left POSTERIOR fascicular Block

The left posterior fascicle (LPF) extends to the posterioinferior papillary muscle. The left anterior fascicle (LAF) extends to the anterosuperior papillary muscle. Normal activation of the LV spreads simultaneously from these two sites near the papillary muscles. The impulses normally travel in opposite directions and cancel each other out on the surface ECG. When a block in the LPF is present, LV activation proceeds from the LAF resulting in changes to the QRS complex:

LPFB ECG Criteria

Rightward axis
Small R waves in leads I and aVL (rS)
Small Q waves in inferior leads (qR)
QRS duration usually normal
Late R wave peak in aVF
Increased QRS voltage in limb leads
No evidence of RVH which is much more common than LPFB

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