A left posterior fascicular block (LPFB) 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 since the posterior fascicle is much more sparsely distributed, so 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 is as follows:
1. Right axis deviation of 90 to 180 degrees
2. The presence of a qR complex in lead III and a rS complex in lead I.
3. Absence of right atrial enlargement and/or right ventricular hypertrophy
Note: The above pattern can appears similar to the S1Q3T3 pattern sometimes present in a patient with a pulmonary embolus.
A left posterior fascicular block can occur in the setting of a bifascicular block as well.
1. Surawicz B et al. ACC/AHA recommendations for the standardization and interpretation of the electrocardiogram. Circulation. 2009;119:e235-240.
2. Chou's Electrocardiography in Clinical Practice: Adult and Pediatric, 6e
By Steven Lome