Left Anterior vs. Posterior Fasicular Blocks

16 Jun

Left Anterior Fasicular Block (more common of the two)

  • Left axis deviation (usually between -45 and -90 degrees)
  • Small Q waves with tall R waves in leads I and aVL
  • Small R waves with deep S waves in leads II, III, aVF

 block1

 

Left Posterior Fasicular Block (almost always associated with RBBB)

  • Right axis deviation (> +90 degrees)
  • Small R waves with deep S waves in leads I and aVL
  • Small Q waves with tall R waves in leads II, III and aVF

 block2

 

Clinical Significance

  • By itself with no symptoms, these blocks are not significant. May indicate underlying CAD which led to impairment or that the patient is at risk for progressing to LBBB.
  • LAFB can be seen in about 4% of acute MI cases. It is the most common type of intraventricular conduction defect seen in acute anterior MI, the LAD usually the involved vessel.
  • LPFB plus RBBB in acute MI associated with high mortality rate.  However, CAD is less common in RBBB with LPFB than in RBBB with LAFB or isolated LAFB.

 

Submitted by Joran Sequeira.

 

References: Biagani et al. Prognostic Significance of Left Anterior Hemiblock in Patients With Suspected Coronary Artery Disease. Journal of the American College of Cardiology. Volume 46, Issue 5, 6 September 2005, Pages 858–863; Elizari et al. Hemiblocks revisited. Circulation. 2007; 115: 1154-1163

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