Introduction

• Bifascicular block is a combination of a right bundle branch block (RBBB) and a block of one of the left bundle branch fascicles – left anterior fascicular block (LAFB) or left posterior fascicular hemiblock (LPFB)

• This means that 2 of the 3 main conducting pathways through the ventricles are blocked

• Bifascicular block occurs in approx. 1% to 2% of the adult population

Causes

• Bifascicular block is often associated with structural heart disease and may be associated with progression to high-grade block or complete heart block
• The rate of progression to complete AV block is 1% to 4% per year
• In patients with syncope the rate goes up to 17% and is also associated with the presence of malignant ventricular arrhythmias 
• Block in either the LAF or LPF of the LBB produces axis deviation - the overall direction of the QRS complex is shifted toward the site of the block

RBBB + Left anterior fascicular block

• more common than RBBB + LPH
• RBB and LAF have a single coronary artery blood supply (left anterior descending)
• may be a sign of a large anteroseptal myocardial infarct

RBBB + Left posterior fascicular block

• is less common due to a dual blood supply (right and left circumflex coronary arteries)
• Even when changes in the ECG are typical, the diagnosis should be considered only if there is no clinical evidence of right ventricle hypertrophy (RVH)

ECG findings

RBBB + Left anterior hemiblock – RBBB pattern with left axis deviation

• the frontal plane axis of the complex should be between −45 and −120 degrees

RBBB + Left posterior hemiblock – RBBB pattern with right axis deviation

• the frontal plane axis of the complex should be ≥+90 degrees

Right bundle branch block (RBBB)

• QRS duration > 120ms
• RsR’ pattern in leads V1 to V2 – M shaped, “rabbit-ear” pattern
• Deep, broad S wave in leads I, aVL and V5 to V6.
• ST segment depression and/or T wave inversion in leads V1 – V3 - altered sequence of repolarization

Left axis deviation

• QRS is POSITIVE (dominant R wave) in Lead I 
• QRS is NEGATIVE (dominant S wave) in leads II, III and aVF

Right axis deviation

• QRS is POSITIVE (dominant R wave) in Lead II, Lead III and aVF 
• QRS is NEGATIVE (dominant S wave) in Lead I

ECG 1 Bifascicular block (Right bundle branch block with Left posterior fascicular block)

• QRS duration 140 ms
• RsR’ / M shaped pattern in leads V1 and V2
• slurred wide S waves in leads I, aVL, V5, V6
• ST segment depression in V1, V2
• right axis deviation + 140° - left posterior fascicular block

ECG 2 Bifascicular block (Right bundle branch block with Left anterior fascicular block)

• QRS duration 128 ms
• RsR’ / M shaped pattern in leads V1 and V2
• slurred wide S waves in leads I, aVL, V5, V6

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Management

• The mortality may be as high as 15% in 2 years, with a 9% risk of sudden death
• Treatment should be considered only if symptoms are present due to progression of AV block (to second-degree or higher AVB)

Olshansky, B. (2017). Bifascicular block management [Graph]. In Arrhythmia Essentials (2nd ed., p. 79).

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References

1. Brian Olshansky, Mina K. Chung, Steven M. Pogwizd, Nora Goldschlager, Chapter 2 - Bradyarrhythmias—Conduction System Abnormalities, Editor(s): Brian Olshansky, Mina K. Chung, Steven M. Pogwizd, Nora Goldschlager, Arrhythmia Essentials (Second Edition), Elsevier, 2017, Pages 28-86, ISBN 9780323399685, https://doi.org/10.1016/B978-0-323-39968-5.00002-0.
2. CAMM, A. J., LÜSCHER, T. F., & SERRUYS, P. W. (2009). The ESC textbook of cardiovascular medicine. Oxford, Oxford University Press
3. STRAUSS, David G., et al. Marriott's Practical Electrocardiography. Wolters Kluwer, 2021

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