Bifascicular Block

Diagnostic criteria

Clinically, bifascicular block presents with one of two ECG patterns:

  • Right bundle branch block (RBBB) with left anterior fascicular block (LAFB), manifested as left axis deviation (LAD)
  • RBBB and left posterior fascicular block (LPFB), manifested as right axis deviation (RAD) in the absence of other causes

*Some authors describe Left bundle branch block (LBBB) as a bifascicular block, as it may indicate LAFB + LPFB. However, clinically the term bifascicular block is reserved for RBBB with either LAFB or LPFB


Bifascicular block involves conduction delay below the atrioventricular node in two of the three fascicles:

  • Conduction to the ventricles is via the single remaining fascicle
  • The ECG will show typical features of RBBB plus either left or right axis deviation.
  • RBBB + LAFB is the most common of the two patterns. This is due to a single coronary artery blood supply (LAD) to the anterior fascicle
  • RBBB + LPFB is less common due to a dual blood supply (right and left circumflex arteries), and this combination may be associated with more extensive underlying cardiac pathology

ECG Bifascicular Block (RBBB + LAFB)
Typical bifascicular block pattern: RBBB combined with LAFB (manifested as LAD)

Clinical significance

Bifascicular block is often associated with structural heart disease (50-80%) and extensive fibrosis of the conducting system. There is a risk of progression to complete heart block with additional damage to the third remaining fascicle, however clinical context is important:

  • Overall rate of progression to complete heart block is 1-4% per year
  • In symptom free patients, these figures are ~1% per year
  • Patients presenting with syncope have a 17% annual risk of progression
  • Syncope or presyncope in the context of a bifascicular block is an indication for admission and monitoring. If other causes of syncope are not identified on work-up, pacemaker insertion is recommended

Main Causes of Bifascicular Block

Causes are similiar to those of RBBB and LAFB/LPFB:

A new-onset bifascicular block in the context of chest pain is highly associated with proximal LAD occlusion, even in the absence of ST-segment changes. In 30% of cases, patients present with no ST elevation and bifascicular block is the only acute ECG finding.

Differential diagnosis
  • Masquerading Bundle Branch Block (MBBB) presents with a mixed complete RBBB and complete LBBB pattern similar to a typical bifascicular block pattern. This rare ECG pattern indicates more extensive fibrosis of the conducting system and limited studies demonstrate a poorer prognosis and higher progression to complete heart block
  • In the context of RBBB, RAD indicating LPFB may be due to other causes such as right ventricular hypertrophy, and these need to be excluded before the ECG is labelled bifascicular block

ECG examples
Example 1


  • RBBB pattern in precordial leads with RSR’ complex in V1-2
  • Prominent LAD indicating LAFB

Example 2

RBBB with LAFB, in the context of chest pain

  • RBBB is seen with RSR’ pattern in V1-3 and slurred S waves in lateral leads
  • There are concordant ST segment changes best seen in V2, and hyper-acute T waves inferiorly.
  • This patient was found to have a 99% proximal LAD occlusion. See OMI: Replacing the STEMI misnomer for further case details

Example 3a
RBBB with LPFB bifascicular block 2
Modified example by Prof. Dr. Johnson Francis


  • RBBB with wide QRS, slurred S wave in lead I and slurred R in V1.
  • Right axis deviation (dominant negative deflection in leads I and aVl) with dominant positive deflection in aVf along with rS pattern in lead I and qR pattern leads III and aVf, suggesting left posterior fascicular block.

Example 3b
Bifascicular block RBBB LPFB RAD

RBBB with LPFB and RAD

Advanced Reading



LITFL Further Reading



Emergency Physician in Prehospital and Retrieval Medicine in Sydney, Australia. He has a passion for ECG interpretation and medical education | ECG Library |

MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner

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