Left Posterior Fascicular Block (LPFB) ECG Review

A left posterior fascicular block ― also known as a left posterior hemiblock, LPHB ― occurs on the ECG when the posterior fascicle of the left bundle branch is no longer able to conduct action potentials.

This is much less common than a left anterior fascicular block, or LAFB, as the posterior fascicle is much more sparsely distributed; thus, a large amount of myocardial tissue must be damaged to block the posterior fascicle. 

The criteria to diagnose a LPFB on a 12-lead ECG include the following:

• Right axis deviation of 90-180 degrees
• Presence of a qR complex in lead III and a rS complex in lead I
• Absence of right atrial enlargement, or RAE, and/or right ventricular hypertrophy, and RVH

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Note: The above pattern can appear similar to the S₁Q₃T₃ pattern sometimes present in patients with a pulmonary embolus.

A LPFB can occur in the setting of a bifascicular block as well.

ECG Examples:

• Left Posterior Fascicular Block (LPFB) ECG
• Bifascicular Block - RBBB + LPFB ECG

References:
1. Chou’s Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
2. Surawicz B, et al. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram. Circulation. 2009; doi:10.1161/CIRCULATIONAHA.108.191095.