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:
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.
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.