more detailed information about left posterior fascicular block

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  • the normal left ventricular activation proceeds first down the left bundle branch, and then simultaneously through its two fascicles (left anterior fascicle and left posterior fascicle)
  • left posterior fascicular block (LPFB), also known as left posterior hemiblock, is much less common than left anterior fascicular block (LAFB) - this is due to a number of factors:
    • the posterior fascicle is a thicker structure than the anterior fascicle
    • posterior fascile has a dual blood supply
    • the anatomic location of the posterior fascicle is away from more turbulent ventricular blood flow
  • ECG changes in LPFB:
    • QRS interval <0.12 seconds
    • right axis deviation
    • rS pattern in the lateral limb leads (I and aVl)
    • qR complex in the inferior leads (II, III, and aVf)
    • no evidence of right ventricular hypertrophy
  • LPFB is a rare finding
    • LPFB is nonspecific for cardiac disease - however coronary artery disease is the most common cause
    • other associations for LPFB include hypertensive heart disease, aortic valvular pathology, and fibrocalcific disease of the cardiac skeleton
    • LPFB is the least common intraventricular conduction block seen in acute myocardial infarction
  • data on established LPFB are insufficient assess prognostic significance


  1. Harrigan RA et al. Electrocardiographic manifestations: bundle branch blocks and fascicular blocks. The Journal of Emergency Medicine 2003;25(1):67–77
  2. Rowlands DJ.Left and right bundle branch block, left anterior and left posterior hemiblock. Eur Heart J. 1984 Mar;5 Suppl A:99-105.

Last reviewed 01/2018