typically requiring monitoring and symptomatic patients often needing pacemaker implantation, especially with trifascicular block which carries a higher risk of progression to complete heart block, as indicated by the 2013 ESC guidelines on cardiac pacing and cardiac resynchronization therapy 1.

Features of Bifascicular and Trifascicular Blocks

• Bifascicular block involves two of the three fascicles (right bundle branch plus either the left anterior or left posterior fascicle), characterized by a wide QRS complex (>120 ms) with specific morphology patterns on ECG.
• Common features include right bundle branch block pattern with either left anterior or left posterior hemiblock.
• Trifascicular block affects all three fascicles, presenting as bifascicular block plus first-degree AV block, or alternating bundle branch blocks.
• These conditions may be asymptomatic but can progress to complete heart block, causing syncope, presyncope, or sudden cardiac death.

Risk Factors and Diagnosis

• Risk factors include coronary artery disease, cardiomyopathy, and degenerative conduction system disease.
• Diagnosis relies on 12-lead ECG showing characteristic patterns of conduction delay.

Management

• Management depends on symptom severity - asymptomatic patients typically require monitoring, while symptomatic patients often need pacemaker implantation, especially with trifascicular block which carries a higher risk of progression to complete heart block, as supported by the 2013 ACCF/AHA/HRS focused update incorporated into the ACCF/AHA/HRS 2008 guidelines for device-based therapy of cardiac rhythm abnormalities 1.
• The 2013 ESC guidelines on cardiac pacing and cardiac resynchronization therapy also recommend pacemaker implantation in patients with alternating bundle branch block, even in the absence of a history of syncope, due to the high risk of rapid progression to AV block 1.
• The European Society of Cardiology guidelines for cardiac pacing and cardiac resynchronization therapy also suggest that permanent pacing is considered appropriate only in asymptomatic patients with bifascicular or trifascicular block who exhibit intermittent second- or third-degree AV block, or signs of a severe conduction disturbance below the level of the AV node, as indicated by the 2007 guidelines 1 and the 2013 guidelines 1.

From the Research

Features of Bifascicular and Trifascicular Blocks

• Bifascicular block is characterized by the presence of two fascicular blocks, which can be either left anterior fascicular block (LAFB) and right bundle branch block (RBBB), or left posterior fascicular block (LPFB) and RBBB 2.
• Trifascicular block is characterized by the presence of three fascicular blocks, which can be either LAFB, LPFB, and RBBB 2.
• Patients with bifascicular block have an increased risk of progression to complete heart block, especially in the setting of an acute myocardial infarction 3, 4.
• The risk of complete heart block is higher in patients with trifascicular block compared to those with bifascicular block 4.
• Bifascicular and trifascicular blocks can be associated with syncope, pacemaker implantation, and increased mortality, especially in patients with underlying heart disease 4, 5, 6.

Clinical Implications

• Patients with bifascicular or trifascicular block may require temporary or permanent pacing, especially if they have symptoms such as syncope or bradycardia 3, 5.
• The presence of bifascicular or trifascicular block can be a predictor of adverse cardiovascular outcomes, including complete heart block, syncope, and mortality 4, 6.
• The type of bundle branch block (BBB) morphology can affect the outcome of patients with syncope and bifascicular block, with left BBB and right BBB having different implications for pacing and prognosis 5.

Diagnostic and Therapeutic Considerations

• Electrocardiographic (ECG) readings can be used to diagnose fascicular blocks, and Cox regression models can be used to estimate the hazard ratios and absolute risks of cardiovascular outcomes 4.
• Anesthesia, monitoring, and surgical techniques can induce conduction defects and bradyarrhythmias in patients with pre-existing bundle branch block, highlighting the need for careful management in these patients 3.
• Permanent pacing may be considered for patients with syncope and bifascicular block, especially if they have a high risk of adverse cardiovascular outcomes 5.